CHAPTER OVERVIEW 12: Sex Strategies Learning Objectives Compare and contrast the costs and benefits of asexual reproduction versus sexual reproduction, and explain hypotheses for the evolution of sexual reproduction. Explain the variety of ways in which sexes are determined across plants and animals. Describe different types of mating systems in plants and animals. Describe the different forms of sexual selection, and explain hypotheses for the evolution of female choice. 12.1: The paradox of sex- sexual versus asexual reproduction 12.2: Sex determination and sex ratios 12.3: Scientist Spotlight - Nettie Stevens 12.4: Mating systems in sexual animals 12.5: Mating Systems in Plants 12.6: Scientist Spotlight - Ernest Everett Just 12.7: Sexual selection Summary The paradox of sexual reproduction is that although it is ubiquitous in multicellular organisms, there are many disadvantages to reproducing sexually when we compare it to asexual reproduction. This chapter explores some of the multitude of hypotheses for why sexual reproduction exists, the variety of ways in which sexes are determined in plants and animals, and some of the variation in mating systems of sexual organisms. This chapter also focuses on sexual selection, a form of natural selection that occurs when traits that improve mating success are favored by selection, even if they cause a decrease in survival. 12: Sex Strategies is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts.
12.1: The paradox of sex- sexual versus asexual reproduction The Paradox of Sex Sexual Reproduction Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete (such as a sperm or egg cell) with a single set of chromosomes (haploid) combines with another to produce a zygote that develops into an organism composed of cells with two sets of chromosomes (diploid) (Maynard Smith & Szathmáry, 1995). Sexual reproduction is the most common life cycle in multicellular eukaryotes, such as animals, fungi and plants. Sexual reproduction does not occur in prokaryotes (organisms without cell nuclei), but they have processes with similar effects such as bacterial conjugation, transformation and transduction, which may have been precursors to sexual reproduction in early eukaryotes. In the production of sex cells in eukaryotes, diploid mother cells divide to produce haploid cells known as gametes in a process called meiosis that involves genetic recombination. The homologous chromosomes pair up so that their DNA sequences are aligned with each other, and this is followed by exchange of genetic information between them. Two rounds of cell division then produce four haploid gametes, each with half the number of chromosomes from each parent cell, but with the genetic information in the parental chromosomes recombined. Two haploid gametes combine into one diploid cell known as a zygote in a process called fertilization. The zygote incorporates genetic material from both gametes. Multiple cell divisions, without change of the number of chromosomes, then form a multicellular diploid phase or generation. The evolution of sexual reproduction is considered paradoxical, because asexually reproducing individuals should be able to outperform sexually reproducing individuals (Otto 2014). This is because every offspring produced by an asexually reproducing individual can produce its own own offspring, while sexually reproducing individuals must produce two sexes: one to fertilize the opposite sex and one to produce/bear offspring. This implies that an asexual population has an intrinsic capacity to grow more rapidly with each generation (Maynard Smith, 1978). This 50% cost is a fitness disadvantage of sexual reproduction (Ridley, 2004). The two-fold cost of sex includes this cost and the fact that any organism can only pass on 50% of its own genes to its offspring. One definite advantage of sexual reproduction is that it impedes the accumulation of genetic mutations (Hussin et al., 2015). The first fossilized evidence of sexual reproduction in eukaryotes is from the Stenian period, about 1.05 billion years ago (Butterfield, 2000; Gibson, 2018).
: In the first stage of sexual reproduction, "meiosis", the number of chromosomes is reduced from a diploid number
(2n) to a haploid number (n). During "fertilization", haploid gametes come together to form a diploid zygote, and the original
Asexual Reproduction Asexual reproduction is a type of reproduction that does not involve the fusion of gametes or change in the number of chromosomes. The offspring that arise by asexual reproduction from either unicellular or multicellular organisms inherit the full set of genes of their single parent. Asexual reproduction is the primary form of reproduction for single-celled organisms such as
archaea and bacteria. Many eukaryotic organisms including plants, animals, and fungi can also reproduce asexually (Engelstädter, 2017). In vertebrates, the most common form of asexual reproduction is parthenogenesis (described below), which is typically used as an alternative to sexual reproduction in times when reproductive opportunities are limited (Dudgeon et al., 2017). While all prokaryotes reproduce without the formation and fusion of gametes, mechanisms for lateral gene transfer such as conjugation, transformation and transduction can be likened to sexual reproduction in the sense of genetic recombination in meiosis (Narra & Ochman, 2006).
: Asexual reproduction in liverworts: a caducous phylloid germinating.
Types of Asexual Reproduction Fission Prokaryotes (archaea and bacteria) reproduce asexually through binary fission, in which the parent organism divides in two to produce two genetically identical daughter organisms. Eukaryotes (such as protists and unicellular fungi) may reproduce in a functionally similar manner by mitosis; most of these are also capable of sexual reproduction. Multiple fission at the cellular level occurs in many protists, e.g. sporozoans and algae. The nucleus of the parent cell divides several times by mitosis, producing several nuclei. The cytoplasm then separates, creating multiple daughter cells (Encyclopædia Britannica, n.d.; Britannica Educational Publishing, 2011; Puranik & Bhate, 2007). Budding
: The yeast Saccharomyces cerevisiae reproducing by budding.
Some cells divide by budding (for example baker's yeast), resulting in a "mother" and a "daughter" cell that is initially smaller than the parent. Budding is also known on a multicellular level; an animal example is the hydra, which reproduces by budding (Leeuwenhoek, 1703). The buds grow into fully matured individuals which eventually break away from the parent organism.
Internal budding is a process of asexual reproduction, favored by parasites such as Toxoplasma gondii. It involves an unusual process in which two (endodyogeny) or more (endopolygeny) daughter cells are produced inside a mother cell, which is then consumed by the offspring prior to their separation (Smyth, 1994).
: Vegetative plantlets of mother-of-thousands, Bryophyllum daigremontianum (Kalanchoe daigremontiana).
Vegetative propagation is a type of asexual reproduction found in plants where new individuals are formed without the production of seeds or spores and thus without syngamy or meiosis (University of California Museum of Paleontology, nd). Examples of vegetative reproduction include the formation of miniaturized plants called plantlets on specialized leaves, for example in kalanchoe (Bryophyllum daigremontianum) and many produce new plants from rhizomes or stolon (for example in strawberry). Other plants reproduce by forming bulbs or tubers (for example tulip bulbs and Dahlia tubers). Some plants produce adventitious shoots and may form a clonal colony. In these examples, all the individuals are clones, and the clonal population may cover a large area (U.S. Forest Service, 2010).
Spore Formation Many multicellular organisms form spores during their biological life cycle in a process called sporogenesis. Exceptions are animals and some protists, which undergo meiosis immediately followed by fertilization. Plants and many algae on the other hand undergo sporic meiosis where meiosis leads to the formation of haploid spores rather than gametes. These spores grow into multicellular individuals (called gametophytes in the case of plants) without a fertilization event. These haploid individuals give rise to gametes through mitosis. Meiosis and gamete formation therefore occur in separate generations or "phases" of the life cycle, referred to as alternation of generations. Since sexual reproduction is often more narrowly defined as the fusion of gametes (fertilization), spore formation in plant sporophytes and algae might be considered a form of asexual reproduction (agamogenesis) despite being the result of meiosis and undergoing a reduction in ploidy. However, both events (spore formation and fertilization) are necessary to complete sexual reproduction in the plant life cycle. Fungi and some algae can also utilize true asexual spore formation, which involves mitosis giving rise to reproductive cells called mitospores that develop into a new organism after dispersal. This method of reproduction is found for example in conidial fungi and the red algae Polysiphonia, and involves sporogenesis without meiosis. Thus the chromosome number of the spore cell is the same as that of the parent producing the spores.
: Linckia guildingi "comet", a starfish regrowing from a single arm.
Fragmentation is a form of asexual reproduction where a new organism grows from a fragment of the parent. Each fragment develops into a mature, fully grown individual. Fragmentation is seen in many organisms. Animals that reproduce asexually include planarians, many annelid worms including polychaetes and some oligochaetes, turbellarians and sea stars (Ruppert et al., 2004). Many fungi and plants reproduce asexually. Some plants have specialized structures for reproduction via fragmentation, such as gemmae in liverworts. Most lichens, which are a symbiotic union of a fungus and photosynthetic algae or cyanobacteria, reproduce through fragmentation to ensure that new individuals contain both symbionts.
Parthenogenesis Parthenogenesis is a form of agamogenesis in which an unfertilized egg develops into a new individual. It has been documented in over 2,000 species (Sköld et al., 2009). Parthenogenesis occurs in the wild in many invertebrates (e.g. water fleas, rotifers, aphids, stick insects, some ants, bees and parasitic wasps) and vertebrates (mostly reptiles, amphibians, and fish). It has also been documented in domestic birds and in genetically altered lab mice (Neuhof et al., 2016; Ramachandran & McDaniel, 2018). Plants can engage in parthenogenesis as well through a process called apomixis. However this process is considered by many to not be an independent reproduction method, but instead a breakdown of the mechanisms behind sexual reproduction (Ozias-Akins & Conner, 2012). Parthenogenetic organisms can be split into two main categories: facultative and obligate.
