Ever-young sex chromosomes in European tree frogs

Anolis sex chromosomes are derived from a single ancestral pair

To explain the frequency and distribution of heteromorphic sex chromosomes in the lizard genus Anolis, we compared the relative roles of sex chromosome conservation versus turnover of sex-determining mechanisms. We used model-based comparative methods to reconstruct karyotype evolution and the presence of heteromorphic sex chromosomes onto a newly generated Anolis phylogeny. We found that heteromorphic sex chromosomes evolved multiple times in the genus. Fluorescent in situ hybridization (FISH) of repetitive DNA showed variable rates of Y chromosome degeneration among Anolis species and identified previously undetected, homomorphic sex chromosomes in two species. We confirmed homology of sex chromosomes in the genus by performing FISH of an X-linked bacterial artificial chromosome (BAC) and quantitative PCR of X-linked genes in multiple Anolis species sampled across the phylogeny. Taken together, these results are consistent with long-term conservation of sex chromosomes in the group. Our results pave the way to address additional questions related to Anolis sex chromosome evolution and describe a conceptual framework that can be used to evaluate the origins and evolution of heteromorphic sex chromosomes in other clades.

The sex chromosomes of frogs: variability and tolerance offer clues to genome evolution and function

Frog sex chromosomes offer an ideal system for advancing our understanding of genome evolution and function because of the variety of sex determination systems in the group, the diversity of sex chromosome maturation states, the ease of experimental manipulation during early development. After briefly reviewing sex chromosome biology generally, we focus on what is known about frog sex determination, sex chromosome evolution, and recent, genomics-facilitated advances in the field. In closing we highlight gaps in our current knowledge of frog sex chromosomes, and suggest priorities for future research that can advance broad knowledge of gene dose and sex chromosome evolution.

Stronger transferability but lower variability in transcriptomic- than in anonymous microsatellites: evidence from Hylid frogs

A simple way to quickly optimize microsatellites in nonmodel organisms is to reuse loci available in closely related taxa; however, this approach can be limited by the stochastic and low cross-amplification success experienced in some groups (e.g. amphibians). An efficient alternative is to develop loci from transcriptome sequences. Transcriptomic microsatellites have been found to vary in their levels of cross-species amplification and variability, but this has to date never been tested in amphibians. Here, we compare the patterns of cross-amplification and levels of polymorphism of 18 published anonymous microsatellites isolated from genomic DNA vs. 17 loci derived from a transcriptome, across nine species of tree frogs (Hyla arborea and Hyla cinerea group). We established a clear negative relationship between divergence time and amplification success, which was much steeper for anonymous than transcriptomic markers, with half-lives (time at which 50% of the markers still amplify) of 1.1 and 37 My, respectively. Transcriptomic markers are significantly less polymorphic than anonymous loci, but remain variable across diverged taxa. We conclude that the exploitation of amphibian transcriptomes for developing microsatellites seems an optimal approach for multispecies surveys (e.g. analyses of hybrid zones, comparative linkage mapping), whereas anonymous microsatellites may be more informative for fine-scale analyses of intraspecific variation. Moreover, our results confirm the pattern that microsatellite cross-amplification is greatly variable among amphibians and should be assessed independently within target lineages. Finally, we provide a bank of microsatellites for Palaearctic tree frogs (so far only available for H. arborea), which will be useful for conservation and evolutionary studies in this radiation.

A large pseudoautosomal region on the sex chromosomes of the frog Silurana tropicalis

Sex chromosome divergence has been documented across phylogenetically diverse species, with amphibians typically having cytologically nondiverged (“homomorphic”) sex chromosomes. With an aim of further characterizing sex chromosome divergence of an amphibian, we used “RAD-tags” and Sanger sequencing to examine sex specificity and heterozygosity in the Western clawed frog Silurana tropicalis (also known as Xenopus tropicalis). Our findings based on approximately 20 million genotype calls and approximately 200 polymerase chain reaction-amplified regions across multiple male and female genomes failed to identify a substantially sized genomic region with genotypic hallmarks of sex chromosome divergence, including in regions known to be tightly linked to the sex-determining region. We also found that expression and molecular evolution of genes linked to the sex-determining region did not differ substantially from genes in other parts of the genome. This suggests that the pseudoautosomal region, where recombination occurs, comprises a large portion of the sex chromosomes of S. tropicalis. These results may in part explain why African clawed frogs have such a high incidence of polyploidization, shed light on why amphibians have a high rate of sex chromosome turnover, and raise questions about why homomorphic sex chromosomes are so prevalent in amphibians.

