Spatial dynamics of evolving dosage compensation in a young sex chromosome system

Multiple karyotype differences between populations of the Hoplias malabaricus (Teleostei; Characiformes), a species complex in the gray area of the speciation process

Neotropical fishes exhibit remarkable karyotype diversity, whose evolution is poorly understood. Here, we studied genetic differences in 60 individuals, from 11 localities of one species, the wolf fish Hoplias malabaricus, from populations that include six different "karyomorphs". These differ in Y-X chromosome differentiation, and, in several cases, by fusions with autosomes that have resulted in multiple sex chromosomes. Other differences are also observed in diploid chromosome numbers and morphologies. In an attempt to start understanding how this diversity was generated, we analyzed within- and between-population differences in a genome-wide sequence data set. We detect clear genotype differences between karyomorphs. Even in sympatry, samples with different karyomorphs differ more in sequence than samples from allopatric populations of the same karyomorph, suggesting that they represent populations that are to some degree reproductively isolated. However, sequence divergence between populations with different karyomorphs is remarkably low, suggesting that chromosome rearrangements may have evolved during a brief evolutionary time. We suggest that the karyotypic differences probably evolved in allopatry, in small populations that would have allowed rapid fixation of rearrangements, and that they became sympatric after their differentiation. Further studies are needed to test whether the karyotype differences contribute to reproductive isolation detected between some H. malabaricus karyomorphs.

Comparative genomics illuminates karyotype and sex chromosome evolution of sharks

Chondrichthyes is an important lineage to reconstruct the evolutionary history of vertebrates. Here, we analyzed genome synteny for six chondrichthyan chromosome-level genomes. Our comparative analysis reveals a slow evolutionary rate of chromosomal changes, with infrequent but independent fusions observed in sharks, skates, and chimaeras. The chondrichthyan common ancestor had a proto-vertebrate-like karyotype, including the presence of 18 microchromosome pairs. The X chromosome is a conversed microchromosome shared by all sharks, suggesting a likely common origin of the sex chromosome at least 181 million years ago. We characterized the Y chromosomes of two sharks that are highly differentiated from the X except for a small young evolutionary stratum and a small pseudoautosomal region. We found that shark sex chromosomes lack global dosage compensation but that dosage-sensitive genes are locally compensated. Our study on shark chromosome evolution enhances our understanding of shark sex chromosomes and vertebrate chromosome evolution.

Why do sex chromosomes progressively lose recombination?

Progressive recombination loss is a common feature of sex chromosomes. Yet, the evolutionary drivers of this phenomenon remain a mystery. For decades, differences in trait optima between sexes (sexual antagonism) have been the favoured hypothesis, but convincing evidence is lacking. Recent years have seen a surge of alternative hypotheses to explain progressive extensions and maintenance of recombination suppression: neutral accumulation of sequence divergence, selection of nonrecombining fragments with fewer deleterious mutations than average, sheltering of recessive deleterious mutations by linkage to heterozygous alleles, early evolution of dosage compensation, and constraints on recombination restoration. Here, we explain these recent hypotheses and dissect their assumptions, mechanisms, and predictions. We also review empirical studies that have brought support to the various hypotheses.

Searching for signatures of sexually antagonistic selection on stickleback sex chromosomes

Intralocus sexually antagonistic selection occurs when an allele is beneficial to one sex but detrimental to the other. This form of selection is thought to be key to the evolution of sex chromosomes but is hard to detect. Here we perform an analysis of phased young sex chromosomes to look for signals of sexually antagonistic selection in the Japan Sea stickleback (Gasterosteus nipponicus). Phasing allows us to date the suppression of recombination on the sex chromosome and provides unprecedented resolution to identify sexually antagonistic selection in the recombining region of the chromosome. We identify four windows with elevated divergence between the X and Y in the recombining region, all in or very near genes associated with phenotypes potentially under sexually antagonistic selection in humans. We are unable, however, to rule out the alternative hypothesis that the peaks of divergence result from demographic effects. Thus, although sexually antagonistic selection is a key hypothesis for the formation of supergenes on sex chromosomes, it remains challenging to detect.

This article is part of the theme issue ‘Genomic architecture of supergenes: causes and evolutionary consequences’.