The Evolution of Sex Sexual reproduction is an adaptive feature which is common to almost all multi-cellular organisms (and also some single-cellular organisms) with many being incapable of reproducing asexually. Prior to the advent of sexual reproduction, the adaptation process whereby genes would change from one generation to the next (genetic mutation) happened very slowly and randomly. Sex evolved as an extremely efficient mechanism for producing variation, and this had the major advantage of enabling organisms to adapt to changing environments. Sex did, however, come with a cost. In reproducing asexually, no time nor energy needs to be expended in choosing a mate. And if the environment has not changed, then there may be little reason for variation, as the organism may already be well adapted. Sex, however, has evolved as the most prolific means of species branching into the tree of life. Diversification into the phylogenetic tree happens much more rapidly via sexual reproduction than it does by way of asexual reproduction. Evolution of sexual reproduction describes how sexually reproducing animals, plants, fungi and protists could have evolved from a common ancestor that was a single-celled eukaryotic species (Letunic & Bork, 2006, 2007, 2011). Sexual reproduction is widespread in the Eukarya, though a few eukaryotic species have secondarily lost the ability to reproduce sexually, such as Bdelloidea, and some plants and animals routinely reproduce asexually (by apomixis and parthenogenesis) without entirely having lost sex. The evolution of sex contains two related yet distinct themes: its origin and its maintenance. The origin of sexual reproduction can be traced to early prokaryotes, around two billion years ago (Gya), when bacteria began exchanging genes via conjugation, transformation, and transduction (Otto, 2014). Though these processes are distinct from true sexual reproduction, they share some basic similarities. In eukaryotes, true sex is thought to have arisen in the Last Eukaryotic Common Ancestor (LECA), possibly via several processes of varying success, and then to have persisted (compare to "LUCA Last Universal Common Ancestor," referring to ancestry shared by plants, animals, and bacteria) (Goodenough & Heitman, 2014). Since hypotheses for the origin of sex are difficult to verify experimentally (outside of evolutionary computation), most current work has focused on the persistence of sexual reproduction over evolutionary time. The maintenance of sexual reproduction (specifically, of its dioecious form) by natural selection in a highly competitive world has long been one of the major mysteries of biology, since both other known mechanisms of reproduction - asexual reproduction and hermaphroditism - possess apparent advantages over it. Asexual reproduction can proceed by budding, fission, or spore formation and does not involve the union of gametes, which accordingly results in a much faster rate of reproduction compared to sexual reproduction, where 50% of offspring are males and unable to produce offspring themselves. In hermaphroditic reproduction, each of the two parent organisms required for the formation of a zygote can provide either the male or the female gamete, which leads to advantages in both size and genetic variance of a population. Sexual reproduction therefore must offer significant fitness advantages because, despite the two-fold cost of sex (see below), it dominates among multicellular forms of life, implying that the fitness of offspring produced by sexual processes outweighs the costs. Sexual reproduction derives from recombination, where parent genotypes are reorganized and shared with the offspring. This stands in contrast to single-parent asexual replication, where the offspring is always identical to the parents (barring mutation). Recombination supplies two fault-tolerance mechanisms at the molecular level: recombinational DNA repair (promoted during meiosis because homologous chromosomes pair at that time) and complementation (also known as heterosis, hybrid vigor or masking of mutations).
: Ladybugs mating; Pollen production is an essential step in sexual reproduction of seed plants.
Historical Perspective The issue of the evolution of sexual reproduction features in the writings of Aristotle, and modern philosophical-scientific thinking on the problem dates from at least Erasmus Darwin (1731-1802) in the 18th century. August Weismann picked up the thread in 1889, arguing that sex serves to generate genetic variation, as detailed in the majority of the explanations below. On the other hand, Charles Darwin (1809-1882) concluded that the effect of hybrid vigor (complementation) "is amply sufficient to account for the ...
genesis of the two sexes". This is consistent with the repair and complementation hypothesis, described below. Since the emergence of the modern evolutionary synthesis in the 20th century, numerous biologists including W. D. Hamilton, Alexey Kondrashov, George C. Williams, Harris Bernstein, Carol Bernstein, Michael M. Cox, Frederic A. Hopf and Richard E. Michod - have suggested competing explanations for how a vast array of different living species maintain sexual reproduction. Disadvantages of Sex and Sexual Reproduction The paradox of the existence of sexual reproduction is that though it is ubiquitous in multicellular organisms, there are ostensibly many inherent disadvantages to reproducing sexually when weighed against the relative advantages of alternative forms of reproduction, such as asexual reproduction. Thus, because sexual reproduction abounds in complex multicellular life, there must be some significant benefit(s) to sex and sexual reproduction that compensates for these fundamental disadvantages. Population Expansion Cost of Sex Among the most limiting disadvantages to the evolution of sexual reproduction by natural selection is that an asexual population can grow much more rapidly than a sexual one with each generation. For example, assume that the entire population of some theoretical species has 100 total organisms consisting of two sexes (i.e. males and females), with 50:50 male-to-female representation, and that only the females of this species can bear offspring. If all capable members of this population procreated once, a total of 50 offspring would be produced (the F1 generation). Contrast this outcome with an asexual species, in which each and every member of an equally sized 100-organism population is capable of bearing young. If all capable members of this asexual population procreated once, a total of 100 offspring would be produced - twice as many as produced by the sexual population in a single generation.
: This diagram illustrates the two-fold cost of sex. If each individual were to contribute to the same number of
offspring (two), (a) the sexual population remains the same size each generation, where the (b) asexual population doubles in size
This idea is sometimes referred to as the two-fold cost of sexual reproduction. It was first described mathematically by John Maynard Smith (1978). In his manuscript, Smith further speculated on the impact of an asexual mutant arising in a sexual population, which suppresses meiosis and allows eggs to develop into offspring genetically identical to the mother by mitotic division (Stearns, 2005). The mutant-asexual lineage would double its representation in the population each generation, all else being equal.
Technically the problem above is not one of sexual reproduction but of having a subset of organisms incapable of bearing offspring. Indeed, some multicellular organisms (isogamous) engage in sexual reproduction but all members of the species are capable of bearing offspring (Hoekstra, 1987). The two-fold reproductive disadvantage assumes that males contribute only genes to their offspring and sexual females waste half their reproductive potential on sons (Stearns, 2005). Thus, in this formulation, the principal cost of sex is that males and females must successfully copulate, which almost always involves expending energy to come together through time and space. Asexual organisms do not need to expend the energy necessary to find a mate.
Genetic Heritability Cost of Sex A sexually reproducing organism only passes on ~50% of its own genetic material to each offspring. This is a consequence of the fact that gametes from sexually reproducing species are haploid. Again, however, this is not applicable to all sexual organisms. There are numerous species which are sexual but do not have a genetic-loss problem because they do not produce males or females. Yeast, for example, are isogamous sexual organisms which have two mating types which fuse and recombine their haploid genomes. Both sexes reproduce during the haploid and diploid stages of their life cycle and have a 100% chance of passing their genes into their offspring (Hoekstra, 1987).
Some species avoid the 50% cost of sexual reproduction, although they have "sex" (in the sense of genetic recombination). In these species (e.g., bacteria, ciliates, dinoflagellates and diatoms), "sex" and reproduction occurs separately (Ridley, 2003; Beukeboom & Perrin, 2014). Advantages of Sex and Sexual Reproduction The concept of sex includes two fundamental phenomena: the sexual process (fusion of genetic information of two individuals) and sexual differentiation (separation of this information into two parts). Depending on the presence or absence of these phenomena, all of the existing forms of reproduction can be classified as asexual, hermaphrodite or dioecious. The sexual process and sexual differentiation are different phenomena, and, in essence, are diametrically opposed. The first creates (increases) diversity of genotypes, and the second decreases it by half. Reproductive advantages of the asexual forms are in quantity of the progeny, and the advantages of the hermaphrodite forms are in maximal diversity. Transition from the hermaphrodite to dioecious state leads to a loss of at least half of the diversity. So, the primary challenge is to explain the advantages given by sexual differentiation, i.e. the benefits of two separate sexes compared to hermaphrodites rather than to explain benefits of sexual forms (hermaphrodite + dioecious) over asexual ones. It has already been understood that since sexual reproduction is not associated with any clear reproductive advantages, as compared with asexual, there should be some important advantages in evolution (Crow, 1994). Advantages Due to Genetic Variation For the advantage due to genetic variation, there are three possible reasons this might happen. First, sexual reproduction can combine the effects of two beneficial mutations in the same individual (i.e. sex aids in the spread of advantageous traits). Also, the necessary mutations do not have to have occurred one after another in a single line of descendants (Goldstein, 2010). Second, sex acts to bring together currently deleterious mutations to create severely unfit individuals that are then eliminated from the population (i.e. sex aids in the removal of deleterious genes). However, in organisms containing only one set of chromosomes, deleterious mutations would be eliminated immediately, and therefore removal of harmful mutations is an unlikely benefit for sexual reproduction. Lastly, sex creates new gene combinations that may be more fit than previously existing ones, or may simply lead to reduced competition among relatives. For the advantage due to DNA repair, there is an immediate large benefit of removing DNA damage by recombinational DNA repair during meiosis, since this removal allows greater survival of progeny with undamaged DNA. The advantage of complementation to each sexual partner is avoidance of the bad effects of their deleterious recessive genes in progeny by the masking effect of normal dominant genes contributed by the other partner. The classes of hypotheses based on the creation of variation are further broken down below. Any number of these hypotheses may be true in any given species (they are not mutually exclusive), and different hypotheses may apply in different species. However, a research framework based on creation of variation has yet to be found that allows one to determine whether the reason for sex is universal for all sexual species, and, if not, which mechanisms are acting in each species. On the other hand, the maintenance of sex based on DNA repair and complementation applies widely to all sexual species. Protection from Major Genetic Mutation In contrast to the view that sex promotes genetic variation, Heng, and Gorelick and Heng reviewed evidence that sex actually acts as a constraint on genetic variation (2007; 2011). They consider that sex acts as a coarse filter, weeding out major genetic changes, such as chromosomal rearrangements, but permitting minor variation, such as changes at the nucleotide or gene level (that are often neutral) to pass through the sexual sieve. Novel Genotypes
: This diagram illustrates how sex might create novel genotypes more rapidly. Two advantageous alleles A and B
occur at random. The two alleles are recombined rapidly in a sexual population (top), but in an asexual population (bottom) the two
alleles must independently arise because of clonal interference.
Sex could be a method by which novel genotypes are created. Because sex combines genes from two individuals, sexually reproducing populations can more easily combine advantageous genes than can asexual populations. If, in a sexual population, two different advantageous alleles arise at different loci on a chromosome in different members of the population, a chromosome containing the two advantageous alleles can be produced within a few generations by recombination. However, should the same two alleles arise in different members of an asexual population, the only way that one chromosome can develop the other allele is to independently gain the same mutation, which would take much longer. Several studies have addressed counterarguments, and the question of whether this model is sufficiently robust to explain the predominance of sexual versus asexual reproduction remains (Birdsell & Wills, 2003).
Ronald Fisher also suggested that sex might facilitate the spread of advantageous genes by allowing them to better escape their genetic surroundings, if they should arise on a chromosome with deleterious genes.