Evolutionary transitions between sex-determining mechanisms: a review of theory

The extraordinary diversity of sex-determining mechanisms found in nature is thought to have arisen by the addition, modification or replacement of regulators at the upstream end of the sex-determining pathway. The spread of a novel regulator of sex determination can manifest itself by an evolutionary transition between environmental and genetic sex determination, for example, or between male and female heterogamety. Both kinds of transition have occurred frequently in the course of evolution. In this paper, various evolutionary forces acting on sex-determining mutations that can bias transitions in one direction or the other are reviewed. Furthermore, the adaptive significance of the main modes of sex determination are discussed, and the common principle underlying ultimate explanations for environmental sex determination, genetic sex determination and maternal control over sex determination in the offspring are highlighted. Most of the current theory concentrates on the population-genetic aspects of sex determination transitions, using models that do not reflect the developmental mechanisms involved in sex determination. However, the increasing availability of molecular data creates opportunities for the future development of mechanistic models that will further clarify how selection and developmental architecture interact to direct the evolution of sex determination genes.

Sex-chromosome turnovers: the hot-potato model

Sex-determining systems often undergo high rates of turnover but for reasons that remain largely obscure. Two recent evolutionary models assign key roles, respectively, to sex-antagonistic (SA) mutations occurring on autosomes and to deleterious mutations accumulating on sex chromosomes. These two models capture essential but distinct key features of sex-chromosome evolution; accordingly, they make different predictions and present distinct limitations. Here we show that a combination of features from the two models has the potential to generate endless cycles of sex-chromosome transitions: SA alleles accruing on a chromosome after it has been co-opted for sex induce an arrest of recombination; the ensuing accumulation of deleterious mutations will soon make a new transition ineluctable. The dynamics generated by these interactions share several important features with empirical data, namely, (i) that patterns of heterogamety tend to be conserved during transitions and (ii) that autosomes are not recruited randomly, with some chromosome pairs more likely than others to be co-opted for sex.

The scope and strength of sex-specific selection in genome evolution

Males and females share the vast majority of their genomes and yet are often subject to different, even conflicting, selection. Genomic and transcriptomic developments have made it possible to assess sex-specific selection at the molecular level, and it is clear that sex-specific selection shapes the evolutionary properties of several genomic characteristics, including transcription, post-transcriptional regulation, imprinting, genome structure and gene sequence. Sex-specific selection is strongly influenced by mating system, which also causes neutral evolutionary changes that affect different regions of the genome in different ways. Here, we synthesize theoretical and molecular work in order to provide a cohesive view of the role of sex-specific selection and mating system in genome evolution. We also highlight the need for a combined approach, incorporating both genomic data and experimental phenotypic studies, in order to understand precisely how sex-specific selection drives evolutionary change across the genome.

Species and population level molecular profiling reveals cryptic recombination and emergent asymmetry in the dimorphic mating locus of C. reinhardtii