Dosage compensation evolution in plants: theories, controversies and mechanisms

In a minority of flowering plants, separate sexes are genetically determined by sex chromosomes. The Y chromosome has a non-recombining region that degenerates, causing a reduced expression of Y genes. In some species, the lower Y expression is accompanied by dosage compensation (DC), a mechanism that re-equalizes male and female expression and/or brings XY male expression back to its ancestral level. Here, we review work on DC in plants, which started as early as the late 1960s with cytological approaches. The use of transcriptomics fired a controversy as to whether DC existed in plants. Further work revealed that various plants exhibit partial DC, including a few species with young and homomorphic sex chromosomes. We are starting to understand the mechanisms responsible for DC in some plants, but in most species, we lack the data to differentiate between global and gene-by-gene DC. Also, it is unknown why some species evolve many dosage compensated genes while others do not. Finally, the forces that drive DC evolution remain mysterious, both in plants and animals. We review the multiple evolutionary theories that have been proposed to explain DC patterns in eukaryotes with XY or ZW sex chromosomes. This article is part of the theme issue 'Sex determination and sex chromosome evolution in land plants'.

Heterogeneous Histories of Recombination Suppression on Stickleback Sex Chromosomes

How consistent are the evolutionary trajectories of sex chromosomes shortly after they form? Insights into the evolution of recombination, differentiation, and degeneration can be provided by comparing closely related species with homologous sex chromosomes. The sex chromosomes of the threespine stickleback (Gasterosteus aculeatus) and its sister species, the Japan Sea stickleback (G. nipponicus), have been well characterized. Little is known, however, about the sex chromosomes of their congener, the blackspotted stickleback (G. wheatlandi). We used pedigrees to obtain experimentally phased whole genome sequences from blackspotted stickleback X and Y chromosomes. Using multispecies gene trees and analysis of shared duplications, we demonstrate that Chromosome 19 is the ancestral sex chromosome and that its oldest stratum evolved in the common ancestor of the genus. After the blackspotted lineage diverged, its sex chromosomes experienced independent and more extensive recombination suppression, greater X-Y differentiation, and a much higher rate of Y degeneration than the other two species. These patterns may result from a smaller effective population size in the blackspotted stickleback. A recent fusion between the ancestral blackspotted stickleback Y chromosome and Chromosome 12, which produced a neo-X and neo-Y, may have been favored by the very small size of the recombining region on the ancestral sex chromosome. We identify six strata on the ancestral and neo-sex chromosomes where recombination between the X and Y ceased at different times. These results confirm that sex chromosomes can evolve large differences within and between species over short evolutionary timescales.

The timing of genetic degeneration of sex chromosomes

Genetic degeneration is an extraordinary feature of sex chromosomes, with the loss of functions of Y-linked genes in species with XY systems, and W-linked genes in ZW systems, eventually affecting almost all genes. Although degeneration is familiar to most biologists, important aspects are not yet well understood, including how quickly a Y or W chromosome can become completely degenerated. I review the current understanding of the time-course of degeneration. Degeneration starts after crossing over between the sex chromosome pair stops, and theoretical models predict an initially fast degeneration rate and a later much slower one. It has become possible to estimate the two quantities that the models suggest are the most important in determining degeneration rates-the size of the sex-linked region, and the time when recombination became suppressed (which can be estimated using Y-X or W-Z sequence divergence). However, quantifying degeneration is still difficult. I review evidence on gene losses (based on coverage analysis) or loss of function (by classifying coding sequences into functional alleles and pseudogenes). I also review evidence about whether small genome regions degenerate, or only large ones, whether selective constraints on the genes in a sex-linked region also strongly affect degeneration rates, and about how long it takes before all (or almost all) genes are lost. This article is part of the theme issue 'Challenging the paradigm in sex chromosome evolution: empirical and theoretical insights with a focus on vertebrates (Part I)'.

Schistosome W-linked genes inform temporal dynamics of sex chromosome evolution and suggest candidate for sex determination

Schistosomes, the human parasites responsible for snail fever, are female-heterogametic. Different parts of their ZW sex chromosomes have stopped recombining in distinct lineages, creating "evolutionary strata" of various ages. While the Z-chromosome is well characterized at the genomic and molecular level, the W-chromosome has remained largely unstudied from an evolutionary perspective, as only a few W-linked genes have been detected outside of the model species Schistosoma mansoni. Here, we characterize the gene content and evolution of the W-chromosomes of S. mansoni and of the divergent species S. japonicum. We use a combined RNA/DNA k-mer based pipeline to assemble around one hundred candidate W-specific transcripts in each of the species. About half of them map to known protein coding genes, the majority homologous to S. mansoni Z-linked genes. We perform an extended analysis of the evolutionary strata present in the two species (including characterizing a previously undetected young stratum in S. japonicum) to infer patterns of sequence and expression evolution of W-linked genes at different time points after recombination was lost. W-linked genes show evidence of degeneration, including high rates of protein evolution and reduced expression. Most are found in young lineage-specific strata, with only a few high expression ancestral W-genes remaining, consistent with the progressive erosion of non-recombining regions. Among these, the splicing factor U2AF2 stands out as a promising candidate for primary sex determination, opening new avenues for understanding the molecular basis of the reproductive biology of this group.