Supporters of these theories respond to the balance argument that the individuals produced by sexual and asexual reproduction may differ in other respects too - which may influence the persistence of sexuality. For example, in the heterogamous water fleas of the genus Cladocera, sexual offspring form eggs which are better able to survive the winter versus those the fleas produce asexually.
Increased Resistance to Parasites One of the most widely discussed theories to explain the persistence of sex is that it is maintained to assist sexual individuals in resisting parasites, also known as the Red Queen Hypothesis (Birdsell & Wills, 2003; Ridley, 1995; MacIntyre & Clegg, 2003; Van Valen, 1973; Hamilton et al., 1990). When an environment changes, previously neutral or deleterious alleles can become favorable. If the environment changed sufficiently rapidly (i.e. between generations), these changes in the environment can make sex advantageous for the individual. Such rapid changes in environment are caused by the co-evolution between hosts and parasites. Imagine, for example that there is one gene in parasites with two alleles p and P conferring two types of parasitic ability, and one gene in hosts with two alleles h and H, conferring two types of parasite resistance, such that parasites with allele p can attach themselves to hosts with the allele h, and P to H. Such a situation will lead to cyclic changes in allele frequency - as p increases in frequency, h will be disfavored. In reality, there will be several genes involved in the relationship between hosts and parasites. In an asexual population of hosts, offspring will only have the different parasitic resistance if a mutation arises. In a sexual population of hosts, however, offspring will have a new combination of parasitic resistance alleles. In other words, like Lewis Carroll's Red Queen, sexual hosts are continually "running" (adapting) to "stay in one place" (resist parasites). Evidence for this explanation for the evolution of sex is provided by comparison of the rate of molecular evolution of genes for kinases and immunoglobulins in the immune system with genes coding other proteins. The genes coding for immune system proteins evolve considerably faster (Kuma et al., 1995; Wolfe & Sharp, 1993). Further evidence for the Red Queen hypothesis was provided by observing long-term dynamics and parasite coevolution in a "mixed" (sexual and asexual) population of snails (Potamopyrgus antipodarum). The number of sexuals, the number of asexuals, and the rates of parasite infection for both were monitored. It was found that clones that were plentiful at the beginning of the study became more susceptible to parasites over time. As parasite infections increased, the once plentiful clones dwindled dramatically in
number. Some clonal types disappeared entirely. Meanwhile, sexual snail populations remained much more stable over time (Jokela, 2009; Parasties may have had a role, 2009). However, Hanley et al. studied mite infestations of a parthenogenetic gecko species and its two related sexual ancestral species (1995). Contrary to expectation based on the Red Queen hypothesis, they found that the prevalence, abundance and mean intensity of mites in sexual geckos was significantly higher than in asexuals sharing the same habitat. In 2011, researchers used the microscopic roundworm Caenorhabditis elegans as a host and the pathogenic bacteria Serratia marcescens to generate a host-parasite coevolutionary system in a controlled environment, allowing them to conduct more than 70 evolution experiments testing the Red Queen Hypothesis. They genetically manipulated the mating system of C. elegans, causing populations to mate either sexually, by self-fertilization, or a mixture of both within the same population. Then they exposed those populations to the S. marcescens parasite. It was found that the self-fertilizing populations of C. elegans were rapidly driven extinct by the coevolving parasites while sex allowed populations to keep pace with their parasites, a result consistent with the Red Queen Hypothesis (Morran et al., 2011; Science Daily, 2011). In natural populations of C. elegans, self-fertilization is the predominant mode of reproduction, but infrequent out-crossing events occur at a rate of about 1% (Barrière & Félix, 2005). DNA Repair and Complementation The repair and complementation hypothesis assumes that genetic recombination is fundamentally a DNA repair process, and that when it occurs during meiosis it is an adaptation for repairing the genomic DNA which is passed on to progeny. Recombinational repair is the only repair process known which can accurately remove double-strand damages in DNA, and such damages are both common in nature and ordinarily lethal if not repaired. For instance, double-strand breaks in DNA occur about 50 times per cell cycle in human cells (naturally occurring DNA damage). Recombinational repair is prevalent from the simplest viruses to the most complex multicellular eukaryotes. It is effective against many different types of genomic damage, and in particular is highly efficient at overcoming double-strand damages. Studies of the mechanism of meiotic recombination indicate that meiosis is an adaptation for repairing DNA (Cox, 2001). These considerations form the basis for the first part of the repair and complementation hypothesis. In some lines of descent from the earliest organisms, the diploid stage of the sexual cycle, which was at first transient, became the predominant stage, because it allowed complementation -- the masking of deleterious recessive mutations (i.e. hybrid vigor or heterosis). Outcrossing, the second fundamental aspect of sex, is maintained by the advantage of masking mutations and the disadvantage of inbreeding (mating with a close relative) which allows expression of recessive mutations (commonly observed as inbreeding depression). This is in accord with Charles Darwin, who concluded that the adaptive advantage of sex is hybrid vigor; or as he put it, "the offspring of two individuals, especially if their progenitors have been subjected to very different conditions, have a great advantage in height, weight, constitutional vigor and fertility over the self fertilized offspring from either one of the same parents," (1876). However, outcrossing may be abandoned in favor of parthenogenesis or selfing (which retain the advantage of meiotic recombinational repair) under conditions in which the costs of mating are very high. For instance, costs of mating are high when individuals are rare in a geographic area, such as when there has been a forest fire and the individuals entering the burned area are the initial ones to arrive. At such times mates are hard to find, and this favors parthenogenic species. In the view of the repair and complementation hypothesis, the removal of DNA damage by recombinational repair produces a new, less deleterious form of informational noise, allelic recombination, as a by-product. This lesser informational noise generates genetic variation, viewed by some as the major effect of sex. Deleterious Mutation Clearance Mutations can have many different effects upon an organism. It is generally believed that the majority of non-neutral mutations are deleterious, which means that they will cause a decrease in the organism's overall fitness (Griffiths, et al., 1999). If a mutation has a deleterious effect, it will then usually be removed from the population by the process of natural selection. Sexual reproduction is believed to be more efficient than asexual reproduction in removing those mutations from the genome (Kondrashov, 1988). There are two main hypotheses which explain how sex may act to remove deleterious genes from the genome.
Evading Harmful Mutation Build-up While DNA is able to recombine to modify alleles, DNA is also susceptible to mutations within the sequence that can affect an organism in a negative manner. Asexual organisms do not have the ability to recombine their genetic information to form new and differing alleles. Once a mutation occurs in the DNA or other genetic carrying sequence, there is no way for the mutation to be removed from the population until another mutation occurs that ultimately deletes the primary mutation. This is rare among organisms. Hermann Joseph Muller introduced the idea that mutations build up in asexual reproducing organisms. Muller described this occurrence by comparing the mutations that accumulate as a ratchet. Each mutation that arises in asexually reproducing organisms turns the ratchet once. The ratchet is unable to be rotated backwards, only forwards. The next mutation that occurs turns the ratchet once more. Additional mutations in a population continually turn the ratchet and the mutations, mostly deleterious, continually accumulate without recombination (Muller, 1964). These mutations are passed onto the next generation because the offspring are exact genetic clones of their parents. The genetic load of organisms and their populations will increase due to the addition of multiple deleterious mutations and decrease the overall reproductive success and fitness. Removal of Deleterious Genes
: Diagram illustrating different relationships between numbers of mutations and fitness. Kondrashov's model requires
synergistic epistasis, which is represented by the red line - each subsequent mutation has a disproportionately large effect on the
organism's fitness (Ridley, 2004; Charlesworth & Charlesworth, 2010).
This hypothesis was proposed by Alexey Kondrashov, and is sometimes known as the deterministic mutation hypothesis (1988). It assumes that the majority of deleterious mutations are only slightly deleterious, and affect the individual such that the introduction of each additional mutation has an increasingly large effect on the fitness of the organism. This relationship between number of mutations and fitness is known as synergistic epistasis.
By way of analogy, think of a car with several minor faults. Each is not sufficient alone to prevent the car from running, but in combination, the faults combine to prevent the car from functioning.
Similarly, an organism may be able to cope with a few defects, but the presence of many mutations could overwhelm its backup mechanisms.
Kondrashov argues that the slightly deleterious nature of mutations means that the population will tend to be composed of individuals with a small number of mutations. Sex will act to recombine these genotypes, creating some individuals with fewer deleterious mutations, and some with more. Because there is a major selective disadvantage to individuals with more mutations, these individuals die out. In essence, sex compartmentalizes the deleterious mutations.
Sources Barrière, A., & Félix, M. A. (2005). High local genetic diversity and low outcrossing rate in Caenorhabditis elegans natural populations. Current Biology, 15(13), 1176-1184. https://doi.org/10.1016/j.cub.2005.06.022 Beukeboom, L., & Perrin, N. (2014). The evolution of sex determination. Oxford University Press. Birdsell, J. A., & Wills, C. (2003). The evolutionary origin and maintenance of sexual recombination: A review of contemporary models. Evolutionary Biology, 33, 27-137. https://doi.org/10.1007/978-1-4757-5190-1_2 Britannica Educational Publishing. (2011). Fungi, algae, and protists. The Rosen Publishing Group.