Heteromorphic sex-determining regions or mating-type loci can contain large regions of non-recombining sequence where selection operates under different constraints than in freely recombining autosomal regions. Detailed studies of these non-recombining regions can provide insights into how genes are gained and lost, and how genetic isolation is maintained between mating haplotypes or sex chromosomes. The Chlamydomonas reinhardtii mating-type locus (MT) is a complex polygenic region characterized by sequence rearrangements and suppressed recombination between its two haplotypes, MT+ and MT−. We used new sequence information to redefine the genetic contents of MT and found repeated translocations from autosomes as well as sexually controlled expression patterns for several newly identified genes. We examined sequence diversity of MT genes from wild isolates of C. reinhardtii to investigate the impacts of recombination suppression. Our population data revealed two previously unreported types of genetic exchange in Chlamydomonas MT—gene conversion in the rearranged domains, and crossover exchanges in flanking domains—both of which contribute to maintenance of genetic homogeneity between haplotypes. To investigate the cause of blocked recombination in MT we assessed recombination rates in crosses where the parents were homozygous at MT. While normal recombination was restored in MT+×MT+ crosses, it was still suppressed in MT−×MT− crosses. These data revealed an underlying asymmetry in the two MT haplotypes and suggest that sequence rearrangements are insufficient to fully account for recombination suppression. Together our findings reveal new evolutionary dynamics for mating loci and have implications for the evolution of heteromorphic sex chromosomes and other non-recombining genomic regions.

Sex chromosomes and mating-type loci are often atypical in their structure and evolutionary dynamics. One distinguishing feature is the absence of recombination that results in genetic isolation and promotes rapid evolution and sometimes degeneration. We investigated gene content, sex-regulated expression, and recombination of mating locus (MT) genes in the unicellular alga Chlamydomonas reinhardtii. Despite the lack of observable recombination in and around Chlamydomonas MT, genes from its two mating types are far more similar to each other than expected for a non-recombining region. This discrepancy is explained by our finding evidence of genetic exchange between the two mating types within wild populations. In addition, we observed an unexpected asymmetry in the recombination behavior of the two mating types that may have contributed to the preferential expansion of one MT haplotype over the other through insertion of new genes. Our data suggest a mechanism to explain the emergence of heteromorphic sex chromosomes in haploid organisms by asymmetric expansion rather than by loss or degeneration as occurs in some Y or W chromosomes from diploid organisms. Our observations support a revised view of recombination in sex-determining regions as a quantitative phenomenon that can significantly affect rates of evolution and sex-linked genetic diversification.

Genetic and genomic interactions of animals with different ploidy levels

Polyploid animals have independently evolved from diploids in diverse taxa across the tree of life. We review a few polyploid animal species or biotypes where recently developed molecular and cytogenetic methods have significantly improved our understanding of their genetics, reproduction and evolution. Mitochondrial sequences that target the maternal ancestor of a polyploid show that polyploids may have single (e.g. unisexual salamanders in the genus Ambystoma) or multiple (e.g. parthenogenetic polyploid lizards in the genus Aspidoscelis) origins. Microsatellites are nuclear markers that can be used to analyze genetic recombinations, reproductive modes (e.g. Ambystoma) and recombination events (e.g. polyploid frogs such as Pelophylax esculentus). Hom(e)ologous chromosomes and rare intergenomic exchanges in allopolyploids have been distinguished by applying genome-specific fluorescent probes to chromosome spreads. Polyploids arise, and are maintained, through perturbations of the 'normal' meiotic program that would include pre-meiotic chromosome replication and genomic integrity of homologs. When possible, asexual, unisexual and bisexual polyploid species or biotypes interact with diploid relatives, and genes are passed from diploid to polyploid gene pools, which increase genetic diversity and ultimately evolutionary flexibility in the polyploid. When diploid relatives do not exist, polyploids can interact with another polyploid (e.g. species of African Clawed Frogs in the genus Xenopus). Some polyploid fish (e.g. salmonids) and frogs (Xenopus) represent independent lineages whose ancestors experienced whole genome duplication events. Some tetraploid frogs (P. esculentus) and fish (Squaliusalburnoides) may be in the process of becoming independent species, but diploid and triploid forms of these 'species' continue to genetically interact with the comparatively few tetraploid populations. Genetic and genomic interaction between polyploids and diploids is a complex and dynamic process that likely plays a crucial role for the evolution and persistence of polyploid animals. See also other articles in this themed issue.