When and how do sex-linked regions become sex chromosomes?

The attention given to heteromorphism and genetic degeneration of "classical sex chromosomes" (Y chromosomes in XY systems, and the W in ZW systems that were studied first and are best described) has perhaps created the impression that the absence of recombination between sex chromosomes is inevitable. I here argue that continued recombination is often to be expected, that absence of recombination is surprising and demands further study, and that the involvement of selection in reduced recombination is not yet well understood. Despite a long history of investigations of sex chromosome pairs, there is a need for more quantitative approaches to studying sex-linked regions. I describe a scheme to help understand the relationships between different properties of sex-linked regions. Specifically, I focus on their sizes (differentiating between small regions and extensive fully sex-linked ones), the times when they evolved, and their differentiation, and review studies using DNA sequencing in nonmodel organisms that are providing information about the processes causing these properties.

The evolution of sex chromosome dosage compensation in animals

The evolution of heteromorphic sex chromosomes shall lead to gene expression dosage problems, as in at least one of the sexes, the sex-linked gene dose has been reduced by half. It has been proposed that the transcriptional output of the whole X or Z chromosome should be doubled for complete dosage compensation in heterogametic sex. However, owing to the variability of the existing methods to determine the transcriptional differences between sex chromosomes and autosomes (S:A ratios) in different studies, we collected more than 500 public RNA-Seq data set from multiple tissues and species in major clades and proposed a unified computational framework for unbiased and comparable measurement of the S:A ratios of multiple species. We also tested the evolution of dosage compensation more directly by assessing changes in the expression levels of the current sex-linked genes relative to those of the ancestral sex-linked genes. We found that in mammals and birds, the S:A ratio is approximately 0.5, whereas in insects, fishes, and flatworms, the S:A ratio is approximately 1.0. Further analysis showed that the fraction of dosage-sensitive housekeeping genes on the X/Z chromosome is significantly correlated with the S:A ratio. In addition, the degree of degeneration of the Y chromosome may be responsible for the change in the S:A ratio in mammals without a dosage compensation mechanism. Our observations offer unequivocal support for the sex chromosome insensitivity hypothesis in animals and suggest that dosage sensitivity states of sex chromosomes are a major factor underlying different evolutionary strategies of dosage compensation.

Differences in the contributions of sex linkage and androgen regulation to sex-biased gene expression in juvenile and adult sticklebacks

Different evolutionary interests between males and females can lead to the evolution of sexual dimorphism. However, intersex genetic correlations due to the shared genome can constrain the evolution of sexual dimorphism, resulting in intra-locus sexual conflict. One of the mechanisms resolving this conflict is sex linkage, which allows males and females to carry different alleles on sex chromosomes. Another is a regulatory mutation causing sex-biased gene expression, which is often mediated by gonadal steroids in vertebrates. How do these two mechanisms differ in the contributions to the resolution of intra-locus sexual conflict? The magnitude of sexual conflict often varies between the juvenile and adult stages. Because gonadal steroids change in titre during development, we hypothesized that gonadal steroids play a role in sexual dimorphism expression only at certain developmental stages, whereas sex linkage is more important for sexual dimorphism expressed throughout life. Our brain transcriptome analysis of juvenile and adult threespine sticklebacks showed that the majority of genes that were sex-biased in both stages were sex-linked. The relative contribution of androgen-dependent regulation to the sex-biased transcriptome increased and that of sex linkage declined in adults compared to juveniles. The magnitude of the sex differences was greater in sex-linked genes than androgen-responsive genes, suggesting that sex linkage is more effective than androgen regulation in the production of large sex differences in gene expression. Overall, our data are consistent with the hypothesis that sex linkage is effective in resolving sexual conflict throughout life, whereas androgen-dependent regulation can contribute to temporary resolution of sexual conflict.

Young sex chromosomes in plants and animals

A major reason for studying plant sex chromosomes is that they may often be 'young' systems. There is considerable evidence for the independent evolution of separate sexes within plant families or genera, in some cases showing that the maximum possible time during which their sex-determining genes have existed must be much shorter than those of several animal taxa. Consequently, their sex-linked regions could either have evolved soon after genetic sex determination arose or considerably later. Plants, therefore, include species with both young and old systems. I review several questions about the evolution of sex-determining systems and sex chromosomes that require studies of young systems, including: the kinds of mutations involved in the transition to unisexual reproduction from hermaphroditism or monoecy (a form of functional hermaphroditism); the times when they arose; and the extent to which the properties of sex-linked regions of genomes reflect responses to new selective situations created by the presence of a sex-determining locus. I also evaluate which questions are best studied in plants, vs other suitable candidate organisms. Studies of young plant systems can help understand general evolutionary processes that are shared with the sex chromosomes of other organisms.