Butterfield, N. J. (2000). Bangiomorpha pubescens n. gen., n. sp.: Implications for the evolution of sex, multicellularity, and the Mesoproterozoic/Neoproterozoic radiation of eukaryotes. Paleobiology, 26(3), 386-404. https://doi.org/10.1666/00948373(2000)026<0386:BPNGNS>2.0.CO;2 Charlesworth, B., & Charlesworth, D. (2010). Elements of evolutionary genetics. Roberts and Company Publishers. Cox, M. M. (2001). Historical overview: Searching for replication help in all of the rec places. Proceedings of the National Academy of Sciences of the United States of America, 98(15), 8173-8180. https://doi.org/10.1073/pnas.131004998 Crow, J. F. (1994). Advantages of sexual reproduction. Developmental Genetics, 15, 205-213. Darwin, C. R. (1876). The effects of cross and self fertilisation in the vegetable kingdom. John Murray. Dudgeon, C. L., Coulton, L., Bone, R., Ovenden, J. R., & Thomas, S. (2017). Switch from sexual to parthenogenetic reproduction in a zebra shark. Scientific Reports, 7(1), 40537. https://doi.org/10.1038/srep40537 Encyclopædia Britannica. (n.d.). Cell reproduction. In Encyclopædia Britannica. Retrieved from https://www.britannica.com/science/cell-reproduction Engelstädter, J. (2017). Asexual but not clonal: Evolutionary processes in automictic populations. Genetics, 206(2), 993-1009. https://doi.org/10.1534/genetics.116.196873 Gibson, T. M. (2018). Precise age of Bangiomorpha pubescens dates the origin of eukaryotic photosynthesis. Geology, 46(2), 135- 138. https://doi.org/10.1130/G39829.1 Goldstein, R. N. (2010). 36 arguments for the existence of God: A work of fiction. Pantheon. Goodenough, U., & Heitman, J. (2014). Origins of eukaryotic sexual reproduction. Cold Spring Harbor Perspectives in Biology, 6(3), a016154. https://doi.org/10.1101/cshperspect.a016154 Gorelick, R., & Heng, H. H. Q. (2011). Sex reduces genetic variation: A multidisciplinary review. Evolution, 65(4), 1088-1098. https://doi.org/10.1111/j.1558-5646.2010.01173.x Griffiths, A. J. F., Gelbart, W. M., Miller, J. H., & Lewontin, R. C. (1999). Gene mutations. In Modern genetic analysis (pp. 197- 234). W. H. Freeman. Hamilton, W. D., Axelrod, R., & Tanese, R. (1990). Sexual reproduction as an adaptation to resist parasites. Proceedings of the National Academy of Sciences, 87(9), 3566-3573. https://doi.org/10.1073/pnas.87.9.3566 Hanley, K. A., Fisher, R. N., & Case, T. J. (1995). Lower mite infestations in an asexual gecko compared with its sexual ancestors. Evolution, 49(3), 418-426. https://doi.org/10.2307/2410266 Heng, H. H. Q. (2007). Elimination of altered karyotypes by sexual reproduction preserves species identity. Genome, 50(5), 517- 524. https://doi.org/10.1139/g07-039 Hoekstra, R. F. (1987). The evolution of sexes. In S. C. Stearns (Ed.), The evolution of sex and its consequences (pp. xx-xx). Springer Basel AG. Hussin, J. G., Hodgkinson, A., Idaghdour, Y., Grenier, J.-C., Goulet, J.-P., Gbeha, E., Hip-Ki, E., & Awadalla, P. (2015). Recombination affects accumulation of damaging and disease-associated mutations in human populations. Nature Genetics, 47(4), 400-404. https://doi.org/10.1038/ng.3216 Jokela, J., Dybdahl, M., & Lively, C. (2009). The maintenance of sex, clonal dynamics, and host-parasite coevolution in a mixed population of sexual and asexual snails. The American Naturalist, 174(S1), S43-S53. https://doi.org/10.1086/599080 Kondrashov, A. S. (1988). Deleterious mutations and the evolution of sexual reproduction. Nature, 336(6198), 435-440. https://doi.org/10.1038/336435a0 Kuma, K., Iwabe, N., & Miyata, T. (1995). Functional constraints against variations on molecules from the tissue-level: Slowly evolving brain-specific genes demonstrated by protein-kinase and immunoglobulin supergene families. Molecular Biology and Evolution, 12(1), 123-130. https://doi.org/10.1093/oxfordjournals.molbev.a040181
Leeuwenhoek, A. V. (1703). Part of a letter from Mr Antony van Leeuwenhoek, F. R. S. concerning green weeds growing in water, and some animalcula found about them. Philosophical Transactions of the Royal Society of London, 23(283), 1304-1311. https://doi.org/10.1098/rstl.1702.0042 Letunic, I., & Bork, P. (2006). Interactive Tree of Life. Retrieved July 23, 2011, from https://itol.embl.de Letunic, I., & Bork, P. (2007). Interactive Tree of Life (iTOL): An online tool for phylogenetic tree display and annotation. Bioinformatics, 23(1), 127-128. https://doi.org/10.1093/bioinformatics/btl529 Letunic, I., & Bork, P. (2011). Interactive Tree of Life v2: Online annotation and display of phylogenetic trees made easy. Nucleic Acids Research, 39(Web Server issue), W475-W478. https://doi.org/10.1093/nar/gkr201 Maynard Smith, J. (1978). The evolution of sex. Cambridge University Press. Maynard Smith, J., & Szathmáry, E. (1995). The major transitions in evolution (p. 149). W. H. Freeman and Company. MacIntyre, R. J., & Clegg, M. T. (Eds.). (2003). Evolutionary Biology. Springer. Morran, L. T., Schmidt, O. G., Gelarden, I. A., Parrish, R., Raymond, C., & Lively, C. M. (2011). Running with the Red Queen: Host-parasite coevolution selects for biparental sex. Science, 333(6039), 216-218. https://doi.org/10.1126/science.1206360 Muller, H. J. (1964). The relation of recombination to mutational advance. Mutation Research, 1, 2-9. https://doi.org/10.1016/0027-5107(64)90047-8 Narra, H. P., & Ochman, H. (2006). Of what use is sex to bacteria? Current Biology, 16(17), R705-R710. https://doi.org/10.1016/j.cub.2006.08.024 Neuhof, M., Levin, M., & Rechavi, O. (2016). Vertically and horizontally-transmitted memories - The fading boundaries between regeneration and inheritance in planaria. Biology Open, 5(9), 1177-1188. https://doi.org/10.1242/bio.020149 Otto, S. (2014). Sexual reproduction and the evolution of sex. Scitable. Retrieved February 28, 2019, from https://www.nature.com/scitable Otto, S. P., & Lenormand, T. (2002). Resolving the paradox of sex and recombination. Nature Reviews Genetics, 3(4), 252-261. https://doi.org/10.1038/nrg761 Ozias-Akins, P., & Conner, J. A. (2012). Regulation of apomixis. In A. Altman & P. M. Hasegawa (Eds.), Plant biotechnology and agriculture (pp. 243-254). Academic Press. Parasites may have had a role in evolution of sex. (2009, July 31). Science Daily. https://www.sciencedaily.com/release...0730141352.htm Puranik, P., & Bhate, A. (2007). Animal forms and functions: Invertebrata. Sarup & Sons Ramachandran, R., & McDaniel, C. D. (2018). Parthenogenesis in birds: A review. Reproduction, 155(6), R245-R257. https://doi.org/10.1530/REP-17-0728 Ridley, M. (1995). The Red Queen: Sex and the evolution of human nature. Penguin. Ridley, M. (2003). Evolution (3rd ed.). Wiley. Ridley, M. (2004). Evolution (3rd ed.). Blackwell Publishing. Ruppert, E. E., Fox, R. S., & Barnes, R. D. (2004). Annelida. In Invertebrate zoology (7th ed., pp. 434-441). Brooks/Cole. Science Daily. (2011, July 9). Sex -- as we know it -- works thanks to ever-evolving host-parasite relationships, biologists find. https://www.sciencedaily.com/release...0708145809.htm Sköld, H. N., Obst, M., Sköld, M., & Åkesson, B. (2009). Stem cells in asexual reproduction of marine invertebrates. In B. Rinkevich & V. Matranga (Eds.), Stem cells in marine organisms (p. 125). Springer. https://doi.org/10.1007/978-90-481-2767-2 Smith, J. M. (1978). The evolution of sex. Cambridge University Press. Smyth, J. D., & Wakelin, D. (1994). Introduction to animal parasitology (3rd ed., pp. 101-102). Cambridge University Press. Stearns, S. C., & Hoekstra, R. F. (2005). Evolution: An introduction (2nd ed.). Oxford University Press.
University of California Museum of Paleontology. (n.d.). Asexual reproduction. UCMP Berkeley. Retrieved August 13, 2010, from https://ucmp.berkeley.edu/fossil/asexual.html U.S. Forest Service. (2010, May 11). Celebrating wildflowers - Fading gold - How aspens grow. FS.fed.us. Archived from the original on September 23, 2010. Van Valen, L. (1973). A new evolutionary law. Evolutionary Theory, 1, 1-30. Wolfe, K. H., & Sharp, P. M. (1993). Mammalian gene evolution: Nucleotide-sequence divergence between mouse and rat. Journal of Molecular Evolution, 37(4), 441-456. https://doi.org/10.1007/BF00178874
Wetzel from Wikipedia: https://en.wikipedia.org/wiki/Sexual_reproduction, https://en.wikipedia.org/wiki/Asexual_reproduction, htt
ps://en.wikipedia.org/wiki/Evolution_of_sexual_reproduction
12.1: The paradox of sex- sexual versus asexual reproduction is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts.
12.2: Sex determination and sex ratios Sex determination A sex-determination system is a biological system that determines the development of sexual characteristics in an organism. Most organisms that create their offspring using sexual reproduction have two sexes. In some species there are hermaphrodites (Rosenfield, 2018). There are also some species that are only one sex due to parthenogenesis, the act of a female reproducing without fertilization. In many species, sex determination is genetic: males and females have different alleles or even different genes that specify their sexual morphology. In animals this is often accompanied by chromosomal differences, generally through combinations of XY, ZW, XO, ZO chromosomes, or haplodiploidy. The sexual differentiation is generally triggered by a main gene (a "sex locus"), with a multitude of other genes following in a domino effect. In other cases, sex of a fetus is determined by environmental variables (such as temperature). The details of some sexdetermination systems are not yet fully understood. Some species such as various plants and fish do not have a fixed sex, and instead go through life cycles and change sex based on genetic cues during corresponding life stages of their type. This could be due to environmental factors such as seasons, temperature, or even social context. In some gonochoric species, a few individuals may have sex characteristics of both sexes, a condition called intersex (Minelli & Fusco, 2019).
: Some chromosomal sex determination systems in animals.
Discovery Sex determination was discovered in the mealworm by the American geneticist Nettie Stevens in 1903 (Nature Education, n.d.; Ogilvie & Choquette, 1981; Smith, 2010).