Recent gene-capture on the UV sex chromosomes of the moss Ceratodon purpureus

Sex chromosomes evolve from ordinary autosomes through the expansion and subsequent degeneration of a region of suppressed recombination that is inherited through one sex. Here we investigate the relative timing of these processes in the UV sex chromosomes of the moss Ceratodon purpureus using molecular population genetic analyses of eight newly discovered sex-linked loci. In this system, recombination is suppressed on both the female-transmitted (U) sex chromosome and the male-transmitted (V) chromosome. Genes on both chromosomes therefore should show the deleterious effects of suppressed recombination and sex-limited transmission, while purifying selection should maintain homologs of genes essential for both sexes on both sex chromosomes. Based on analyses of eight sex-linked loci, we show that the nonrecombining portions of the U and V chromosomes expanded in at least two events (~0.6-1.3 MYA and ~2.8-3.5 MYA), after the divergence of C. purpureus from its dioecious sister species, Trichodon cylindricus and Cheilothela chloropus. Both U- and V-linked copies showed reduced nucleotide diversity and limited population structure, compared to autosomal loci, suggesting that the sex chromosomes experienced more recent selective sweeps that the autosomes. Collectively these results highlight the dynamic nature of gene composition and molecular evolution on nonrecombining portions of the U and V sex chromosomes.

Maintenance of ancestral sex chromosomes in Palearctic tree frogs: direct evidence from Hyla orientalis

Contrasting with the situation found in birds and mammals, sex chromosomes are generally homomorphic in poikilothermic vertebrates. This homomorphy was recently shown to result from occasional X-Y recombinations (not from turnovers) in several European species of tree frogs (Hyla arborea, H. intermedia and H. molleri). Because of recombination, however, alleles at sex-linked loci were rarely diagnostic at the population level; support for sex linkage had to rely on multilocus associations, combined with occasional sex differences in allelic frequencies. Here, we use direct evidence, obtained from anatomical and histological analyses of offspring with known pedigrees, to show that the Eastern tree frog (H. orientalis) shares the same pair of sex chromosomes, with identical patterns of male heterogamety and complete absence of X-Y recombination in males. Conservation of an ancestral pair of sex chromosomes, regularly rejuvenated via occasional X-Y recombination, seems thus a widespread pattern among Hyla species. Sibship analyses also identified discrepancies between genotypic and phenotypic sex among offspring, associated with abnormal gonadal development, suggesting a role for sexually antagonistic genes on the sex chromosomes.

Within-population polymorphism of sex-determination systems in the common frog (Rana temporaria)

In sharp contrast with birds and mammals, the sex chromosomes of ectothermic vertebrates are often undifferentiated, for reasons that remain debated. A linkage map was recently published for Rana temporaria (Linnaeus, 1758) from Fennoscandia (Eastern European lineage), with a proposed sex-determining role for linkage group 2 (LG2). We analysed linkage patterns in lowland and highland populations from Switzerland (Western European lineage), with special focus on LG2. Sibship analyses showed large differences from the Fennoscandian map in terms of recombination rates and loci order, pointing to large-scale inversions or translocations. All linkage groups displayed extreme heterochiasmy (total map length was 12.2 cM in males, versus 869.8 cM in females). Sex determination was polymorphic within populations: a majority of families (with equal sex ratios) showed a strong correlation between offspring phenotypic sex and LG2 paternal haplotypes, whereas other families (some of which with female-biased sex ratios) did not show any correlation. The factors determining sex in the latter could not be identified. This coexistence of several sex-determination systems should induce frequent recombination of X and Y haplotypes, even in the absence of male recombination. Accordingly, we found no sex differences in allelic frequencies on LG2 markers among wild-caught male and female adults, except in one high-altitude population, where nonrecombinant Y haplotypes suggest sex to be entirely determined by LG2. Multifactorial sex determination certainly contributes to the lack of sex-chromosome differentiation in amphibians.