Identification of the master sex determining gene in Northern pike (Esox lucius) reveals restricted sex chromosome differentiation

Teleost fishes, thanks to their rapid evolution of sex determination mechanisms, provide remarkable opportunities to study the formation of sex chromosomes and the mechanisms driving the birth of new master sex determining (MSD) genes. However, the evolutionary interplay between the sex chromosomes and the MSD genes they harbor is rather unexplored. We characterized a male-specific duplicate of the anti-Müllerian hormone (amh) as the MSD gene in Northern Pike (Esox lucius), using genomic and expression evidence as well as by loss-of-function and gain-of-function experiments. Using RAD-Sequencing from a family panel, we identified Linkage Group (LG) 24 as the sex chromosome and positioned the sex locus in its sub-telomeric region. Furthermore, we demonstrated that this MSD originated from an ancient duplication of the autosomal amh gene, which was subsequently translocated to LG24. Using sex-specific pooled genome sequencing and a new male genome sequence assembled using Nanopore long reads, we also characterized the differentiation of the X and Y chromosomes, revealing a small male-specific insertion containing the MSD gene and a limited region with reduced recombination. Our study reveals an unexpectedly low level of differentiation between a pair of sex chromosomes harboring an old MSD gene in a wild teleost fish population, and highlights both the pivotal role of genes from the amh pathway in sex determination, as well as the importance of gene duplication as a mechanism driving the turnover of sex chromosomes in this clade.

In stark contrast to mammals and birds, a high proportion of teleosts have homomorphic sex chromosomes and display a high diversity of sex determining genes. Yet, population level knowledge of both the sex chromosome and the master sex determining gene is only available for the Japanese medaka, a model species. Here we identified and provided functional proofs of an old duplicate of anti-Müllerian hormone (Amh), a member of the Tgf- β family, as the male master sex determining gene in the Northern pike, Esox lucius. We found that this duplicate, named amhby (Y-chromosome-specific anti-Müllerian hormone paralog b), was translocated to the sub-telomeric region of the new sex chromosome, and now amhby shows strong sequence divergence as well as substantial expression pattern differences from its autosomal paralog, amha. We assembled a male genome sequence using Nanopore long reads and identified a restricted region of differentiation within the sex chromosome pair in a wild population. Our results provide insight on the conserved players in sex determination pathways, the mechanisms of sex chromosome turnover, and the diversity of levels of differentiation between homomorphic sex chromosomes in teleosts.

Sex-Biased Gene Expression and Dosage Compensation on the Artemia franciscana Z-Chromosome

Males and females of Artemia franciscana, a crustacean commonly used in the aquarium trade, are highly dimorphic. Sex is determined by a pair of ZW chromosomes, but the nature and extent of differentiation of these chromosomes is unknown. Here, we characterize the Z chromosome by detecting genomic regions that show lower genomic coverage in female than in male samples, and regions that harbor an excess of female-specific SNPs. We detect many Z-specific genes, which no longer have homologs on the W, but also Z-linked genes that appear to have diverged very recently from their existing W-linked homolog. We assess patterns of male and female expression in two tissues with extensive morphological dimorphism, gonads, and heads. In agreement with their morphology, sex-biased expression is common in both tissues. Interestingly, the Z chromosome is not enriched for sex-biased genes, and seems to in fact have a mechanism of dosage compensation that leads to equal expression in males and in females. Both of these patterns are contrary to most ZW systems studied so far, making A. franciscana an excellent model for investigating the interplay between the evolution of sexual dimorphism and dosage compensation, as well as Z chromosome evolution in general.

Dosage Compensation throughout the Schistosoma mansoni Lifecycle: Specific Chromatin Landscape of the Z Chromosome

Differentiated sex chromosomes are accompanied by a difference in gene dose between X/Z-specific and autosomal genes. At the transcriptomic level, these sex-linked genes can lead to expression imbalance, or gene dosage can be compensated by epigenetic mechanisms and results into expression level equalization. Schistosoma mansoni has been previously described as a ZW species (i.e., female heterogamety, in opposition to XY male heterogametic species) with a partial dosage compensation, but underlying mechanisms are still unexplored. Here, we combine transcriptomic (RNA-Seq) and epigenetic data (ChIP-Seq against H3K4me3, H3K27me3, and H4K20me1 histone marks) in free larval cercariae and intravertebrate parasitic stages. For the first time, we describe differences in dosage compensation status in ZW females, depending on the parasitic status: free cercariae display global dosage compensation, whereas intravertebrate stages show a partial dosage compensation. We also highlight regional differences of gene expression along the Z chromosome in cercariae, but not in the intravertebrate stages. Finally, we feature a consistent permissive chromatin landscape of the Z chromosome in both sexes and stages. We argue that dosage compensation in schistosomes is characterized by chromatin remodeling mechanisms in the Z-specific region.

Exaggerated heterochiasmy in a fish with sex-linked male coloration polymorphisms

It is often stated that polymorphisms for mutations affecting fitness of males and females in opposite directions [sexually antagonistic (SA) polymorphisms] are the main selective force for the evolution of recombination suppression between sex chromosomes. However, empirical evidence to discriminate between different hypotheses is difficult to obtain. We report genetic mapping results in laboratory-raised families of the guppy (Poecilia reticulata), a sexually dimorphic fish with SA polymorphisms for male coloration genes, mostly on the sex chromosomes. Comparison of the genetic and physical maps shows that crossovers are distributed very differently in the two sexes (heterochiasmy); in male meiosis, they are restricted to the termini of all four chromosomes studied, including chromosome 12, which carries the sex-determining locus. Genome resequencing of male and female guppies from a population also indicates sex linkage of variants across almost the entire chromosome 12. More than 90% of the chromosome carrying the male-determining locus is therefore transmitted largely through the male lineage. A lack of heterochiasmy in a related fish species suggests that it originated recently in the lineage leading to the guppy. Our findings do not support the hypothesis that suppressed recombination evolved in response to the presence of SA polymorphisms. Instead, a low frequency of recombination on a chromosome that carries a male-determining locus and has not undergone genetic degeneration has probably facilitated the establishment of male-beneficial coloration polymorphisms.

The DNA Methylation Landscape of Stickleback Reveals Patterns of Sex Chromosome Evolution and Effects of Environmental Salinity

Epigenetic mechanisms such as DNA methylation are a key component of dosage compensation on sex chromosomes and have been proposed as an important source of phenotypic variation influencing plasticity and adaptive evolutionary processes, yet little is known about the role of DNA methylation in an ecological or evolutionary context in vertebrates. The threespine stickleback (Gasterosteus aculeatus) is an ecological and evolutionary model system that has been used to study mechanisms involved in the evolution of adaptive phenotypes in novel environments as well as the evolution heteromorphic sex chromosomes and dosage compensation in vertebrates. Using whole genome bisulfite sequencing, we compared genome-wide DNA methylation patterns between threespine stickleback males and females and between stickleback reared at different environmental salinities. Apparent hypermethylation of the younger evolutionary stratum of the stickleback X chromosome in females relative to males suggests a potential role of DNA methylation in the evolution of heteromorphic sex chromosomes. We also demonstrate that rearing salinity has genome-wide effects on DNA methylation levels, which has the potential to lead to the accumulation of epigenetic variation between natural populations in different environments.

Evolution of gene dosage on the Z-chromosome of schistosome parasites

XY systems usually show chromosome-wide compensation of X-linked genes, while in many ZW systems, compensation is restricted to a minority of dosage-sensitive genes. Why such differences arose is still unclear. Here, we combine comparative genomics, transcriptomics and proteomics to obtain a complete overview of the evolution of gene dosage on the Z-chromosome of Schistosoma parasites. We compare the Z-chromosome gene content of African (Schistosoma mansoni and S. haematobium) and Asian (S. japonicum) schistosomes and describe lineage-specific evolutionary strata. We use these to assess gene expression evolution following sex-linkage. The resulting patterns suggest a reduction in expression of Z-linked genes in females, combined with upregulation of the Z in both sexes, in line with the first step of Ohno's classic model of dosage compensation evolution. Quantitative proteomics suggest that post-transcriptional mechanisms do not play a major role in balancing the expression of Z-linked genes.

Evolution of recombination rates between sex chromosomes

In species with genetic sex-determination, the chromosomes carrying the sex-determining genes have often evolved non-recombining regions and subsequently evolved the full set of characteristics denoted by the term 'sex chromosomes'. These include size differences, creating chromosomal heteromorphism, and loss of gene functions from one member of the chromosome pair. Such characteristics and changes have been widely reviewed, and underlie molecular genetic approaches that can detect sex chromosome regions. This review deals mainly with the evolution of new non-recombining regions, focusing on how certain evolutionary situations select for suppressed recombination (rather than the proximate mechanisms causing suppressed recombination between sex chromosomes). Particularly important is the likely involvement of sexually antagonistic polymorphisms in genome regions closely linked to sex-determining loci. These may be responsible for the evolutionary strata of sex chromosomes that have repeatedly formed by recombination suppression evolving across large genome regions. More studies of recently evolved non-recombining sex-determining regions should help to test this hypothesis empirically, and may provide evidence about whether other situations can sometimes lead to sex-linked regions evolving. Similarities with other non-recombining genome regions are discussed briefly, to illustrate common features of the different cases, though no general properties apply to all of them.This article is part of the themed issue 'Evolutionary causes and consequences of recombination rate variation in sexual organisms'.

Evolution of Sex Chromosome Dosage Compensation in Animals: A Beautiful Theory, Undermined by Facts and Bedeviled by Details

Many animals with genetic sex determination harbor heteromorphic sex chromosomes, where the heterogametic sex has half the gene dose of the homogametic sex. This imbalance, if reflected in the abundance of transcripts or proteins, has the potential to deleteriously disrupt interactions between X-linked and autosomal loci in the heterogametic sex. Classical theory predicts that molecular mechanisms will evolve to provide dosage compensation that recovers expression levels comparable to ancestral expression prior to sex chromosome divergence. Such dosage compensating mechanisms may also, secondarily, result in balanced sex-linked gene expression between males and females. However, numerous recent studies addressing sex chromosome dosage compensation (SCDC) in a diversity of animals have yielded a surprising array of patterns concerning dosage compensation in the heterogametic sex, as well as dosage balance between sexes. These results substantially contradict longstanding theory, catalyzing both novel perspectives and new approaches in dosage compensation research. In this review, we summarize the theory, analytical approaches, and recent results concerning evolutionary patterns of SCDC in animals. We also discuss methodological challenges and discrepancies encountered in this research, which often underlie conflicting results. Finally, we discuss what outstanding questions and opportunities exist for future research on SCDC.

Global Dosage Compensation Is Ubiquitous in Lepidoptera, but Counteracted by the Masculinization of the Z Chromosome

While chromosome-wide dosage compensation of the X chromosome has been found in many species, studies in ZW clades have indicated that compensation of the Z is more localized and/or incomplete. In the ZW Lepidoptera, some species show complete compensation of the Z chromosome, while others lack full equalization, but what drives these inconsistencies is unclear. Here, we compare patterns of male and female gene expression on the Z chromosome of two closely related butterfly species, Papilio xuthus and Papilio machaon, and in multiple tissues of two moths species, Plodia interpunctella and Bombyx mori, which were previously found to differ in the extent to which they equalize Z-linked gene expression between the sexes. We find that, while some species and tissues seem to have incomplete dosage compensation, this is in fact due to the accumulation of male-biased genes and the depletion of female-biased genes on the Z chromosome. Once this is accounted for, the Z chromosome is fully compensated in all four species, through the up-regulation of Z expression in females and in some cases additional down-regulation in males. We further find that both sex-biased genes and Z-linked genes have increased rates of expression divergence in this clade, and that this can lead to fast shifts in patterns of gene expression even between closely related species. Taken together, these results show that the uneven distribution of sex-biased genes on sex chromosomes can confound conclusions about dosage compensation and that Z chromosome-wide dosage compensation is not only possible but ubiquitous among Lepidoptera.

Evolution and conservation of Characidium sex chromosomes

Fish species exhibit substantial variation in the degree of genetic differentiation between sex chromosome pairs, and therefore offer the opportunity to study the full range of sex chromosome evolution. We used restriction-site associated DNA sequencing (RAD-seq) to study the sex chromosomes of Characidium gomesi, a species with conspicuous heteromorphic ZW/ZZ sex chromosomes. We screened 9863 single-nucleotide polymorphisms (SNPs), corresponding to ~1 marker/100 kb distributed across the genome for sex-linked variation. With this data set, we identified 26 female-specific RAD loci, putatively located on the W chromosome, as well as 148 sex-associated SNPs showing significant differentiation (average F_ST=0.144) between males and females, and therefore in regions of more recent divergence between the Z and W chromosomes. In addition, we detected 25 RAD loci showing extreme heterozygote deficiency in females but which were in Hardy-Weinberg equilibrium in males, consistent with degeneration of the W chromosome and therefore female hemizygosity. We validated seven female-specific and two sex-associated markers in a larger sample of C. gomesi, of which three localised to the W chromosome, thereby providing useful markers for sexing wild samples. Validated markers were evaluated in other populations and species of the genus Characidium, this exploration suggesting a rapid turnover of W-specific repetitive elements. Together, our analyses point to a complex origin for the sex chromosome of C. gomesi and highlight the utility of RAD-seq for studying the composition and evolution of sex chromosomes systems in wild populations.

Regulatory Architecture of Gene Expression Variation in the Threespine Stickleback Gasterosteus aculeatus

Much adaptive evolutionary change is underlain by mutational variation in regions of the genome that regulate gene expression rather than in the coding regions of the genes themselves. An understanding of the role of gene expression variation in facilitating local adaptation will be aided by an understanding of underlying regulatory networks. Here, we characterize the genetic architecture of gene expression variation in the threespine stickleback (Gasterosteus aculeatus), an important model in the study of adaptive evolution. We collected transcriptomic and genomic data from 60 half-sib families using an expression microarray and genotyping-by-sequencing, and located expression quantitative trait loci (eQTL) underlying the variation in gene expression in liver tissue using an interval mapping approach. We identified eQTL for several thousand expression traits. Expression was influenced by polymorphism in both cis- and trans-regulatory regions. Trans-eQTL clustered into hotspots. We did not identify master transcriptional regulators in hotspot locations: rather, the presence of hotspots may be driven by complex interactions between multiple transcription factors. One observed hotspot colocated with a QTL recently found to underlie salinity tolerance in the threespine stickleback. However, most other observed hotspots did not colocate with regions of the genome known to be involved in adaptive divergence between marine and freshwater habitats.

Maternal stress has divergent effects on gene expression patterns in the brains of male and female threespine stickleback

Maternal stress can have long-term effects on neurodevelopment that can influence offspring performance and population evolutionary trajectories. To examine the mechanistic basis for these neurodevelopmental effects of maternal stress, we used RNA-seq to assess differential gene expression across the brain transcriptome of adult male and female threespine stickleback (Gasterosteus aculeatus) from stressed and unstressed mothers. We identified sexually divergent effects of maternal stress on the brain transcriptome. In males, genes that were upregulated by maternal stress were enriched for processes involved in synaptic function and organization and steroid hormone-mediated signalling pathways, whereas in females genes that were upregulated by maternal stress were enriched for processes involved in protein translation and metabolic functions. The expression of several genes involved in the hypothalamic-pituitary-interrenal response to stress and epigenetic processes such as the regulation of DNA methylation patterns and miRNAs increased in males and not in females. These data suggest that maternal stress has markedly different effects on cellular pathways in the brains of male and female offspring of mothers that are exposed to stress, which could have important implications when assessing the long-term ecological and evolutionary impacts of stress across generations.

Dosage compensation of the sex chromosomes and autosomes

Males are XY and females are XX in most mammalian species. Other species such as birds have a different sex chromosome make-up: ZZ in males and ZW in females. In both types of organisms one of the sex chromosomes, Y or W, has degenerated due to lack of recombination with its respective homolog X or Z. Since autosomes are present in two copies in diploid organisms the heterogametic sex has become a natural "aneuploid" with haploinsufficiency for X- or Z-linked genes. Specific mechanisms have evolved to restore a balance between critical gene products throughout the genome and between males and females. Some of these mechanisms were co-opted from and/or added to compensatory processes that alleviate autosomal aneuploidy. Surprisingly, several modes of dosage compensation have evolved. In this review we will consider the evidence for dosage compensation and the molecular mechanisms implicated.

Genetic conflict between sexual signalling and juvenile survival in the three-spined stickleback

Background

Secondary sexual traits and mating preferences may evolve in part because the offspring of attractive males inherit attractiveness and other genetically correlated traits such as fecundity and viability. A problem regarding these indirect genetic mechanisms is how sufficient genetic variation in the traits subject to sexual selection is maintained within a population. Here we explored the additive genetic correlations between carotenoid-based male ornament colouration, female fecundity and juvenile survival rate in the three-spined stickleback (Gasterosteus aculeatus) to test the possibility that attractiveness genes reduce important fitness components in the bearers not expressing the sexual trait.

Results

Male sexual attractiveness (i.e., red nuptial colouration) as well as female fecundity and juvenile viability showed heritable variations in the three-spined stickleback. Thus, females can gain indirect benefits by mating with an attractive male. There was a strong positive genetic correlation between female fecundity and juvenile viability. However, red sexual signal of male sticklebacks was negatively genetically correlated with juvenile survival, suggesting genetic conflict between attractiveness and viability. There was no significant correlation between attractiveness of brothers and fecundity of sisters, suggesting no intra-locus sexual conflict.

Conclusions

The negative effects of mating with a colourful male on offspring viability may contribute to maintaining the heritable variation under strong directional sexual selection. The strength of indirect sexual selection may be weaker than previously thought due to the hidden genetic conflicts.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-016-0613-4) contains supplementary material, which is available to authorized users.

Evolution of vertebrate sex chromosomes and dosage compensation

Differentiated sex chromosomes in mammals and other vertebrates evolved independently but in strikingly similar ways. Vertebrates with differentiated sex chromosomes share the problems of the unequal expression of the genes borne on sex chromosomes, both between the sexes and with respect to autosomes. Dosage compensation of genes on sex chromosomes is surprisingly variable - and can even be absent - in different vertebrate groups. Systems that compensate for different gene dosages include a wide range of global, regional and gene-by-gene processes that differ in their extent and their molecular mechanisms. However, many elements of these control systems are similar across distant phylogenetic divisions and show parallels to other gene silencing systems. These dosage systems cannot be identical by descent but were probably constructed from elements of ancient silencing mechanisms that are ubiquitous among vertebrates and shared throughout eukaryotes.

The X Chromosome of Hemipteran Insects: Conservation, Dosage Compensation and Sex-Biased Expression

Insects of the order Hemiptera (true bugs) use a wide range of mechanisms of sex determination, including genetic sex determination, paternal genome elimination, and haplodiploidy. Genetic sex determination, the prevalent mode, is generally controlled by a pair of XY sex chromosomes or by an XX/X0 system, but different configurations that include additional sex chromosomes are also present. Although this diversity of sex determining systems has been extensively studied at the cytogenetic level, only the X chromosome of the model pea aphid Acyrthosiphon pisum has been analyzed at the genomic level, and little is known about X chromosome biology in the rest of the order.

In this study, we take advantage of published DNA- and RNA-seq data from three additional Hemiptera species to perform a comparative analysis of the gene content and expression of the X chromosome throughout this clade. We find that, despite showing evidence of dosage compensation, the X chromosomes of these species show female-biased expression, and a deficit of male-biased genes, in direct contrast to the pea aphid X. We further detect an excess of shared gene content between these very distant species, suggesting that despite the diversity of sex determining systems, the same chromosomal element is used as the X throughout a large portion of the order.

Conservation of Regional Variation in Sex-Specific Sex Chromosome Regulation

Regional variation in sex-specific gene regulation has been observed across sex chromosomes in a range of animals and is often a function of sex chromosome age. The avian Z chromosome exhibits substantial regional variation in sex-specific regulation, where older regions show elevated levels of male-biased expression. Distinct sex-specific regulation also has been observed across the male hypermethylated (MHM) region, which has been suggested to be a region of nascent dosage compensation. Intriguingly, MHM region regulatory features have not been observed in distantly related avian species despite the hypothesis that it is situated within the oldest region of the avian Z chromosome and is therefore orthologous across most birds. This situation contrasts with the conservation of other aspects of regional variation in gene expression observed on the avian sex chromosomes but could be the result of sampling bias. We sampled taxa across the Galloanserae, an avian clade spanning 90 million years, to test whether regional variation in sex-specific gene regulation across the Z chromosome is conserved. We show that the MHM region is conserved across a large portion of the avian phylogeny, together with other sex-specific regulatory features of the avian Z chromosome. Our results from multiple lines of evidence suggest that the sex-specific expression pattern of the MHM region is not consistent with nascent dosage compensation.

Purifying Selection Maintains Dosage-Sensitive Genes during Degeneration of the Threespine Stickleback Y Chromosome

Sex chromosomes are subject to unique evolutionary forces that cause suppression of recombination, leading to sequence degeneration and the formation of heteromorphic chromosome pairs (i.e., XY or ZW). Although progress has been made in characterizing the outcomes of these evolutionary processes on vertebrate sex chromosomes, it is still unclear how recombination suppression and sequence divergence typically occur and how gene dosage imbalances are resolved in the heterogametic sex. The threespine stickleback fish (Gasterosteus aculeatus) is a powerful model system to explore vertebrate sex chromosome evolution, as it possesses an XY sex chromosome pair at relatively early stages of differentiation. Using a combination of whole-genome and transcriptome sequencing, we characterized sequence evolution and gene expression across the sex chromosomes. We uncovered two distinct evolutionary strata that correspond with known structural rearrangements on the Y chromosome. In the oldest stratum, only a handful of genes remain, and these genes are under strong purifying selection. By comparing sex-linked gene expression with expression of autosomal orthologs in an outgroup, we show that dosage compensation has not evolved in threespine sticklebacks through upregulation of the X chromosome in males. Instead, in the oldest stratum, the genes that still possess a Y chromosome allele are enriched for genes predicted to be dosage sensitive in mammals and yeast. Our results suggest that dosage imbalances may have been avoided at haploinsufficient genes by retaining function of the Y chromosome allele through strong purifying selection.