Chromosomal systems XX/XY sex chromosomes
The XX/XY sex-determination system is the most familiar, as it is found in humans. The XX/XY system is found in most other mammals, as well as some insects. In this system, most females have two of the same kind of sex chromosome (XX), while most males have two distinct sex chromosomes (XY). The X and Y sex chromosomes are different in shape and size from each other, unlike the rest of the chromosomes (autosomes), and are sometimes called allosomes. In some species, such as humans, organisms remain sex indifferent for a period of time after fertilization; in others, however, such as fruit flies, sexual differentiation occurs as soon as the egg is fertilized (Hake, 2008).
X-centered sex determination Some species, such as fruit flies, use the presence of two X chromosomes to determine femaleness (Penalva & Sánchez, 2003). Species that use the number of Xs to determine sex are nonviable with an extra X chromosome.
XX/X0 sex chromosomes In this variant of the XY system, females have two copies of the sex chromosome (XX) but males have only one (X0). The 0 denotes the absence of a second sex chromosome. Generally in this method, the sex is determined by amount of genes expressed across the two chromosomes. This system is observed in a number of insects, including the grasshoppers and crickets of order Orthoptera and in cockroaches (order Blattodea). A small number of mammals also lack a Y chromosome. These include the Amami spiny rat (Tokudaia osimensis) and the Tokunoshima spiny rat (Tokudaia tokunoshimensis) and Sorex araneus, a shrew species. Transcaucasian mole voles (Ellobius lutescens) also have a form of XO determination, in which both sexes lack a second sex chromosome (Chandra, 1999). The mechanism of sex determination is not yet understood (Kuroiwa et al., 2011). The nematode C. elegans is male with one sex chromosome (X0); with a pair of chromosomes (XX) it is a hermaphrodite (Majerus, 2003, p. 60). Its main sex gene is XOL, which encodes XOL-1 and also controls the expression of the genes TRA-2 and HER-1. These genes reduce male gene activation and increase it, respectively (Kuwabara et al., 1992).
: Heredity of sex chromosomes in XO sex determination.
ZW/ZZ sex chromosomes The ZW sex-determination system is found in birds, some reptiles, and some insects and other organisms. The ZW sexdetermination system is reversed compared to the XY system: females have two different kinds of chromosomes (ZW), and males have two of the same kind of chromosomes (ZZ). In the chicken, this was found to be dependent on the expression of DMRT1 (Smith et al., 2009). In birds, the genes FET1 and ASW are found on the W chromosome for females, similar to how the Y
chromosome contains SRY (Hake, 2008). However, not all species depend upon the W for their sex. For example, there are moths and butterflies that are ZW, but some have been found female with ZO, as well as female with ZZW (Majerus, 2003, p. 60). Also, while mammals deactivate one of their extra X chromosomes when female, it appears that in the case of Lepidoptera, the males produce double the normal amount of enzymes, due to having two Z's (Majerus, 2003, p. 60). Because the use of ZW sex determination is varied, it is still unknown how exactly most species determine their sex (Majerus, 2003, p. 60). However, reportedly, the silkworm Bombyx mori uses a single female-specific piRNA as the primary determiner of sex (Kiuchi et al., 2014). Despite the similarities between the ZW and XY systems, these sex chromosomes evolved separately. In the case of the chicken, their Z chromosome is more similar to humans' autosome 9 (Stiglec et al., 2007). The chicken's Z chromosome also seems to be related to the X chromosome of the platypus (Grützner, 2004). When a ZW species, such as the Komodo dragon, reproduces parthenogenetically, usually only males are produced. This is due to the fact that the haploid eggs double their chromosomes, resulting in ZZ or WW. The ZZ become males, but the WW are not viable and are not brought to term (BBC News, 2006). In both XY and ZW sex determination systems, the sex chromosome carrying the critical factors is often significantly smaller, carrying little more than the genes necessary for triggering the development of a given sex (Annenberg Media, 2004). Haplodiploidy Haplodiploidy is found in insects belonging to Hymenoptera, such as ants and bees. Sex determination is controlled by the zygosity of a complementary sex determiner (csd) locus. Unfertilized eggs develop into haploid individuals which have a single, hemizygous copy of the csd locus and are therefore males. Fertilized eggs develop into diploid individuals which, due to high variability in the csd locus, are generally heterozygous females. In rare instances diploid individuals may be homozygous, these develop into sterile males. The gene acting as a csd locus has been identified in the honeybee and several candidate genes have been proposed as a csd locus for other Hymenopterans (Beye et al., 2003; Privman, 2013; Miyakawa, 2018). Most females in the Hymenoptera order can decide the sex of their offspring by holding received sperm in their spermatheca and either releasing it into their oviduct or not. This allows them to create more workers, depending on the status of the colony (Van Wilgenburg, 2006).
Temperature-dependent Many other sex-determination systems exist. In some species of reptiles, including alligators, some turtles, and the tuatara, sex is determined by the temperature at which the egg is incubated during a temperature-sensitive period. There are no examples of temperature-dependent sex determination (TSD) in birds. Megapodes had formerly been thought to exhibit this phenomenon, but were found to actually have different temperature-dependent embryo mortality rates for each sex (Göth & Booth, 2005). For some species with TSD, sex determination is achieved by exposure to hotter temperatures resulting in the offspring being one sex and cooler temperatures resulting in the other. This type of TSD is called Pattern I. For others species using TSD, it is exposure to temperatures on both extremes that results in offspring of one sex, and exposure to moderate temperatures that results in offspring of the opposite sex, called Pattern II TSD. The specific temperatures required to produce each sex are known as the femalepromoting temperature and the male-promoting temperature (Torres Maldonado et al, 2002). When the temperature stays near the threshold during the temperature sensitive period, the sex ratio is varied between the two sexes (Bull, 1980). Some species' temperature standards are based on when a particular enzyme is created. These species that rely upon temperature for their sex determination do not have the SRY gene, but have other genes such as DAX1, DMRT1, and SOX9 that are expressed or not expressed depending on the temperature (Torres Maldonado et al., 2002). The sex of some species, such as the Nile tilapia,
Australian skink lizard, and Australian dragon lizard, is initially determined by chromosomes, but can later be changed by the temperature of incubation (Schartl, 2004). It is unknown how exactly temperature-dependent sex determination evolved (Valenzuela & Janzen, 2001). It could have evolved through certain sexes being more suited to certain areas that fit the temperature requirements. For example, a warmer area could be more suitable for nesting, so more females are produced to increase the amount that nest next season (Valenzuela & Janzen, 2001). Environmental sex determination preceded the genetically determined systems of birds and mammals; it is thought that a temperature-dependent amniote was the common ancestor of amniotes with sex chromosomes (Janzen & Phillips, 2006).
: All alligators determine the sex of their offspring by the temperature of the nest.
Other systems There are other environmental sex determination systems including location-dependent determination systems as seen in the marine worm Bonellia viridis - larvae become males if they make physical contact with a female, and females if they end up on the bare sea floor. This is triggered by the presence of a chemical produced by the females, bonellin (Gilbert, 2006). Some species, such as some snails, practice sex change: adults start out male, then become female. In tropical clown fish, the dominant individual in a group becomes female while the other ones are male, and bluehead wrasses (Thalassoma bifasciatum) are the reverse. Some species, however, have no sex-determination system. Hermaphrodite species include the common earthworm and certain species of snails. A few species of fish, reptiles, and insects reproduce by parthenogenesis and are female altogether. There are some reptiles, such as the boa constrictor and Komodo dragon that can reproduce both sexually and asexually, depending on whether a mate is available (Watts et al., 2006). Other unusual systems include those of the green swordtail (a polyfactorial system with the sex-determining genes on several chromosomes); the juvenile hermaphroditism of zebrafish, with an unknown trigger; and the platyfish, which has W, X, and Y chromosomes. This allows WY, WX, or XX females and YY or XY males (Schartl, 2004).
Sex ratios The sex ratio is the ratio of males to females in a population. In most sexually reproducing species, the ratio tends to be 1:1. This tendency is explained by Fisher's principle (Hamilton, 1967). For various reasons, however, many species deviate from anything like an even sex ratio, either periodically or permanently. Examples include parthenogenic species, periodically mating organisms such as aphids, some eusocial wasps such as Polistes fuscatus and Polistes exclamans, bees, ants, and termites (Kobayashi et al., 2013). In most species, the sex ratio varies according to the age profile of the population (Coney & Mackey, 1998). Fisher's principle Fisher's principle explains why for most species, the sex ratio is approximately 1:1. Bill Hamilton expounded Fisher's argument in his 1967 paper on "Extraordinary sex ratios" as follows, given the assumption of equal parental expenditure on offspring of both sexes (Hamilton, 1967). 1. Suppose male births are less common than female. 2. A newborn male then has better mating prospects than a newborn female, and therefore can expect to have more offspring. 3. Therefore parents genetically disposed to produce males tend to have more than average numbers of grandchildren born to them. 4. Therefore the genes for male-producing tendencies spread, and male births become more common. 5. As the 1:1 sex ratio is approached, the advantage associated with producing males dies away. 6. The same reasoning holds if females are substituted for males throughout. Therefore 1:1 is the equilibrium ratio.
In modern language, the 1:1 ratio is the evolutionarily stable strategy (ESS) (Smith & Price, 1973). This ratio has been observed in many species, including the bee Macrotera portalis. A study performed by Danforth observed no significant difference in the number of males and females from the 1:1 sex ratio (Danforth, 1991).