Progressive recombination suppression and differentiation in recently evolved neo-sex chromosomes

Recombination suppression leads to the structural and functional differentiation of sex chromosomes and is thus a crucial step in the process of sex chromosome evolution. Despite extensive theoretical work, the exact processes and mechanisms of recombination suppression and differentiation are not well understood. In threespine sticklebacks (Gasterosteus aculeatus), a different sex chromosome system has recently evolved by a fusion between the Y chromosome and an autosome in the Japan Sea lineage, which diverged from the ancestor of other lineages approximately 2 Ma. We investigated the evolutionary dynamics and differentiation processes of sex chromosomes based on comparative analyses of these divergent lineages using 63 microsatellite loci. Both chromosome-wide differentiation patterns and phylogenetic inferences with X and Y alleles indicated that the ancestral sex chromosomes were extensively differentiated before the divergence of these lineages. In contrast, genetic differentiation appeared to have proceeded only in a small region of the neo-sex chromosomes. The recombination maps constructed for the Japan Sea lineage indicated that recombination has been suppressed or reduced over a large region spanning the ancestral and neo-sex chromosomes. Chromosomal regions exhibiting genetic differentiation and suppressed or reduced recombination were detected continuously and sequentially in the neo-sex chromosomes, suggesting that differentiation has gradually spread from the fusion point following the extension of recombination suppression. Our study illustrates an ongoing process of sex chromosome differentiation, providing empirical support for the theoretical model postulating that recombination suppression and differentiation proceed in a gradual manner in the very early stage of sex chromosome evolution.

Should Y stay or should Y go: the evolution of non-recombining sex chromosomes

Gradual degradation seems inevitable for non-recombining sex chromosomes. This has been supported by the observation of degenerated non-recombining sex chromosomes in a variety of species. The human Y chromosome has also degenerated significantly during its evolution, and theories have been advanced that the Y chromosome could disappear within the next ~5 million years, if the degeneration rate it has experienced continues. However, recent studies suggest that this is unlikely. Conservative evolutionary forces such as strong purifying selection and intrachromosomal repair through gene conversion balance the degeneration tendency of the Y chromosome and maintain its integrity after an initial period of faster degeneration. We discuss the evidence both for and against the extinction of the Y chromosome. We also discuss potential insights gained on the evolution of sex-determining chromosomes by studying simpler sex-determining chromosomal regions of unicellular and multicellular microorganisms.

A trans-species missense SNP in Amhr2 is associated with sex determination in the tiger pufferfish, Takifugu rubripes (fugu)

Heterogametic sex chromosomes have evolved independently in various lineages of vertebrates. Such sex chromosome pairs often contain nonrecombining regions, with one of the chromosomes harboring a master sex-determining (SD) gene. It is hypothesized that these sex chromosomes evolved from a pair of autosomes that diverged after acquiring the SD gene. By linkage and association mapping of the SD locus in fugu (Takifugu rubripes), we show that a SNP (C/G) in the anti-Müllerian hormone receptor type II (Amhr2) gene is the only polymorphism associated with phenotypic sex. This SNP changes an amino acid (His/Asp384) in the kinase domain. While females are homozygous (His/His384), males are heterozygous. Sex in fugu is most likely determined by a combination of the two alleles of Amhr2. Consistent with this model, the medaka hotei mutant carrying a substitution in the kinase domain of Amhr2 causes a female phenotype. The association of the Amhr2 SNP with phenotypic sex is conserved in two other species of Takifugu but not in Tetraodon. The fugu SD locus shows no sign of recombination suppression between X and Y chromosomes. Thus, fugu sex chromosomes represent an unusual example of proto–sex chromosomes. Such undifferentiated X-Y chromosomes may be more common in vertebrates than previously thought.

Diverse systems of sex determination have evolved independently in the animal and plant kingdoms. In vertebrates, so far four master sex-determining (SD) genes, Sry, Dmrt1, Dmy, and Dm-W, have been identified. These genes code for transcription factors and are located on only one of the sex chromosomes surrounded by nonrecombining regions. It is hypothesized that these sex chromosomes evolved from a pair of homologous chromosomes that diverged after acquiring the SD gene. We investigated the SD locus in fugu by high-resolution genetic mapping and association mapping. We found that a SNP that changes an amino acid (His/Asp384) in the kinase domain of anti-Müllerian hormone receptor type II (Amhr2) is perfectly associated with phenotypic sex. A combination of the two alleles of the SNP (homozygous females and heterozygous males) is likely to be responsible for sex determination in fugu. While these alleles are conserved in two other species of Takifugu, they are absent in the freshwater pufferfish, Tetraodon. Furthermore, Fugu Amhr2 lies in a region that shows no evidence for recombination suppression between X and Y chromosomes. Thus, fugu sex chromosomes represent an unusual example of a pre-differentiated phase of sex chromosomes in vertebrates.