References Annenberg Media. (n.d.). Evolution of the Y chromosome. Archived from the original on November 4, 2004. Retrieved April 1, 2008. BBC News. (2006, December 20). Virgin births for giant lizards. https://news.bbc.co.uk/2/hi/science/...re/6196225.stm Beye, M., Hasselmann, M., Fondrk, M. K., Page, R. E., & Omholt, S. W. (2003). The gene csd is the primary signal for sexual development in the honeybee and encodes an SR-type protein. Cell, 114(4), 419-429. https://doi.org/10.1016/S00928674(03)00606-8 Bull, J. J. (1980). Sex determination in reptiles. The Quarterly Review of Biology, 55(1), 3-21. https://doi.org/10.1086/411613 Chandra, H. S. (1999). Another way of looking at the enigma of sex determination in Ellobius lutescens. Current Science, 76(8), 1072. Coney, N. S., & Mackey, W. C. (1998). The woman as final arbiter: A case for the facultative character of the human sex ratio. Journal of Sex Research, 35(2), 169-175. https://doi.org/10.1080/00224499809551930 Danforth, B. (1991). Female foraging and intranest behavior of a communal bee, Perdita portalis (Hymenoptera: Andrenidae). Annals of the Entomological Society of America, 84(5), 537-548. https://doi.org/10.1093/aesa/84.5.537 Gilbert, S. F. (2006). Developmental biology (8th ed., pp. 550-553). Sinauer Associates. Göth, A., & Booth, D. T. (2005). Temperature-dependent sex ratio in a bird. Biology Letters, 1(1), 31-33. https://doi.org/10.1098/rsbl.2004.0247 Grützner, F., Rens, W., Tsend-Ayush, E., El-Mogharbel, N., O'Brien, P. C. M., Jones, R. C., Ferguson-Smith, M. A., & Marshall, J. A. (2004). In the platypus, a meiotic chain of ten sex chromosomes shares genes with the bird Z and mammal X chromosomes. Nature, 432(7019), 913-917. https://doi.org/10.1038/nature03021 Hake, L. (2008). Genetic mechanisms of sex determination. Nature Education, 1(1). Retrieved December 8, 2011. Hamilton, W. D. (1967). Extraordinary sex ratios. Science, 156(3774), 477-488. https://doi.org/10.1126/science.156.3774.477 Janzen, F. J., & Phillips, P. C. (2006). Exploring the evolution of environmental sex determination, especially in reptiles. Journal of Evolutionary Biology, 19(6), 1775-1784. https://doi.org/10.1111/j.1420-9101.2006.01138.x Kiuchi, T., Koga, H., Kawamoto, M., Shoji, K., Sakai, H., Arai, Y., Ishihara, G., Kawaoka, S., Sugano, S., Shimada, T., Suzuki, Y., Suzuki, M., & Katsuma, S. (2014). A single female-specific piRNA is the primary determiner of sex in the silkworm. Nature, 509(7502), 633-636. https://doi.org/10.1038/nature13315 Kobayashi, K., Hasegawa, E., Yamamoto, Y., Kawatsu, K., Vargo, E. L., Yoshimura, J., & Matsuura, K. (2013). Sex ratio biases in termites provide evidence for kin selection. Nature Communications, 4, 2048. https://doi.org/10.1038/ncomms3048 Kuroiwa, A., Handa, S., Nishiyama, C., Chiba, E., Yamada, F., Abe, S., & Matsuda, Y. (2011). Additional copies of CBX2 in the genomes of males of mammals lacking SRY, the Amami spiny rat (Tokudaia osimensis) and the Tokunoshima spiny rat (Tokudaia tokunoshimensis). Chromosome Research, 19(5), 635-644. https://doi.org/10.1007/s10577-011-9223-6 Kuwabara, P. E., Okkema, P. G., & Kimble, J. (1992). tra-2 encodes a membrane protein and may mediate cell communication in the Caenorhabditis elegans sex determination pathway. Molecular Biology of the Cell, 3(4), 461-473. https://doi.org/10.1091/mbc.3.4.461 Majerus, M. E. N. (2003). Sexual conflict in insects and arachnids (p. 60). Princeton University Press.
Minelli, A., & Fusco, G. (2019). The biology of reproduction (pp. 116-117). Cambridge University Press. Miyakawa, M. O., Tsuchida, K., & Miyakawa, H. (2018). The doublesex gene integrates multi-locus complementary sex determination signals in the Japanese ant, Vollenhovia emeryi. Insect Biochemistry and Molecular Biology, 94, 42-49. https://doi.org/10.1016/j.ibmb.2018.01.006 Nature Education. (n.d.). Nettie Stevens: A discoverer of sex chromosomes. Scitable by Nature Education. Retrieved June 7, 2018, from https://www.nature.com/scitable Ogilvie, M. B., & Choquette, C. J. (1981). Nettie Maria Stevens (1861-1912): Her life and contributions to cytogenetics. Proceedings of the American Philosophical Society, 125(4), 292-311. https://www.jstor.org/stable/986332 Penalva, L. O. F., & Sánchez, L. (2003). RNA binding protein sex-lethal (Sxl) and control of Drosophila sex determination and dosage compensation. Microbiology and Molecular Biology Reviews, 67(3), 343-359. https://doi.org/10.1128/MMBR.67.3.343359.2003 Privman, E., Wurm, Y., & Keller, L. (2013). Duplication and concerted evolution in a master sex determiner under balancing selection. Proceedings of the Royal Society B: Biological Sciences, 280(1758), 20122968. https://doi.org/10.1098/rspb.2012.2968 Rosenfield, K. A. (2018). Hermaphrodite. In J. Vonk & T. Shackelford (Eds.), Encyclopedia of animal cognition and behavior (pp. 1-2). Springer. https://doi.org/10.1007/978-3-319-47829-6_329-1 Schartl, M. (2004). A comparative view on sex determination in medaka. Mechanisms of Development, 121(7-8), 639-645. https://doi.org/10.1016/j.mod.2004.03.001 Smith, C. A., Roeszler, K. N., Ohnesorg, T., Cummins, D. M., Farlie, P. G., Doran, T. J., & Sinclair, A. H. (2009). The avian Zlinked gene DMRT1 is required for male sex determination in the chicken. Nature, 461(7261), 267-271. https://doi.org/10.1038/nature08298 Smith, J. M., & Price, G. R. (1973). The logic of animal conflict. Nature, 246(5427), 15-18. https://doi.org/10.1038/246015a0 Smith, K. (2010). Nettie Maria Stevens (1861-1912). The Embryo Project Encyclopedia. Retrieved June 7, 2018, from https://embryo.asu.edu Stiglec, R., Ezaz, T., & Graves, J. A. M. (2007). A new look at the evolution of avian sex chromosomes. Cytogenetic and Genome Research, 117(1-4), 103-109. https://doi.org/10.1159/000103170 Torres Maldonado, L. C., Landa Piedra, A., Moreno Mendoza, N., Marmolejo Valencia, A., Meza Martínez, A., & Merchant Larios, H. (2002). Expression profiles of Dax1, Dmrt1, and Sox9 during temperature sex determination in gonads of the sea turtle Lepidochelys olivacea. General and Comparative Endocrinology, 129(1), 20-26. https://doi.org/10.1016/s0016-6480(02)00511-7 Valenzuela, N., & Janzen, F. J. (2001). Nest-site philopatry and the evolution of temperature-dependent sex determination. Evolutionary Ecology Research, 3, 779-794. Van Wilgenburg, E., Driessen, G., & Beukeboom, L. W. (2006). Single locus complementary sex determination in Hymenoptera: An "unintelligent" design? Frontiers in Zoology, 3(1), 1. https://doi.org/10.1186/1742-9994-3-1 Watts, P. C., Buley, K. R., Sanderson, S., Boardman, W., Ciofi, C., & Gibson, R. (2006). Parthenogenesis in Komodo dragons. Nature, 444(7122), 1021-1022. https://doi.org/10.1038/4441021a
determination_system, https://en.wikipedia.org/wiki/Sex_ratio
12.2: Sex determination and sex ratios is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts.
12.3: Scientist Spotlight - Nettie Stevens Famously, King Henry VIII had six wives in his efforts to have a male heir. He disposed of each for various acts of disrespect, but most especially, for not providing him a son. Throughout history, the birth of a female child, over the oft-preferred male child, has lain with the attitudes or defiance of women. Aptly, it would be a woman to discover the determining factor of sex at birth and free the future of women from such accusations. Within the scientific community, sex had been long-debated as either an inherited trait or one influenced by embryonic environmental influence, and it was ultimately Dr. Nettie Stevens who uncovered the truth.
: Photograph of Nettie Stevens, taken at Carnegie Institution of Washington and kept in the Bryn Mawr College Photo Archives. Image available in the public domain.
Stevens was a native of Vermont, born in 1861 and dying at the age of 51 in 1912. She studied biology at Westfield State College, where she was only one of three women to graduate between 1872 and 1883. After a few years of teaching, she went on to earn an MA at Stanford University and a PhD at Bryn Mawr College. In the early 1900s, she shifted her research from morphology to cytology and regeneration, leading her to the topic of embryo and chromosome variations. By 1905, she published her findings on yellow mealworms (Tenebrio molitor) and the X and Y chromosomes as the cause of an individual's sex at birth. Two X chromosomes (XX) results in a female, while one of each (XY) results in a male. It is now known that the first X is inherited from the egg, while the second (either X or Y) is inherited from the sperm. In her paper, Stevens concluded, "this seems to be a clear case of sex-determination...by a definite difference in the character of the elements of one pair of chromosomes..., the spermatozoa which contain the small chromosome determining the male sex, while those that contain 10 chromosomes of equal size determining the female sex" (1905, p. 13). Not only did Stevens' discovery resolve the debate surrounding sex origins, it was the first time that scientists could link a phenotype to a specific chromosome. Stevens' reputation and contributions to the field of genetics are often overlooked by subsequent findings on the topic during the same time period, with more credit given to her male contemporary Dr. Edmund Wilson, but the value of her discoveries have been incalculable.
: Title page of Nettie Stevens academic paper, outlining her research and her findings on the genetics of assigned birth sex.
: Carl Zeiss Jena 8261 Microscope used by Nettie Maria Stevens. This image was provided to Wikimedia Commons by Bryn Mawr College as part of a cooperation project (licensed under CC BY-SA 3.0).
As culture continues to evolve, modern scientists now know that there are more than the simple XX and XY, male and female binary, that Stevens had originally written about. Today, our concepts of sex pertain to the biological sense which Stevens focused on, though intersex has been added to the male or female classifications and we recognize in the present day that assigned birth at sex does not necessarily indicate gender identity. With science as an accumulated field, guided by the culture it exists within, our concepts of sex and gender will continue to adjust with time, using foundational knowledge like that afforded to us by Dr. Nettie Stevens as a stabilizer for the heights we will reach.