Gene conversion occurs within the mating-type locus of Cryptococcus neoformans during sexual reproduction

Meiotic recombination of sex chromosomes is thought to be repressed in organisms with heterogametic sex determination (e.g. mammalian X/Y chromosomes), due to extensive divergence and chromosomal rearrangements between the two chromosomes. However, proper segregation of sex chromosomes during meiosis requires crossing-over occurring within the pseudoautosomal regions (PAR). Recent studies reveal that recombination, in the form of gene conversion, is widely distributed within and may have played important roles in the evolution of some chromosomal regions within which recombination was thought to be repressed, such as the centromere cores of maize. Cryptococcus neoformans, a major human pathogenic fungus, has an unusually large mating-type locus (MAT, >100 kb), and the MAT alleles from the two opposite mating-types show extensive nucleotide sequence divergence and chromosomal rearrangements, mirroring characteristics of sex chromosomes. Meiotic recombination was assumed to be repressed within the C. neoformans MAT locus. A previous study identified recombination hot spots flanking the C. neoformans MAT, and these hot spots are associated with high GC content. Here, we investigated a GC-rich intergenic region located within the MAT locus of C. neoformans to establish if this region also exhibits unique recombination behavior during meiosis. Population genetics analysis of natural C. neoformans isolates revealed signals of homogenization spanning this GC-rich intergenic region within different C. neoformans lineages, consistent with a model in which gene conversion of this region during meiosis prevents it from diversifying within each lineage. By analyzing meiotic progeny from laboratory crosses, we found that meiotic recombination (gene conversion) occurs around the GC-rich intergenic region at a frequency equal to or greater than the meiotic recombination frequency observed in other genomic regions. We discuss the implications of these findings with regards to the possible functional and evolutionary importance of gene conversion within the C. neoformans MAT locus and, more generally, in fungi.

Recombination has been thought to be repressed within sex chromosomes, as well as within the mating-type (MAT) loci in many fungi, due to the highly diverged and rearranged nature between alleles defining opposite sexes or mating-types. However, it has long been appreciated that recombination can occur within these presumptive recombinational “cold spots,” and recent studies reveal that recombination, including gene conversion, can occur at a frequency higher than previously appreciated and could play important roles in shaping evolution of these chromosomal regions. Here, we provide evidence that, during sexual reproduction of the human pathogenic fungus Cryptococcus neoformans, recombination (gene conversion) occurs across a GC-rich intergenic region within the MAT locus. The frequency of this gene conversion is comparable to those of typical meiotic recombination events observed in other chromosomal regions. This is in accord with population genetics analyses, which indicate homogenization between alleles of opposite mating-types within the intergenic region. Gene conversion within these highly rearranged chromosomal regions may serve to ensure proper meiosis and/or rejuvenate genes/chromosomal regions within MAT that are otherwise facing irreversible evolutionary decay. In conclusion, our study provides further experimental evidence that at least some recombinational “cold spots” are not that cold, after all.