References Brush, S.G. (1978). Nettie M. Stevens and the discovery of sex determination by chromosomes. The University of Chicago Press, 69(2), pp.162-172. https://www.jstor.org/stable/230427. The Editors of Encyclopaedia Britannica. (n.d.). Henry VIII. Britannica. https://www.britannica.com/biography/Henry-VIII-kingof-England. Accessed 1/26/2024. The Editors of Encyclopaedia Britannica. (n.d.). Nettie Stevens. Britannica. https://www.britannica.com/biography/Nettie-Stevens. Accessed 1/26/2024. Stevens, N.M. (1905). Studies in spermatogenesis. Carnegie Institution of Washington, 36, pp. 33-74.
12.3: Scientist Spotlight - Nettie Stevens is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts.
12.4: Mating systems in sexual animals The content for this subtopic is found in an external page. Please click the link below to access this information. Mating systems in sexual animals 12.4: Mating systems in sexual animals is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts.
12.5: Mating Systems in Plants Asexual Reproduction Many plants are facultatively sexual rather than obligately sexual. Asexual reproduction is a type of reproduction where the offspring comes from one parent only, thus, inheriting the characteristics of the parent. Asexual reproduction in plants occurs in two fundamental forms vegetative reproduction and agamospermy (Barrett, 2008). Vegetative reproduction involves a vegetative piece of the original plant producing new individuals by budding, tillering, etc. and is distinguished from apomixis, which is a replacement of sexual reproduction, and in some cases involves seeds. Apomixis occurs in many plant species such as dandelions (Taraxacum species) and also in some non-plant organisms. For apomixis and similar processes in non-plant organisms, see parthenogenesis. Natural vegetative reproduction is a process mostly found in perennial plants, and typically involves structural modifications of the stem or roots and in a few species leaves. Most plant species that employ vegetative reproduction do so as a means to perennialize the plants, allowing them to survive from one season to the next and often facilitating their expansion in size. A plant that persists in a location through vegetative reproduction of individuals constitutes a clonal colony. A single ramet, or apparent individual, of a clonal colony is genetically identical to all others in the same colony. The distance that a plant can move during vegetative reproduction is limited, though some plants can produce ramets from branching rhizomes or stolons that cover a wide area, often in only a few growing seasons. In a sense, this process is not one of reproduction but one of survival and expansion of biomass of the individual. When an individual organism increases in size via cell multiplication and remains intact, the process is called vegetative growth. However, in vegetative reproduction, the new plants that result are new individuals in almost every respect except genetic. A major disadvantage of vegetative reproduction, is the transmission of pathogens from parent to offspring. It is uncommon for pathogens to be transmitted from the plant to its seeds (in sexual reproduction or in apomixis), though there are occasions when it occurs (Fritz & Simms, 1992). Seeds generated by apomixis are a means of asexual reproduction, involving the formation and dispersal of seeds that do not originate from the fertilization of the embryos. Hawkweeds (Hieracium), dandelions (Taraxacum), some species of Citrus and Kentucky blue grass (Poa pratensis) all use this form of asexual reproduction. Pseudogamy occurs in some plants that have apomictic seeds, where pollination is often needed to initiate embryo growth, though the pollen contributes no genetic material to the developing offspring (O'Neill, 2003). Other forms of apomixis occur in plants also, including the generation of a plantlet in replacement of a seed or the generation of bulbils instead of flowers, where new cloned individuals are produced. Sexual Reproduction Sexual reproduction involves two fundamental processes: meiosis, which rearranges the genes and reduces the number of chromosomes, and fertilisation, which restores the chromosome to a complete diploid number. In between these two processes, different types of plants and algae vary, but many of them, including all land plants, undergo alternation of generations, with two different multicellular structures (phases), a gametophyte and a sporophyte. In mosses and liverworts, the gametophyte is relatively large, and the sporophyte is a much smaller structure that is never separated from the gametophyte. In ferns, gymnosperms, and flowering plants (angiosperms), the gametophytes are relatively small and the sporophyte is much larger. In gymnosperms and flowering plants the megagametophyte is contained within the ovule (that may develop into a seed) and the microgametophyte is contained within a pollen grain. In the evolution of early plants, abiotic means, including water and much later, wind, transported sperm for reproduction. The first plants were aquatic, and released sperm freely into the water to be carried with the currents. Ancestral land plants like liverworts and mosses have motile sperm that swam in a thin film of water or were splashed in water droplets. As taller and more complex plants evolved, modifications in the alternation of generations evolved. In the Paleozoic era progymnosperms reproduced by using spores dispersed on the wind and many gymnosperms and some angiosperms still rely on wind for gamete dispersal. The seed plants including seed ferns, conifers and cordaites have pollen grains that contain the male gametes for protection of the sperm during the process of transfer from the male to female parts. Angiosperms, or flowering plants, are the most derived and most abundant plant species and they rely on flowers producing pollen and ovules for reproduction.
Self-Fertilization and Self-Incompatibility Many species of plants, particularly those which produce both staminate and pistillate flowers or produce `perfect' bisexual flowers, also have the ability to reproduce sexually with themselves. This is advantageous particularly if pollination services are unreliable or unpredictable, as it ensures the plant still has some fitness. Self-pollination is a form of pollination in which pollen from the same plant arrives at the stigma of a flower (in flowering plants) or at the ovule (in gymnosperms). The term selfing that is often used as a synonym, is not limited to self-pollination, but also applies to other types of self-fertilization. Plants may either be obligately self-fertilizing, or facultatively so. In facultatively selfing plants, there may be mechanisms which delay selfing, such as stamens that are initially reflexed but move to come into contact with the stigma. About 42% of flowering plants exhibit a mixed mating system in nature (Goodwillie et al., 2005). In the most common kind of system, individual plants produce a single flower type and fruits may contain self-pollinated, out-crossed or a mixture of progeny types. Another mixed mating system is referred to as dimorphic cleistogamy. In this system a single plant produces both open, potentially out-crossed and closed, obligately selfpollinated cleistogamous flowers (Munguía-Rosas et al., 2013). Still other species are self-incompatible, and will reject their own pollen grains if they land on their own stigmatic surface. These plants are obligately outcrossing, and must successfully sexually reproduce with another member of their species. In plants with SI, when a pollen grain produced in a plant reaches a stigma of the same plant or another plant with a matching allele or genotype, the process of pollen germination, pollen-tube growth, ovule fertilization, or embryo development is inhibited, and consequently no seeds are produced. SI is one of the most important means of preventing inbreeding and promoting the generation of new genotypes in plants and it is considered one of the causes of the spread and success of angiosperms on the earth.
Plants that use insects or other animals to move pollen from one flower to the next have developed greatly modified flower parts to attract pollinators and to facilitate the movement of pollen from one flower to the insect and from the insect back to the next flower. Flowers of wind-pollinated plants tend to lack petals and or sepals; typically large amounts of pollen are produced and pollination often occurs early in the growing season before leaves can interfere with the dispersal of the pollen. Many trees and all grasses and sedges are wind-pollinated.
Plants have a number of different means to attract pollinators including color, scent, heat, nectar glands, edible pollen and flower
shape. Along with modifications involving the above structures two other conditions play a very important role in the sexual
reproduction of flowering plants, the first is the timing of flowering and the other is the size or number of flowers produced. Often
plant species have a few large, very showy flowers while others produce many small flowers, often flowers are collected together
into large inflorescences to maximize their visual effect, becoming more noticeable to passing pollinators. Flowers are attraction
strategies and sexual expressions are functional strategies used to produce the next generation of plants, with pollinators and plants
having co-evolved, often to some extraordinary degrees, very often rendering mutual benefit. Specialization can be advantageous
because it results in more consistent pollination services. As a result, the specialized pollinator and plant can exert strong selective
pressure on each other, leading to coevolution. Examples of this include the coevolution between figs and fig wasps, or yucca and
yucca moths, wherein the yucca moths are both the obligate pollinators and the antagonistic herbivores of yucca. Another visually
striking example is the co-evolution of long floral corollas and long beaks or proboscii in pollinators
A photo of a hummingbird approaches a long trumpeted flower with its long bill that can reach inside for nectar. A photo of a hummingbird reaching its long, thin beak toward similarly shaped trumpeted flowers.
: (A) Sword-billed Hummingbird in Ecuador ("Sword-Billed Hummingbird" by Andrew Morffew is licensed under
CC BY 2.0); (B) Rufous Hummingbird pollinating Scarlet Gilia in the Rocky Mountains ("Rufous Hummingbird" by Smallman12q
Plants which share pollination methods or pollinators often accumulate suites of related traits called pollination syndromes. These have evolved in response to natural selection imposed by different pollen vectors, which can be abiotic (wind and water) or biotic, such as birds, bees, flies, etc. through a process called pollinator-mediated selection (Faegri & van der Pijl, 1980; Proctor et al., 1996). These traits include flower shape, size, colour, odor, reward type and amount, nectar composition, and timing of flowering. For example, tubular red flowers with copious nectar often attract birds; foul smelling flowers attract carrion flies or beetles, etc. Different species which use the same pollinators may either flower synchronously, to attract pollinators more successfully, or asynchronously, to avoid heterospecific pollen transfer (or stigma gunking). The latter is an example of niche partitioning.
The largest family of flowering plants is the orchids (Orchidaceae), estimated by some specialists to include up to 35,000 species, which often have highly specialized flowers that attract particular insects for pollination (Flora of North America, n.d.). The stamens are modified to produce pollen in clusters called pollinia, which become attached to insects that crawl into the flower. The flower shapes may force insects to pass by the pollen, which is "glued" to the insect. Some orchids are even more highly specialized, with flower shapes that mimic the shape of insects to attract them to attempt to 'mate' with the flowers, a few even have scents that mimic insect pheromones ( ).
: A bee-mimic orchid evolved to resemble a sexually receptive female bee and attract naïve male bees to pollinate it ("Bee Orchid (Ophrys apifera)" by Ian Capper is licensed under CC BY 2.0).