Is the Y chromosome disappearing?--both sides of the argument

On August 31, 2011 at the 18th International Chromosome Conference in Manchester, Jenny Graves took on Jenn Hughes to debate the demise (or otherwise) of the mammalian Y chromosome. Sex chromosome evolution is an example of convergence; there are numerous examples of XY and ZW systems with varying degrees of differentiation and isolated examples of the Y disappearing in some lineages. It is agreed that the Y was once genetically identical to its partner and that the present-day human sex chromosomes retain only traces of their shared ancestry. The euchromatic portion of the male-specific region of the Y is ~1/6 of the size of the X and has only ~1/12 the number of genes. The big question however is whether this degradation will continue or whether it has reached a point of equilibrium. Jenny Graves argued that the Y chromosome is subject to higher rates of variation and inefficient selection and that Ys (and Ws) degrade inexorably. She argued that there is evidence that the Y in other mammals has undergone lineage-specific degradation and already disappeared in some rodent lineages. She also pointed out that there is practically nothing left of the original human Y and the added part of the human Y is degrading rapidly. Jenn Hughes on the other hand argued that the Y has not disappeared yet and it has been around for hundreds of millions of years. She stated that it has shown that it can outsmart genetic decay in the absence of "normal" recombination and that most of its genes on the human Y exhibit signs of purifying selection. She noted that it has added at least eight different genes, many of which have subsequently expanded in copy number, and that it has not lost any genes since the human and chimpanzee diverged ~6 million years ago. The issue was put to the vote with an exact 50/50 split among the opinion of the audience; an interesting (though perhaps not entirely unexpected) skew however was noted in the sex ratio of those for and against the notion.

The evolution of XY recombination: sexually antagonistic selection versus deleterious mutation load

Recombination arrest between X and Y chromosomes, driven by sexually antagonistic genes, is expected to induce their progressive differentiation. However, in contrast to birds and mammals (which display the predicted pattern), most cold-blooded vertebrates have homomorphic sex chromosomes. Two main hypotheses have been proposed to account for this, namely high turnover rates of sex-determining systems and occasional XY recombination. Using individual-based simulations, we formalize the evolution of XY recombination (here mediated by sex reversal; the "fountain-of-youth" model) under the contrasting forces of sexually antagonistic selection and deleterious mutations. The shift between the domains of elimination and accumulation occurs at much lower selection coefficients for the Y than for the X. In the absence of dosage compensation, mildly deleterious mutations accumulating on the Y depress male fitness, thereby providing incentives for XY recombination. Under our settings, this occurs via "demasculinization" of the Y, allowing recombination in XY (sex-reversed) females. As we also show, this generates a conflict with the X, which coevolves to oppose sex reversal. The resulting rare events of XY sex reversal are enough to purge the Y from its load of deleterious mutations. Our results support the "fountain of youth" as a plausible mechanism to account for the maintenance of sex-chromosome homomorphy.

Small but mighty: the evolutionary dynamics of W and Y sex chromosomes

Although sex chromosomes have been the focus of a great deal of scientific scrutiny, most interest has centred on understanding the evolution and relative importance of X and Z chromosomes. By contrast, the sex-limited W and Y chromosomes have received far less attention, both because of their generally degenerate nature and the difficulty in studying non-recombining and often highly heterochromatic genomic regions. However, recent theory and empirical evidence suggest that the W and Y chromosomes play a far more important role in sex-specific fitness traits than would be expected based on their size alone, and this importance may explain the persistence of some Y and W chromosomes in the face of powerful degradative forces. In addition to their role in fertility and fecundity, the sex-limited nature of these genomic regions results in unique evolutionary forces acting on Y and W chromosomes, implicating them as potentially major contributors to sexual selection and speciation. Recent empirical studies have borne out these predictions and revealed that some W and Y chromosomes play a vital role in key sex-specific evolutionary processes.

Evolution of sex and mating loci: an expanded view from Volvocine algae

Sexual reproduction in Volvocine algae coevolved with the acquisition of multicellularity. Unicellular genera such as Chlamydomonas and small colonial genera from this group have classical mating types with equal-sized gametes, while larger multicellular genera such as Volvox have differentiated males and females that produce sperm and eggs respectively. Newly available sequence from the Volvox and Chlamydomonas genomes and mating loci open up the potential to investigate how sex-determining regions co-evolve with major changes in development and sexual reproduction. The expanded size and sequence divergence between the male and female haplotypes of the Volvox mating locus (MT) not only provide insights into how the colonial Volvocine algae might have evolved sexual dimorphism, but also raise questions about why the putative ancestral-like MT locus in Chlamydomonas shows less divergence between haplotypes than expected.