Examples of Pollination Syndromes Wind: Flowers may be small and inconspicuous, as well as green and not showy. They produce enormous numbers of relatively small pollen grains (hence wind-pollinated plants may be allergens, but seldom are animal-pollinated plants allergenic). Their stigmas may be large and feathery to catch the pollen grains. Water: Water-pollinated plants are aquatic and pollen is released into the water. Water currents therefore act as a pollen vector in a similar way to wind currents. Their flowers tend to be small and inconspicuous with many pollen grains and large, feathery stigmas to catch the pollen. However, this is relatively uncommon (only 2% of pollination is hydrophily) and most aquatic plants are insect-pollinated, with flowers that emerge into the air. Vallisneria is an example. Bee: Some bee flowers tend to be yellow or blue, often with ultraviolet nectar guides and scent. Nectar, pollen, or both are offered as rewards in varying amounts. The sugar in the nectar tends to be sucrose-dominated. A few bees collect oil from special glands on the flower (Martins et al., 2014). Butterfly: Butterfly-pollinated flowers tend to be large and showy, pink or lavender in colour, frequently have a landing area, and are usually scented. Since butterflies do not digest pollen (with one exception), more nectar is offered than pollen. The flowers have simple nectar guides with the nectaries usually hidden in narrow tubes or spurs, reached by the long tongue of the butterflies. Moth: Among the more important moth pollinators are the hawk moths (Sphingidae). Their behaviour is similar to hummingbirds: they hover in front of flowers with rapid wingbeats. Most are nocturnal or crepuscular. Moth-pollinated flowers tend to be white, night-opening, large and showy with tubular corollas and a strong, sweet scent produced in the evening, night or early morning. Much nectar is produced to fuel the high metabolic rates needed to power their flight. Bat: There are major differences between bat pollination in the Americas as opposed to the Afro-Eurasia. Afro-Eurasian pollinating bats are large fruit bats of the family Pteropodidae which do not have the ability to hover and must perch in the plant to lap the nectar; these bats furthermore do not have the ability to echolocate (Fleming et al., 2009). Bat-pollinated flowers in this part of the world tend to be large and showy, white or light coloured, open at night and have strong musty odours. They are often large balls of stamens. In the Americas pollinating bats are tiny creatures called glossophagines which have both the ability to hover as well as echolocate, and have extremely long tongues. Plants in this part of the world are often pollinated by
both bats and hummingbirds, and have long tubular flowers (Fleming et al., 2009). In one essay, von Helversen et al. speculate that maybe some bell-shaped flowers have evolved to attract bats in the Americas, as the bell-shape might reflect the sonar pulses emitted by the bats in a recognisable pattern (2003). Fly: Myophilous plants tend not to emit a strong scent, are typically purple, violet, blue, and white, and have open dishes or tubes (Kastinger & Weber, 2001). Sapromyophilous plants try to attract flies which normally visit dead animals or dung. Flowers mimic the odor of such objects. The plant provides them with no reward and they leave quickly unless it has traps to slow them down. Such plants are far less common than myophilous ones (Jones & Jones, 2001). Beetle: Beetle-pollinated flowers are usually large, greenish or off-white in color and heavily scented. Scents may be spicy, fruity, or similar to decaying organic material. Most beetle-pollinated flowers are flattened or dish shaped, with pollen easily accessible, although they may include traps to keep the beetle longer. The plant's ovaries are usually well protected from the biting mouthparts of their pollinators (Gullan & Cranston, 2005). A number of cantharophilous plants are thermogenic, with flowers that can increase their temperature. This heat is thought to help further spread the scent, but the infrared light produced by this heat may also be visible to insects during the dark night, and act as a shining beacon to attract them (Korotkova & Barthlott, 2009). Bird: Flowers pollinated by specialist nectarivores tend to be large, red or orange tubes with a lot of dilute nectar, secreted during the day. Since birds do not have a strong response to scent, they tend to be odorless. Flowers pollinated by generalist birds are often shorter and wider. Hummingbirds are often associated with pendulous flowers, whereas passerines (perching birds) need a landing platform so flowers and surrounding structures are often more robust. Also, many plants have anthers placed in the flower so that pollen rubs against the birds head/back as the bird reaches in for nectar. ...and many more! References Barrett, S. C. H. (2008). Major evolutionary transitions in flowering plant reproduction (p. 157). University of Chicago Press. Faegri, K., & van der Pijl, L. (1980). The principles of pollination ecology (3rd ed.). Pergamon Press. Fleming, T. H., Geiselman, C., & Kress, W. J. (2009). The evolution of bat pollination: A phylogenetic perspective. Annals of Botany, 104(6), 1017-1043. https://doi.org/10.1093/aob/mcp197 Flora of North America. (n.d.). Orchidaceae. eFloras.org. http://www.efloras.org Fritz, R. E., & Simms, E. L. (1992). Plant resistance to herbivores and pathogens: Ecology, evolution, and genetics (p. 359). University of Chicago Press. Goodwillie, C., Kalisz, S., & Eckert, C. G. (2005). The evolutionary enigma of mixed mating systems in plants: Occurrence, theoretical explanations, and empirical evidence. Annual Review of Ecology, Evolution, and Systematics, 36, 47-79. https://doi.org/10.1146/annurev.ecol....091704.175539 Gullan, P. J., & Cranston, P. S. (2005). The insects: An outline of entomology (p. 282). Blackwell Publishing Ltd. Jones, G. D., & Jones, S. D. (2001). The uses of pollen and its implication for entomology. Neotropical Entomology, 30(3), 314- 349. https://doi.org/10.1590/S1519-566X2001000300001 Kastinger, C., & Weber, A. (2001). Bee-flies (Bombylius spp., Bombyliidae, Diptera) and the pollination of flowers. Flora, 196(1), 3-25. https://doi.org/10.1016/S0367-2530(17)30015-4 Korotkova, N., & Barthlott, W. (2009). On the thermogenesis of the Titan arum (Amorphophallus titanum). Plant Signaling & Behavior, 4(11), 1096-1098. https://doi.org/10.4161/psb.4.11.9872 Martins, A. C., Melo, G. A. R., & Renner, S. S. (2014). The corbiculate bees arose from New World oil-collecting bees: Implications for the origin of pollen baskets. Molecular Phylogenetics and Evolution, 80, 88-94. https://doi.org/10.1016/j.ympev.2014.07.003 Munguía-Rosas, M. A., Campos-Navarrete, M. J., & Parra-Tabla, V. (2013). The effect of pollen source vs. flower type on progeny performance and seed predation under contrasting light environments in a cleistogamous herb. PLOS ONE, 8(11), e80934.
Contributors and Attributions Modified by Castilleja Olmsted (University of Pittsburgh) from the following sources: https://en.wikipedia.org/wiki/Plant_reproduction https://en.wikipedia.org/wiki/Self-incompatibility https://en.wikipedia.org/wiki/Self-pollination https://en.wikipedia.org/wiki/Pollination_syndrome Original text by Castilleja Olmsted 12.5: Mating Systems in Plants is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts.
12.6: Scientist Spotlight - Ernest Everett Just For those who enjoy them sunny side up, scrambled, or hard boiled, the word "egg" may bring chickens to mind. However, eggs don't always take the form of smooth, oval-shaped objects that constitute your breakfast, nor do they only come from birds. Scientifically speaking, an "egg" is a gamete, or a sex cell, central to the process of sexual reproduction. This means that eggs are common to all sexually reproducing organisms, even the ones that don't lay the eggs we're familiar with. One type of reproduction, called sexual reproduction, occurs when a sperm cell fertilizes an egg cell. The fertilized egg cell is called a "zygote". When two organisms are involved, the zygote contains genetic material from two different parents. However, sexual reproduction can also occur when one hermaphroditic organism (possessing male and female gametes) self-fertilizes. No matter the number of contributors, the defining mechanism of sexual reproduction is when a sperm cell fertilizes an egg cell. This process occurs on a microscopic level, so how do we know what fertilization looks like? One answer to this broad question was unearthed by embryologist Ernest Everett Just. Just was born in Charleston, South Carolina in 1883, and his father died 4 years later (Selassie 2007). He went on to graduate from Dartmouth College, teach English and biology at Howard University, and earn a PhD in zoology at the University of Chicago (Byrnes and Newman 2014). Prior to his time in Chicago, Just conducted research at the Woods Hole Biological Marine Laboratory, where his work with marine invertebrates revealed that the ectoplasm (egg surface) significantly influences the fertilization and development of eggs (Just 1919, 1922; Wellner 2010). Throughout his career, Just authored more than seventy papers and two books. Despite his major contributions to our understanding of fertilization and evolutionary developmental biology, his work was "...largely forgotten and invisible to the world of biology" (Byrnes and Newman 2014). As an African American man in the early 20th century, Just belonged to a historically marginalized group of people, and he is only now receiving recognition for his pioneering work.
: "Ernest Everett Just" is available in the public domain.
References Scientist Spotlight Inspiration from the Scientist Spotlights Initiative Byrnes, M.W., & Newman, S.A. (2014). Ernest Everett Just: Egg and embryo as excitable systems. <https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4277254/>. Accessed October 5, 2021. Just, Ernest Everett. 1919. The Fertilization Reaction in Echinarachnius Parma. Biological Bulletin, 36(1), pp. 1-10. Just, Ernest Everett. 1922. Initiation of development in the egg of Arbacia. II. Fertilization of eggs in various stages of artificially induced mitosis. Biological Bulletin, 43(6), pp. 401-410. Selassie I, & Gabriel, W. (2007). Ernest Everett Just (1883-1941). <https://www.blackpast.org/african-am...ett-1883-1941/>. Accessed October 5, 2021. Wellner, K. (2010). Ernest Everett Just (1883-1941). Embryo Project Encyclopedia. <http://embryo.asu.edu/handle/10776/2039>. Accessed October 5, 2021. 12.6: Scientist Spotlight - Ernest Everett Just is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts.
12.7: Sexual selection The content for this subtopic is found in an external page. Please click the link below to access this information. Sexual selection Chapter summary The paradox of sexual reproduction is that although it is ubiquitous in multicellular organisms, there are many disadvantages to reproducing sexually when we compare it to asexual reproduction. This chapter explores some of the multitude of hypotheses for why sexual reproduction exists, the variety of ways in which sexes are determined in plants and animals, and some of the variation in mating systems of sexual organisms. This chapter also focuses on sexual selection, a form of natural selection that occurs when traits that improve mating success are favored by selection, even if they cause a decrease in survival. 12.7: Sexual selection is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts.