Sexual antagonism and X inactivation--the SAXI hypothesis

Evolutionarily new genes in humans with disease phenotypes reveal functional enrichment patterns shaped by adaptive innovation and sexual selection

New genes (or young genes) are genetic novelties pivotal in mammalian evolution. Their phenotypic impacts and evolutionary pattern over time, however, remain elusive in humans due to the technical and ethical complexities in functional studies. By combining human gene age dating and Mendelian disease phenotyping, our research reveals a gradual increase in disease gene proportions with gene age. Logistic regression modeling indicates that this increase could be related to longer protein lengths and higher burdens of deleterious de novo germline variants (DNVs) for older genes. We also find a steady integration of new genes with biomedical phenotypes into the human genome over macroevolutionary timescales (~0.07% per million years). Despite this stable pace, we observe distinct patterns in phenotypic enrichment, pleiotropy, and selective pressures across gene ages. Notably, young genes show significant enrichment in diseases related to the male reproductive system, indicating strong sexual selection. Young genes also exhibit disease-related functions in tissues and systems potentially linked to human phenotypic innovations, such as increased brain size, musculoskeletal phenotypes, and color vision. We further reveal a logistic growth pattern of pleiotropy over evolutionary time, indicating a diminishing marginal growth of new functions for older genes due to intensifying selective constraints over time. We propose a "pleiotropy-barrier" model that delineates higher potentials of phenotypic innovation for young genes than for older genes, a process subject to natural selection. Our study demonstrates that evolutionary new genes are critical in influencing human reproductive evolution and adaptive phenotypic innovations driven by sexual and natural selection, with low pleiotropy as a selective advantage.

Single-cell RNA-seq of Drosophila miranda testis reveals the evolution and trajectory of germline sex chromosome regulation

Although sex chromosomes have evolved from autosomes, they often have unusual regulatory regimes that are sex- and cell-type-specific such as dosage compensation (DC) and meiotic sex chromosome inactivation (MSCI). The molecular mechanisms and evolutionary forces driving these unique transcriptional programs are critical for genome evolution but have been, in the case of MSCI in Drosophila, subject to continuous debate. Here, we take advantage of the younger sex chromosomes in D. miranda (XR and the neo-X) to infer how former autosomes acquire sex-chromosome-specific regulatory programs using single-cell and bulk RNA sequencing and ribosome profiling, in a comparative evolutionary context. We show that contrary to mammals and worms, the X down-regulation through germline progression is most consistent with the shutdown of DC instead of MSCI, resulting in half gene dosage at the end of meiosis for all 3 X's. Moreover, lowly expressed germline and meiotic genes on the neo-X are ancestrally lowly expressed, instead of acquired suppression after sex linkage. For the young neo-X, DC is incomplete across all tissue and cell types and this dosage imbalance is rescued by contributions from Y-linked gametologs which produce transcripts that are translated to compensate both gene and protein dosage. We find an excess of previously autosomal testis genes becoming Y-specific, showing that the neo-Y and its masculinization likely resolve sexual antagonism. Multicopy neo-sex genes are predominantly expressed during meiotic stages of spermatogenesis, consistent with their amplification being driven to interfere with mendelian segregation. Altogether, this study reveals germline regulation of evolving sex chromosomes and elucidates the consequences these unique regulatory mechanisms have on the evolution of sex chromosome architecture.

Evidence of Nonrandom Patterns of Functional Chromosome Organization in Danaus plexippus

Our understanding on the interplay between gene functionality and gene arrangement at different chromosome scales relies on a few Diptera and the honeybee, species with quality reference genome assemblies, accurate gene annotations, and abundant transcriptome data. Using recently generated 'omic resources in the monarch butterfly Danaus plexippus, a species with many more and smaller chromosomes relative to Drosophila species and the honeybee, we examined the organization of genes preferentially expressed at broadly defined developmental stages (larva, pupa, adult males, and adult females) at both fine and whole-chromosome scales. We found that developmental stage-regulated genes do not form more clusters, but do form larger clusters, than expected by chance, a pattern consistent across the gene categories examined. Notably, out of the 30 chromosomes in the monarch genome, 12 of them, plus the fraction of the chromosome Z that corresponds to the ancestral Z in other Lepidoptera, were found enriched for developmental stage-regulated genes. These two levels of nonrandom gene organization are not independent as enriched chromosomes for developmental stage-regulated genes tend to harbor disproportionately large clusters of these genes. Further, although paralogous genes were overrepresented in gene clusters, their presence is not enough to explain two-thirds of the documented cases of whole-chromosome enrichment. The composition of the largest clusters often included paralogs from more than one multigene family as well as unrelated single-copy genes. Our results reveal intriguing patterns at the whole-chromosome scale in D. plexippus while shedding light on the interplay between gene expression and chromosome organization beyond Diptera and Hymenoptera.

scRNA-seq Reveals Novel Genetic Pathways and Sex Chromosome Regulation in Tribolium Spermatogenesis

Spermatogenesis is critical to sexual reproduction yet evolves rapidly in many organisms. High-throughput single-cell transcriptomics promises unparalleled insight into this important process but understanding can be impeded in nonmodel systems by a lack of known genes that can reliably demarcate biologically meaningful cell populations. Tribolium castaneum, the red flour beetle, lacks known markers for spermatogenesis found in insect species like Drosophila melanogaster. Using single-cell sequencing data collected from adult beetle testes, we implement a strategy for elucidating biologically meaningful cell populations by using transient expression stage identification markers, weighted principal component clustering, and SNP-based haploid/diploid phasing. We identify populations that correspond to observable points in sperm differentiation and find species specific markers for each stage. Our results indicate that molecular pathways underlying spermatogenesis in Coleoptera are substantially diverged from those in Diptera. We also show that most genes on the X chromosome experience meiotic sex chromosome inactivation. Temporal expression of Drosophila MSL complex homologs coupled with spatial analysis of potential chromatin entry sites further suggests that the dosage compensation machinery may mediate escape from meiotic sex chromosome inactivation and postmeiotic reactivation of the X chromosome.

Evolutionarily new genes in humans with disease phenotypes reveal functional enrichment patterns shaped by adaptive innovation and sexual selection

New genes (or young genes) are structural novelties pivotal in mammalian evolution. Their phenotypic impact on humans, however, remains elusive due to the technical and ethical complexities in functional studies. Through combining gene age dating with Mendelian disease phenotyping, our research reveals that new genes associated with disease phenotypes steadily integrate into the human genome at a rate of ~ 0.07% every million years over macroevolutionary timescales. Despite this stable pace, we observe distinct patterns in phenotypic enrichment, pleiotropy, and selective pressures between young and old genes. Notably, young genes show significant enrichment in the male reproductive system, indicating strong sexual selection. Young genes also exhibit functions in tissues and systems potentially linked to human phenotypic innovations, such as increased brain size, bipedal locomotion, and color vision. Our findings further reveal increasing levels of pleiotropy over evolutionary time, which accompanies stronger selective constraints. We propose a "pleiotropy-barrier" model that delineates different potentials for phenotypic innovation between young and older genes subject to natural selection. Our study demonstrates that evolutionary new genes are critical in influencing human reproductive evolution and adaptive phenotypic innovations driven by sexual and natural selection, with low pleiotropy as a selective advantage.

Orphan gene in Littorina: An unexpected role of symbionts in the host evolution

Mechanisms of reproductive isolation between closely related sympatric species are of high evolutionary significance as they may function as initial drivers of speciation and protect species integrity afterwards. Proteins involved in the establishment of reproductive barriers often evolve fast and may be key players in cessation of gene flow between the incipient species. The five Atlantic Littorina (Neritrema) species represent a notable example of recent radiation. The geographic ranges of these young species largely overlap and the mechanisms of reproductive isolation are poorly understood. In this study, we performed a detailed analysis of the reproductive protein LOSP, previously identified in Littorina. We showed that this protein is evolutionary young and taxonomically restricted to the genus Littorina. It has high sequence variation both within and between Littorina species, which is compatible with its presumable role in the reproductive isolation. The strongest differences in the LOSP structure were detected between Littorina subgenera with distinctive repetitive motifs present exclusively in the Neritrema species, but not in L. littorea. Moreover, the sequence of these repetitive structural elements demonstrates a high homology with genetic elements of bacteria, identified as components of Littorina associated microbiomes. We suggest that these elements were acquired from a symbiotic bacterial donor via horizontal genetic transfer (HGT), which is indirectly confirmed by the presence of multiple transposable elements in the LOSP flanking and intronic regions. Furthermore, we hypothesize that this HGT-driven evolutionary innovation promoted LOSP function in reproductive isolation, which might be one of the factors determining the intensive cladogenesis in the Littorina (Neritrema) lineage in contrast to the anagenesis in the L. littorea clade.

The de novo assembly of a European wild boar genome revealed unique patterns of chromosomal structural variations and segmental duplications

The rapid progress of sequencing technology has greatly facilitated the de novo genome assembly of pig breeds. However, the assembly of the wild boar genome is still lacking, hampering our understanding of chromosomal and genomic evolution during domestication from wild boars into domestic pigs. Here, we sequenced and de novo assembled a European wild boar genome (ASM2165605v1) using the long‐range information provided by 10× Linked‐Reads sequencing. We achieved a high‐quality assembly with contig N50 of 26.09 Mb. Additionally, 1.64% of the contigs (222) with lengths from 107.65 kb to 75.36 Mb covered 90.3% of the total genome size of ASM2165605v1 (~2.5 Gb). Mapping analysis revealed that the contigs can fill 24.73% (93/376) of the gaps present in the orthologous regions of the updated pig reference genome (Sscrofa11.1). We further improved the contigs into chromosome level with a reference‐assistant scaffolding method. Using the ‘assembly‐to‐assembly’ approach, we identified intra‐chromosomal large structural variations (SVs, length >1 kb) between ASM2165605v1 and Sscrofa11.1 assemblies. Interestingly, we found that the number of SV events on the X chromosome deviated significantly from the linear models fitting autosomes (R² > 0.64, p < 0.001). Specifically, deletions and insertions were deficient on the X chromosome by 66.14 and 58.41% respectively, whereas duplications and inversions were excessive on the X chromosome by 71.96 and 107.61% respectively. We further used the large segmental duplications (SDs, >1 kb) events as a proxy to understand the large‐scale inter‐chromosomal evolution, by resolving parental‐derived relationships for SD pairs. We revealed a significant excess of SD movements from the X chromosome to autosomes (p < 0.001), consistent with the expectation of meiotic sex chromosome inactivation. Enrichment analyses indicated that the genes within derived SD copies on autosomes were significantly related to biological processes involving nervous system, lipid biosynthesis and sperm motility (p < 0.01). Together, our analyses of the de novo assembly of ASM2165605v1 provides insight into the SVs between European wild boar and domestic pig, in addition to the ongoing process of meiotic sex chromosome inactivation in driving inter‐chromosomal interaction between the sex chromosome and autosomes.

Meiotic sex chromosome inactivation and the XY body: a phase separation hypothesis

In mammalian male meiosis, the heterologous X and Y chromosomes remain unsynapsed and, as a result, are subject to meiotic sex chromosome inactivation (MSCI). MSCI is required for the successful completion of spermatogenesis. Following the initiation of MSCI, the X and Y chromosomes undergo various epigenetic modifications and are transformed into a nuclear body termed the XY body. Here, we review the mechanisms underlying the initiation of two essential, sequential processes in meiotic prophase I: MSCI and XY-body formation. The initiation of MSCI is directed by the action of DNA damage response (DDR) pathways; downstream of the DDR, unique epigenetic states are established, leading to the formation of the XY body. Accumulating evidence suggests that MSCI and subsequent XY-body formation may be driven by phase separation, a physical process that governs the formation of membraneless organelles and other biomolecular condensates. Thus, here we gather literature-based evidence to explore a phase separation hypothesis for the initiation of MSCI and the formation of the XY body.

On the Origin and Evolution of Drosophila New Genes during Spermatogenesis

The origin of functional new genes is a basic biological process that has significant contribution to organismal diversity. Previous studies in both Drosophila and mammals showed that new genes tend to be expressed in testes and avoid the X chromosome, presumably because of meiotic sex chromosome inactivation (MSCI). Here, we analyze the published single-cell transcriptome data of Drosophila adult testis and find an enrichment of male germline mitotic genes, but an underrepresentation of meiotic genes on the X chromosome. This can be attributed to an excess of autosomal meiotic genes that were derived from their X-linked mitotic progenitors, which provides direct cell-level evidence for MSCI in Drosophila. We reveal that new genes, particularly those produced by retrotransposition, tend to exhibit an expression shift toward late spermatogenesis compared with their parental copies, probably due to the more intensive sperm competition or sexual conflict. Our results dissect the complex factors including age, the origination mechanisms and the chromosomal locations that influence the new gene origination and evolution in testes, and identify new gene cases that show divergent cell-level expression patterns from their progenitors for future functional studies.

Rapid Gene evolution in an ancient post-transcriptional and translational regulatory system compensates for meiotic X chromosomal inactivation

It is conventionally assumed that conserved pathways evolve slowly with little participation of gene evolution. Nevertheless, it has been recently observed that young genes can take over fundamental functions in essential biological processes, for example, development and reproduction. It is unclear how newly duplicated genes are integrated into ancestral networks and reshape the conserved pathways of important functions. Here, we investigated origination and function of two autosomal genes that evolved recently in Drosophila: Poseidon and Zeus, which were created by RNA-based duplications from the X-linked CAF40, a subunit of the conserved CCR4–NOT deadenylase complex involved in posttranscriptional and translational regulation. Knockdown and knockout assays show that the two genes quickly evolved critically important functions in viability and male fertility. Moreover, our transcriptome analysis demonstrates that the three genes have a broad and distinct effect in the expression of hundreds of genes, with almost half of the differentially expressed genes being perturbed exclusively by one paralog, but not the others. Co-immunoprecipitation and tethering assays show that the CAF40 paralog Poseidon maintains the ability to interact with the CCR4–NOT deadenylase complex and might act in posttranscriptional mRNA regulation. The rapid gene evolution in the ancient posttranscriptional and translational regulatory system may be driven by evolution of sex chromosomes to compensate for the meiotic X chromosomal inactivation (MXCI) in Drosophila.

Phylogeny and sex chromosome evolution of palaeognathae

Many paleognaths (ratites and tinamous) have a pair of homomorphic ZW sex chromosomes in contrast to the highly differentiated sex chromosomes of most other birds. To understand the evolutionary causes for the different tempos of sex chromosome evolution, we produced female genomes of 12 paleognathous species and reconstructed the phylogeny and the evolutionary history of paleognathous sex chromosomes. We uncovered that Palaeognathae sex chromosomes had undergone stepwise recombination suppression and formed a pattern of "evolutionary strata". Nine of the 15 studied species' sex chromosomes have maintained homologous recombination in their long pseudoautosomal regions extending more than half of the entire chromosome length. We found that in older strata, the W chromosome suffered more serious functional gene loss. Their homologous Z-linked regions, compared with other genomic regions, have produced an excess of species-specific autosomal duplicated genes that evolved female-specific expression, in contrast to their broadly expressed progenitors. We speculate the "defeminization" of Z chromosome with underrepresentation of female-biased genes and slow divergence of sex chromosomes of paleognaths might be related to their distinctive mode of sexual selection targeting females that evolved in their common ancestors.

Dynamic sex chromosome expression in Drosophila male germ cells

Given their copy number differences and unique modes of inheritance, the evolved gene content and expression of sex chromosomes is unusual. In many organisms the X and Y chromosomes are inactivated in spermatocytes, possibly as a defense mechanism against insertions into unpaired chromatin. In addition to current sex chromosomes, Drosophila has a small gene-poor X-chromosome relic (4th) that re-acquired autosomal status. Here we use single cell RNA-Seq on fly larvae to demonstrate that the single X and pair of 4th chromosomes are specifically inactivated in primary spermatocytes, based on measuring all genes or a set of broadly expressed genes in testis we identified. In contrast, genes on the single Y chromosome become maximally active in primary spermatocytes. Reduced X transcript levels are due to failed activation of RNA-Polymerase-II by phosphorylation of Serine 2 and 5.

Analysis of new retrogenes provides insight into dog adaptive evolution

The origin and subsequent evolution of new genes have been considered as an important source of genetic and phenotypic diversity in organisms. Dog breeds show great phenotypic diversity for morphological, physiological, and behavioral traits. However, the contributions of newly originated retrogenes, which provide important genetic bases for dog species differentiation and adaptive traits, are largely unknown. Here, we analyzed the dog genome to identify new RNA-based duplications and comprehensively investigated their origin, evolution, functions in adaptive traits, and gene movement processes. First, we totally identified 3,025 retrocopies including 476 intact retrogenes, 2,518 retropseudogenes, and 31 chimerical retrogenes. Second, selective pressure along with ESTs expression analysis showed that most of the intact retrogenes were significantly under stronger purifying selection and subjected to more functional constraints when compared to retropseudogenes. Furthermore, a large number of retrocopies and chimerical retrogenes that occurred approximately 22 million years ago implied a burst of retrotransposition in the dog genome after the divergence time between dog and its closely related species red fox. Interestingly, GO and pathway analyses showed that new retrogenes had expanded in glutathione biosynthetic/metabolic process which likely provided important genetic basis for dogs' adaptation to scavenge human waste dumps. Finally, consistent with the results in human and mouse, a significant excess of functional retrogenes movement on and off the X chromosome in the dog confirmed a general pattern of gene movement process in mammals which was likely driven by natural selection or sexual antagonism. Together, these results increase our understanding that new retrogenes can reshape the dog genome and provide further exploration of the molecular mechanisms underlying the dogs' adaptive evolution.

Excess of retrogene traffic in pig X chromosome

Despite the insights into retrogene evolution in multiple species provided by trans-traffic retrogenes (trans-retrogenes), cis-traffic retrogenes (cis-retrogenes) are hitherto understudied and thus limit our understanding of genetic novelty. Here we used the pig genome as a model to compare cis- and trans-retrogenes. We found that cis- and trans-retrogenes have similar patterns in terms of excessive X-chromosome representation, testis expression preference, and metabolic functional enrichment. Despite these similarities, lower levels of selective pressure were found in cis- than in trans-retrogenes. The evolutionary dating evidence revealed that more cis-retrogenes have originated after pig-cattle divergence than trans-retrogenes, suggesting a more recent origination of cis-retrogenes. The gene family clustering confirmed that excesses of X-derived cis-retrogenes may be driven by their rapid expansion in the recent times. Thus, the different selective constraints between cis- and trans-retrogenes may be due to different evolutionary time-scales rather than any differential functional importance. Thus, this study highlights the ongoing functional contributions from cis- and trans-traffic retrogenes during the reshaping of pig genome.

Genes Relocated Between Drosophila Chromosome Arms Evolve Under Relaxed Selective Constraints Relative to Non-Relocated Genes

Gene duplication creates a second copy of a gene either in tandem to the ancestral locus or dispersed to another chromosomal location. When the ancestral copy of a dispersed duplicate is lost from the genome, it creates the appearance that the gene was "relocated" from the ancestral locus to the derived location. Gene relocations may be as common as canonical dispersed duplications in which both the ancestral and derived copies are retained. Relocated genes appear to be under more selective constraints than the derived copies of canonical duplications, and they are possibly as conserved as single-copy non-relocated genes. To test this hypothesis, we combined comparative genomics, population genetics, gene expression, and functional analyses to assess the selection pressures acting on relocated, duplicated, and non-relocated single-copy genes in Drosophila genomes. We find that relocated genes evolve faster than single-copy non-relocated genes, and there is no evidence that this faster evolution is driven by positive selection. In addition, relocated genes are less essential for viability and male fertility than single-copy non-relocated genes, suggesting that relocated genes evolve fast because of relaxed selective constraints. However, relocated genes evolve slower than the derived copies of canonical dispersed duplicated genes. We therefore conclude that relocated genes are under more selective constraints than canonical duplicates, but are not as conserved as single-copy non-relocated genes.

Regulation of the X Chromosome in the Germline and Soma of Drosophila melanogaster Males

During the evolution of heteromorphic sex chromosomes, the sex-specific Y chromosome degenerates, while the X chromosome evolves new mechanisms of regulation. Using bioinformatic and experimental approaches, we investigate the expression of the X chromosome in Drosophila melanogaster. We observe nearly complete X chromosome dosage compensation in male somatic tissues, but not in testis. The X chromosome contains disproportionately fewer genes with high expression in testis than the autosomes, even after accounting for the lack of dosage compensation, which suggests that another mechanism suppresses their expression in the male germline. This is consistent with studies of reporter genes and transposed genes, which find that the same gene has higher expression when autosomal than when X-linked. Using a new reporter gene that is expressed in both testis and somatic tissues, we find that the suppression of X-linked gene expression is limited to genes with high expression in testis and that the extent of the suppression is positively correlated with expression level.

Rapid evolution of female-biased genes among four species of Anopheles malaria mosquitoes

Understanding how phenotypic differences between males and females arise from the sex-biased expression of nearly identical genomes can reveal important insights into the biology and evolution of a species. Among Anopheles mosquito species, these phenotypic differences include vectorial capacity, as it is only females that blood feed and thus transmit human malaria. Here, we use RNA-seq data from multiple tissues of four vector species spanning the Anopheles phylogeny to explore the genomic and evolutionary properties of sex-biased genes. We find that, in these mosquitoes, in contrast to what has been found in many other organisms, female-biased genes are more rapidly evolving in sequence, expression, and genic turnover than male-biased genes. Our results suggest that this atypical pattern may be due to the combination of sex-specific life history challenges encountered by females, such as blood feeding. Furthermore, female propensity to mate only once in nature in male swarms likely diminishes sexual selection of post-reproductive traits related to sperm competition among males. We also develop a comparative framework to systematically explore tissue- and sex-specific splicing to document its conservation throughout the genus and identify a set of candidate genes for future functional analyses of sex-specific isoform usage. Finally, our data reveal that the deficit of male-biased genes on the X Chromosomes in Anopheles is a conserved feature in this genus and can be directly attributed to chromosome-wide transcriptional regulation that de-masculinizes the X in male reproductive tissues.

Specific down-regulation of spermatogenesis genes targeted by 22G RNAs in hybrid sterile males associated with an X-Chromosome introgression

Hybrid incompatibility (HI) prevents gene flow between species, thus lying at the heart of speciation genetics. One of the most common HIs is male sterility. Two superficially contradictory observations exist for hybrid male sterility. First, an introgression on the X Chromosome is more likely to produce male sterility than on autosome (so-called large-X theory); second, spermatogenesis genes are enriched on the autosomes but depleted on the X Chromosome (demasculinization of X Chromosome). Analysis of gene expression in Drosophila hybrids suggests a genetic interaction between the X Chromosome and autosomes that is essential for male fertility. However, the prevalence of such an interaction and its underlying mechanism remain largely unknown. Here we examine the interaction in nematode species by contrasting the expression of both coding genes and transposable elements (TEs) between hybrid sterile males and its parental nematode males. We use two lines of hybrid sterile males, each carrying an independent introgression fragment from Caenorhabditis briggsae X Chromosome in an otherwise Caenorhabditis nigoni background, which demonstrate similar defects in spermatogenesis. We observe a similar pattern of down-regulated genes that are specific for spermatogenesis between the two hybrids. Importantly, the down-regulated genes caused by the X Chromosome introgressions show a significant enrichment on the autosomes, supporting an epistatic interaction between the X Chromosome and autosomes. We investigate the underlying mechanism of the interaction by measuring small RNAs and find that a subset of 22G RNAs specifically targeting the down-regulated spermatogenesis genes is significantly up-regulated in hybrids, suggesting that perturbation of small RNA-mediated regulation may contribute to the X-autosome interaction.

The Constrained Maximal Expression Level Owing to Haploidy Shapes Gene Content on the Mammalian X Chromosome

X chromosomes are unusual in many regards, not least of which is their nonrandom gene content. The causes of this bias are commonly discussed in the context of sexual antagonism and the avoidance of activity in the male germline. Here, we examine the notion that, at least in some taxa, functionally biased gene content may more profoundly be shaped by limits imposed on gene expression owing to haploid expression of the X chromosome. Notably, if the X, as in primates, is transcribed at rates comparable to the ancestral rate (per promoter) prior to the X chromosome formation, then the X is not a tolerable environment for genes with very high maximal net levels of expression, owing to transcriptional traffic jams. We test this hypothesis using The Encyclopedia of DNA Elements (ENCODE) and data from the Functional Annotation of the Mammalian Genome (FANTOM5) project. As predicted, the maximal expression of human X-linked genes is much lower than that of genes on autosomes: on average, maximal expression is three times lower on the X chromosome than on autosomes. Similarly, autosome-to-X retroposition events are associated with lower maximal expression of retrogenes on the X than seen for X-to-autosome retrogenes on autosomes. Also as expected, X-linked genes have a lesser degree of increase in gene expression than autosomal ones (compared to the human/Chimpanzee common ancestor) if highly expressed, but not if lowly expressed. The traffic jam model also explains the known lower breadth of expression for genes on the X (and the Z of birds), as genes with broad expression are, on average, those with high maximal expression. As then further predicted, highly expressed tissue-specific genes are also rare on the X and broadly expressed genes on the X tend to be lowly expressed, both indicating that the trend is shaped by the maximal expression level not the breadth of expression per se. Importantly, a limit to the maximal expression level explains biased tissue of expression profiles of X-linked genes. Tissues whose tissue-specific genes are very highly expressed (e.g., secretory tissues, tissues abundant in structural proteins) are also tissues in which gene expression is relatively rare on the X chromosome. These trends cannot be fully accounted for in terms of alternative models of biased expression. In conclusion, the notion that it is hard for genes on the Therian X to be highly expressed, owing to transcriptional traffic jams, provides a simple yet robustly supported rationale of many peculiar features of X’s gene content, gene expression, and evolution.

Laurence Hurst, Lukasz Huminiecki, and the FANTOM5 consortium propose a new explanation for the peculiar expression properties of genes on the human X chromosome, based on the premise that very high expression levels cannot be achieved on a haploid-expressed chromosome.

Genes located on the human X chromosome are not a random mix of genes: they tend to be expressed in relatively few tissues or are specific for a particular set of tissues, e.g., brain regions. Prior attempts to explain this skewed gene content have hypothesized that the X chromosome might be peculiar because it has to balance mutations that are advantageous to one sex but deleterious to the other, or because it has to shut down during the process of sperm manufacture in males. Here we suggest and test a third possible explanation: that genes on the X chromosome are limited in their transcription levels and thus tend to be genes that are lowly or specifically expressed. We consider the suggestion that since these genes can only be expressed from one chromosome, as males only have one X, the ability to express a gene at very high rates is limited owing to potential transcriptional traffic jams. As predicted, we find that human X-located genes have maximal expression rates far below that of genes residing on autosomes. When we look at genes that have moved onto or off the X chromosome during recent evolution, we find the maximal expression is higher when not on the X chromosome. We also find that X-located genes that are relatively highly expressed are not able to increase their expression level further. Our model explains both the enrichment for tissue specificity and the paucity of certain tissues with X-located genes. Genes underrepresented on the X are either expressed in many tissues—such genes tend to have high maximal expression—or are from tissues that require a lot of transcription (e.g., fast secreting tissues like the liver). Just as many of the findings cannot be explained by the two earlier models, neither can the traffic jam model explain all the peculiar features of the genes found on the X chromosome. Indeed, we find evidence of a reproduction-related bias in X-located genes, even after allowing for the traffic jam problem.

Genetics of dioecy and causal sex chromosomes in plants

Dioecy (separate male and female individuals) ensures outcrossing and is more prevalent in animals than in plants. Although it is common in bryophytes and gymnosperms, only 5% of angiosperms are dioecious. In dioecious higher plants, flowers borne on male and female individuals are, respectively deficient in functional gynoecium and androecium. Dioecy is inherited via three sex chromosome systems: XX/XY, XX/X0 and WZ/ZZ, such that XX or WZ is female and XY, X0 or ZZ are males. The XX/XY system generates the rarer XX/X0 and WZ/ZZ systems. An autosome pair begets XY chromosomes. A recessive loss-of-androecium mutation (ana) creates X chromosome and a dominant gynoecium-suppressing (GYS) mutation creates Y chromosome. The ana/ANA and gys/GYS loci are in the sex-determining region (SDR) of the XY pair. Accumulation of inversions, deleterious mutations and repeat elements, especially transposons, in the SDR of Y suppresses recombination between X and Y in SDR, making Y labile and increasingly degenerate and heteromorphic from X. Continued recombination between X and Y in their pseudoautosomal region located at the ends of chromosomal arms allows survival of the degenerated Y and of the species. Dioecy is presumably a component of the evolutionary cycle for the origin of new species. Inbred hermaphrodite species assume dioecy. Later they suffer degenerate-Y-led population regression. Cross-hybridization between such extinguishing species and heterologous species, followed by genome duplication of segregants from hybrids, give rise to new species.

Temporal genomic evolution of bird sex chromosomes

BACKGROUND

Sex chromosomes exhibit many unusual patterns in sequence and gene expression relative to autosomes. Birds have evolved a female heterogametic sex system (male ZZ, female ZW), through stepwise suppression of recombination between chrZ and chrW. To address the broad patterns and complex driving forces of Z chromosome evolution, we analyze here 45 newly available bird genomes and four species' transcriptomes, over their course of recombination loss between the sex chromosomes.

RESULTS

We show Z chromosomes in general have a significantly higher substitution rate in introns and synonymous protein-coding sites than autosomes, driven by the male-to-female mutation bias ('male-driven evolution' effect). Our genome-wide estimate reveals that the degree of such a bias ranges from 1.6 to 3.8 among different species. G + C content of third codon positions exhibits the same trend of gradual changes with that of introns, between chrZ and autosomes or regions with increasing ages of becoming Z-linked, therefore codon usage bias in birds is probably driven by the mutational bias. On the other hand, Z chromosomes also evolve significantly faster at nonsynonymous sites relative to autosomes ('fast-Z' evolution). And species with a lower level of intronic heterozygosities tend to evolve even faster on the Z chromosome. Further analysis of fast-evolving genes' enriched functional categories and sex-biased expression patterns support that, fast-Z evolution in birds is mainly driven by genetic drift. Finally, we show in species except for chicken, gene expression becomes more male-biased within Z-linked regions that have became hemizygous in females for a longer time, suggesting a lack of global dosage compensation in birds, and the reported regional dosage compensation in chicken has only evolved very recently.

CONCLUSIONS

In conclusion, we uncover that the sequence and expression patterns of Z chromosome genes covary with their ages of becoming Z-linked. In contrast to the mammalian X chromosomes, such patterns are mainly driven by mutational bias and genetic drift in birds, due to the opposite sex-biased inheritance of Z vs. X.

Drosophila sperm proteome evolution: Insights from comparative genomic approaches

Despite their conserved functional role in sexually reproducing organisms, spermatozoa are a diverse and rapidly evolving cell type. This phenomenon is largely attributed to sexual selection in polygamous species where sperm from multiple males compete to fertilize a limited number of oocytes. Drosophila have proven to be a particularly informative model system for the study of spermatogenesis and in this review we discuss how the characterization of the Drosophila melanogaster sperm proteome has advanced our understanding of the evolutionary genomics of sperm form and function. We summarize the molecular evolutionary characteristics of sperm genes and highlight recent evidence demonstrating the importance of novel gene creation in the evolution of sperm function and competitive ability. Comparative proteomic evidence is also provided, supporting an overall functional conservation between the Drosophila and mouse sperm proteomes. This analysis reveals a diverse repertoire of proteins functioning in proteolytic pathways, as well as the presence of proteins of the complement and innate immunity systems. We propose that these pathways may have functional relevance to post-mating female immunological responses as well as coevolved interactions with pathways expressed in the female reproductive tract, including those involved in sperm-oocyte recognition and fertilization.

Chromosomal distribution of cytonuclear genes in a dioecious plant with sex chromosomes

The coordination between nuclear and organellar genes is essential to many aspects of eukaryotic life, including basic metabolism, energy production, and ultimately, organismal fitness. Although nuclear genes are biparentally inherited, mitochondrial and chloroplast genes are almost exclusively maternally inherited, and this asymmetry may lead to a bias in the chromosomal distribution of nuclear genes whose products act in the mitochondria or chloroplasts. In particular, because X-linked genes have a higher probability of cotransmission with organellar genes (2/3) compared with autosomal genes (1/2), selection for coadaptation has been predicted to lead to an overrepresentation of nuclear-mitochondrial and nuclear-chloroplast genes on the X chromosome relative to autosomes. In contrast, the occurrence of sexually antagonistic organellar mutations might lead to selection for movement of cytonuclear genes from the X chromosome to autosomes to reduce male mutation load. Recent broad-scale comparative studies of N-mt distributions in animals have found evidence for these hypotheses in some species, but not others. Here, we use transcriptome sequences to conduct the first study of the chromosomal distribution of cytonuclear interacting genes in a plant species with sex chromosomes (Rumex hastatulus; Polygonaceae). We found no evidence of under- or overrepresentation of either N-mt or N-cp genes on the X chromosome, and thus no support for either the coadaptation or the sexual-conflict hypothesis. We discuss how our results from a species with recently evolved sex chromosomes fit into an emerging picture of the evolutionary forces governing the chromosomal distribution of nuclear-mitochondrial and nuclear-chloroplast genes.

Meiotic sex chromosome inactivation in Drosophila

In several different taxa, there is indubitable evidence of transcriptional silencing of the X and Y chromosomes in male meiotic cells of spermatogenesis. However, the so called meiotic sex chromosome inactivation (MSCI) has been recently a hot bed for debate in Drosophila melanogaster. This review covers cytological and genetic observations, data from transgenic constructs with testis-specific promoters, global expression profiles obtained from mutant, wild-type, larvae and adult testes as well as from cells of different stages of spermatogenesis. There is no dispute on that D. melanogaster spermatogenesis presents a down-regulation of X chromosome that does not result from the lack of dosage compensation. However, the issue is currently focused on the level of reduction of X-linked expression, the precise time it occurs and how many genes are affected. The deep examination of data and experiments in this review exposes the limitations intrinsic to the methods of studying MSCI in D. melanogaster. The current methods do not allow us to affirm anything else than the X chromosome down-regulation in meiosis (MSCI). Therefore, conclusion about level, degree or precise timing is inadequate until new approaches are implemented to know the details of MSCI or other processes involved for D. melanogaster model.

Presence of extensive Wolbachia symbiont insertions discovered in the genome of its host Glossina morsitans morsitans

Tsetse flies (Glossina spp.) are the cyclical vectors of Trypanosoma spp., which are unicellular parasites responsible for multiple diseases, including nagana in livestock and sleeping sickness in humans in Africa. Glossina species, including Glossina morsitans morsitans (Gmm), for which the Whole Genome Sequence (WGS) is now available, have established symbiotic associations with three endosymbionts: Wigglesworthia glossinidia, Sodalis glossinidius and Wolbachia pipientis (Wolbachia). The presence of Wolbachia in both natural and laboratory populations of Glossina species, including the presence of horizontal gene transfer (HGT) events in a laboratory colony of Gmm, has already been shown. We herein report on the draft genome sequence of the cytoplasmic Wolbachia endosymbiont (cytWol) associated with Gmm. By in silico and molecular and cytogenetic analysis, we discovered and validated the presence of multiple insertions of Wolbachia (chrWol) in the host Gmm genome. We identified at least two large insertions of chrWol, 527,507 and 484,123 bp in size, from Gmm WGS data. Southern hybridizations confirmed the presence of Wolbachia insertions in Gmm genome, and FISH revealed multiple insertions located on the two sex chromosomes (X and Y), as well as on the supernumerary B-chromosomes. We compare the chrWol insertions to the cytWol draft genome in an attempt to clarify the evolutionary history of the HGT events. We discuss our findings in light of the evolution of Wolbachia infections in the tsetse fly and their potential impacts on the control of tsetse populations and trypanosomiasis.

African trypanosomes are transmitted to man and animals by tsetse fly, a blood sucking insect. Tsetse flies include all Glossina species with the genome of Glossina morsitans morsitans (Gmm) being sequenced under the International Glossina Genome Initiative. The endosymbionts Wigglesworthia glossinidia, Sodalis glossinidius and Wolbachia pipientis (Wolbachia) have been found to establish symbiotic associations with Gmm. Wolbachia is known to be present in natural and laboratory populations of Glossina species. In this study we report the genome sequence of the Wolbachia strain that is associated with Gmm. With the aid of in silico and molecular and cytogenetic analyses, multiple insertions of the Wolbachia genome were revealed and confirmed in Gmm chromosome. Comparison of the cytoplasmic Wolbachia draft genome and the chromosomal insertions enabled us to infer the evolutionary history of the Wolbachia horizontal transfer events. These findings are discussed in relation to their impact on the development of Wolbachia-based strategies for the control of tsetse flies and trypanosomiasis.

Regulation of the X chromosomes in Caenorhabditis elegans

Dosage compensation, which regulates the expression of genes residing on the sex chromosomes, has provided valuable insights into chromatin-based mechanisms of gene regulation. The nematode Caenorhabditis elegans has adopted various strategies to down-regulate and even nearly silence the X chromosomes. This article discusses the different chromatin-based strategies used in somatic tissues and in the germline to modulate gene expression from the C. elegans X chromosomes and compares these strategies to those used by other organisms to cope with similar X-chromosome dosage differences.

A long-term demasculinization of X-linked intergenic noncoding RNAs in Drosophila melanogaster

Recent studies have revealed key roles of noncoding RNAs in sex-related pathways, but little is known about the evolutionary forces acting on these noncoding RNAs. Profiling the transcriptome of Drosophila melanogaster with whole-genome tiling arrays found that 15% of male-biased transcribed fragments are intergenic noncoding RNAs (incRNAs), suggesting a potentially important role for incRNAs in sex-related biological processes. Statistical analysis revealed a paucity of male-biased incRNAs and coding genes on the X chromosome, suggesting that similar evolutionary forces could be affecting the genomic organization of both coding and noncoding genes. Expression profiling across germline and somatic tissues further suggested that both male meiotic sex chromosome inactivation (MSCI) and sexual antagonism could contribute to the chromosomal distribution of male-biased incRNAs. Comparative sequence analysis showed that the evolutionary age of male-biased incRNAs is a significant predictor of their chromosomal locations. In addition to identifying abundant sex-biased incRNAs in the fly genome, our work unveils a global picture of the complex interplay between noncoding RNAs and sexual chromosome evolution.

Gene expression and DNA methylation status of chicken primordial germ cells

DNA methylation reprogramming of primordial germ cells (PGCs) in mammals establishes monoallelic expression of imprinting genes, maintains retrotransposons in an inactive state, inactivates one of the two X chromosomes, and suppresses gene expression. However, the roles of DNA methylation in chickens PGCs are unknown. In this study, we found a 1.5-fold or greater difference in the expression of 261 transcripts when comparing PGCs and chicken embryonic fibroblasts (CEFs) using an Affymetrix GeneChip Chicken Genome Array. In addition, we analyzed the methylation patterns of the regions ~5-kb upstream of 261 sorted genes, 51 of which were imprinting homologous loci and 49 of which were X-linked homologous loci in chicken using the MeDIP Array by Roche NimbleGen. Seven hypomethylated and five hypermethylated regions within the 5-kb upstream regions of 261 genes were found in PGCs when compared with CEFs. These differentially methylated regions were restrictively matched to differentially expressed genes in PGCs. We also detected 203 differentially methylated regions within imprinting and X-linked homologous regions between male PGCs and female PGCs. These differentially methylated regions may be directly or indirectly associated with gene expression during early embryonic development, and the epigenetic difference could be evolutionally conserved between mammals and birds.

New genes as drivers of phenotypic evolution

During the course of evolution, genomes acquire novel genetic elements as sources of functional and phenotypic diversity, including new genes that originated in recent evolution. In the past few years, substantial progress has been made in understanding the evolution and phenotypic effects of new genes. In particular, an emerging picture is that new genes, despite being present in the genomes of only a subset of species, can rapidly evolve indispensable roles in fundamental biological processes, including development, reproduction, brain function and behaviour. The molecular underpinnings of how new genes can develop these roles are starting to be characterized. These recent discoveries yield fresh insights into our broad understanding of biological diversity at refined resolution.

Sex-linked transcriptional divergence in the hermaphrodite fungus Neurospora tetrasperma

In the filamentous ascomycete Neurospora tetrasperma, a large (approx. 7 Mbp) region of suppressed recombination surrounds the mating-type (mat) locus. While the remainder of the genome is largely homoallelic, this region of recombinational suppression, extending over 1500 genes, is associated with sequence divergence. Here, we used microarrays to examine how the molecular phenotype of gene expression level is linked to this divergent region, and thus to the mating type. Culturing N. tetrasperma on agar media that induce sexual/female or vegetative/male tissue, we found 196 genes significantly differentially expressed between mat A and mat a mating types. Our data show that the genes exhibiting mat-linked expression are enriched in the region genetically linked to mating type, and sequence and expression divergence are positively correlated. Our results indicate that the phenotype of mat A strains is optimized for traits promoting sexual/female development and the phenotype of mat a strains for vegetative/male development. This discovery of differentially expressed genes associated with mating type provides a link between genotypic and phenotypic divergence in this taxon and illustrates a fungal analogue to sexual dimorphism found among animals and plants.

Sex-biased gene expression during head development in a sexually dimorphic stalk-eyed fly

Stalk-eyed flies (family Diopsidae) are a model system for studying sexual selection due to the elongated and sexually dimorphic eye-stalks found in many species. These flies are of additional interest because their X chromosome is derived largely from an autosomal arm in other flies. To identify candidate genes required for development of dimorphic eyestalks and investigate how sex-biased expression arose on the novel X, we compared gene expression between males and females using oligonucleotide microarrays and RNA from developing eyestalk tissue or adult heads in the dimorphic diopsid, Teleopsis dalmanni. Microarray analysis revealed sex-biased expression for 26% of 3,748 genes expressed in eye-antennal imaginal discs and concordant sex-biased expression for 86 genes in adult heads. Overall, 415 female-biased and 482 male-biased genes were associated with dimorphic eyestalk development but not differential expression in the adult head. Functional analysis revealed that male-biased genes are disproportionately associated with growth and mitochondrial function while female-biased genes are associated with cell differentiation and patterning or are novel transcripts. With regard to chromosomal effects, dosage compensation occurs by elevated expression of X-linked genes in males. Genes with female-biased expression were more common on the X and less common on autosomes than expected, while male-biased genes exhibited no chromosomal pattern. Rates of protein evolution were lower for female-biased genes but higher for genes that moved on or off the novel X chromosome. These findings cannot be due to meiotic sex chromosome inactivation or by constraints associated with dosage compensation. Instead, they could be consistent with sexual conflict in which female-biased genes on the novel X act primarily to reduce eyespan in females while other genes increase eyespan in both sexes. Additional information on sex-biased gene expression in other tissues and related sexually monomorphic species could confirm this interpretation.

The contribution of female meiotic drive to the evolution of neo-sex chromosomes

Sex chromosomes undergo rapid turnover in certain taxonomic groups. One of the mechanisms of sex chromosome turnover involves fusions between sex chromosomes and autosomes. Sexual antagonism, heterozygote advantage, and genetic drift have been proposed as the drivers for the fixation of this evolutionary event. However, all empirical patterns of the prevalence of multiple sex chromosome systems across different taxa cannot be simply explained by these three mechanisms. In this study, we propose that female meiotic drive may contribute to the evolution of neo-sex chromosomes. The results of this study showed that in mammals, the XY(1) Y(2) sex chromosome system is more prevalent in species with karyotypes of more biarmed chromosomes, whereas the X(1) X(2) Y sex chromosome system is more prevalent in species with predominantly acrocentric chromosomes. In species where biarmed chromosomes are favored by female meiotic drive, X-autosome fusions (XY(1) Y(2) sex chromosome system) will be also favored by female meiotic drive. In contrast, in species with more acrocentric chromosomes, Y-autosome fusions (X(1) X(2) Y sex chromosome system) will be favored just because of the biased mutation rate toward chromosomal fusions. Further consideration should be given to female meiotic drive as a mechanism in the fixation of neo-sex chromosomes.

Little evidence for demasculinization of the Drosophila X chromosome among genes expressed in the male germline

Male-biased genes—those expressed at higher levels in males than in females—are underrepresented on the X chromosome of Drosophila melanogaster. Several evolutionary models have been posited to explain this so-called demasculinization of the X. Here, we show that the apparent paucity of male-biased genes on the X chromosome is attributable to global X-autosome differences in expression in Drosophila testes, owing to a lack of sex chromosome dosage compensation in the male germline, but not to any difference in the density of testis-specific or testis-biased genes on the X chromosome. First, using genome-wide gene expression data from 20 tissues, we find no evidence that genes with testis-specific expression are underrepresented on the X chromosome. Second, using contrasts in gene expression profiles among pairs of tissues, we recover a statistical underrepresentation of testis-biased genes on the X but find that the pattern largely disappears once we account for the lack of dosage compensation in the Drosophila male germline. Third, we find that computationally “demasculinizing” the autosomes is not sufficient to produce an expression profile similar to that of the X chromosome in the testes. Our findings thus show that the lack of sex chromosome dosage compensation in Drosophila testes can explain the apparent signal of demasculinization on the X, whereas evolutionary demasculinization of the X cannot explain its overall reduced expression in the testes.

Segmental dataset and whole body expression data do not support the hypothesis that non-random movement is an intrinsic property of Drosophila retrogenes

Background

Several studies in Drosophila have shown excessive movement of retrogenes from the X chromosome to autosomes, and that these genes are frequently expressed in the testis. This phenomenon has led to several hypotheses invoking natural selection as the process driving male-biased genes to the autosomes. Metta and Schlötterer (BMC Evol Biol 2010, 10:114) analyzed a set of retrogenes where the parental gene has been subsequently lost. They assumed that this class of retrogenes replaced the ancestral functions of the parental gene, and reported that these retrogenes, although mostly originating from movement out of the X chromosome, showed female-biased or unbiased expression. These observations led the authors to suggest that selective forces (such as meiotic sex chromosome inactivation and sexual antagonism) were not responsible for the observed pattern of retrogene movement out of the X chromosome.

Results

We reanalyzed the dataset published by Metta and Schlötterer and found several issues that led us to a different conclusion. In particular, Metta and Schlötterer used a dataset combined with expression data in which significant sex-biased expression is not detectable. First, the authors used a segmental dataset where the genes selected for analysis were less testis-biased in expression than those that were excluded from the study. Second, sex-biased expression was defined by comparing male and female whole-body data and not the expression of these genes in gonadal tissues. This approach significantly reduces the probability of detecting sex-biased expressed genes, which explains why the vast majority of the genes analyzed (parental and retrogenes) were equally expressed in both males and females. Third, the female-biased expression observed by Metta and Schlötterer is mostly found for parental genes located on the X chromosome, which is known to be enriched with genes with female-biased expression. Fourth, using additional gonad expression data, we found that autosomal genes analyzed by Metta and Schlötterer are less up regulated in ovaries and have higher chance to be expressed in meiotic cells of spermatogenesis when compared to X-linked genes.

Conclusions

The criteria used to select retrogenes and the sex-biased expression data based on whole adult flies generated a segmental dataset of female-biased and unbiased expressed genes that was unable to detect the higher propensity of autosomal retrogenes to be expressed in males. Thus, there is no support for the authors’ view that the movement of new retrogenes, which originated from X-linked parental genes, was not driven by selection. Therefore, selection-based genetic models remain the most parsimonious explanations for the observed chromosomal distribution of retrogenes.

Gene duplication, tissue-specific gene expression and sexual conflict in stalk-eyed flies (Diopsidae)

Gene duplication provides an essential source of novel genetic material to facilitate rapid morphological evolution. Traits involved in reproduction and sexual dimorphism represent some of the fastest evolving traits in nature, and gene duplication is intricately involved in the origin and evolution of these traits. Here, we review genomic research on stalk-eyed flies (Diopsidae) that has been used to examine the extent of gene duplication and its role in the genetic architecture of sexual dimorphism. Stalk-eyed flies are remarkable because of the elongation of the head into long stalks, with the eyes and antenna laterally displaced at the ends of these stalks. Many species are strongly sexually dimorphic for eyespan, and these flies have become a model system for studying sexual selection. Using both expressed sequence tag and next-generation sequencing, we have established an extensive database of gene expression in the developing eye-antennal imaginal disc, the adult head and testes. Duplicated genes exhibit narrower expression patterns than non-duplicated genes, and the testes, in particular, provide an abundant source of gene duplication. Within somatic tissue, duplicated genes are more likely to be differentially expressed between the sexes, suggesting gene duplication may provide a mechanism for resolving sexual conflict.

Evidence of a paucity of genes that interact with the mitochondrion on the X in mammals

Mitochondria are essential organelles whose replication, development, and physiology are dependent upon coordinated gene interactions with both the mitochondrial and the nuclear genomes. The evolution of coadapted (CA) nuclear-mitochondrial gene combinations would be facilitated if such nuclear genes were located on the X-chromosome instead of on the autosomes because of the increased probability of cotransmission. Here, we test the prediction of the CA hypothesis by investigating the chromosomal distribution of nuclear genes that interact with mitochondria. Using the online genome database BIOMART, we compared the density of genes that have a mitochondrion cellular component annotation across chromosomes in 16 vertebrates. We find a strong and highly significant genomic pattern against the CA hypothesis: nuclear genes interacting with the mitochondrion are significantly underrepresented on the X-chromosome in mammals but not in birds. We interpret our findings in terms of sexual conflict as a mechanism that may generate the observed pattern. Our finding extends single-gene theory for the evolution of sexually antagonistic genes to nuclear-mitochondrial gene combinations.

Re-analysis of the larval testis data on meiotic sex chromosome inactivation revealed evidence for tissue-specific gene expression related to the drosophila X chromosome

BACKGROUND

Meiotic sex chromosome inactivation (MSCI) during spermatogenesis has been proposed as one of the evolutionary driving forces behind both the under-representation of male-biased genes on, and the gene movement out of, the X chromosome in Drosophila. However, the relevance of MSCI in shaping sex chromosome evolution is controversial. Here we examine two aspects of a recent study on testis gene expression (Mikhaylova and Nurminsky, BMC Biol 2011, 9:29) that failed to support the MSCI in Drosophila. First, Mikhaylova and Nurminsky found no differences between X-linked and autosomal genes based on the transcriptional profiling of the early testis development, and thus concluded that MSCI does not occur in D. melanogaster. Second, they also analyzed expression data from several D. melanogaster tissues and concluded that under-representation on the X chromosome is not an exclusive property of testis-biased genes, but instead, a general property of tissue-specific genes.

RESULTS

By re-analyzing the Mikhaylova and Nurminsky's testis data and the expression data on several D. melanogaster tissues, we made two major findings that refuted their original claims. First, the developmental testis data has generally greater experimental error than conventional analyses, which reduced significantly the power to detect chromosomal differences in expression. Nevertheless, our re-analysis observed significantly lower expression of the X chromosome in the genomic transcriptomes of later development stages of the testis, which is consistent with the MSCI hypothesis. Second, tissue-specific genes are also in general enriched with genes more expressed in testes than in ovaries, that is testis-biased genes. By completely excluding from the analyses the testis-biased genes, which are known to be under-represented in the X, we found that all the other tissue-specific genes are randomly distributed between the X chromosome and the autosomes.

CONCLUSIONS

Our findings negate the original study of Mikhaylova and Nurminsky, which concluded a lack of MSCI and generalized the pattern of paucity in the X chromosome for tissue-specific genes in Drosophila. Therefore, MSCI and other selection-based models such as sexual antagonism, dosage compensation, and meiotic-drive continue to be viable models as driving forces shaping the genomic distribution of male-related genes in Drosophila.

Demasculinization of the Anopheles gambiae X chromosome

Background

In a number of organisms sex-biased genes are non-randomly distributed between autosomes and the shared sex chromosome X (or Z). Studies on Anopheles gambiae have produced conflicting results regarding the underrepresentation of male-biased genes on the X chromosome and it is unclear to what extent sexual antagonism, dosage compensation or X-inactivation in the male germline, the evolutionary forces that have been suggested to affect the chromosomal distribution of sex-biased genes, are operational in Anopheles.

Results

We performed a meta-analysis of sex-biased gene expression in Anopheles gambiae which provides evidence for a general underrepresentation of male-biased genes on the X-chromosome that increased in significance with the observed degree of sex-bias. A phylogenomic comparison between Drosophila melanogaster, Aedes aegypti and Culex quinquefasciatus also indicates that the Anopheles X chromosome strongly disfavours the evolutionary conservation of male-biased expression and that novel male-biased genes are more likely to arise on autosomes. Finally, we demonstrate experimentally that transgenes situated on the Anopheles gambiae X chromosome are transcriptionally silenced in the male germline.

Conclusion

The data presented here support the hypothesis that the observed demasculinization of the Anopheles X chromosome is driven by X-chromosome inactivation in the male germline and by sexual antagonism. The demasculinization appears to be the consequence of a loss of male-biased expression, rather than a failure in the establishment or the extinction of male-biased genes.

The genetics of sex chromosomes: evolution and implications for hybrid incompatibility

Heteromorphic sex chromosomes, where one sex has two different types of sex chromosomes, face very different evolutionary consequences than do autosomes. Two important features of sex chromosomes arise from being present in only one copy in one of the sexes: dosage compensation and the meiotic silencing of sex chromosomes. Other differences arise because sex chromosomes spend unequal amounts of time in each sex. Thus, the impact of evolutionary processes (mutation, selection, genetic drift, and meiotic drive) differs substantially between each sex chromosome, and between the sex chromosomes and the autosomes. Sex chromosomes also play a disproportionate role in Haldane's rule and other important patterns related to hybrid incompatibility, and thus speciation. We review the consequences of sex chromosomes on hybrid incompatibility. A theme running through this review is that epigenetic processes, notably those related to chromatin, may be more important to the evolution of sex chromosomes and the evolution of hybrid incompatibility than previously recognized.

Retrogenes moved out of the z chromosome in the silkworm

Previous studies on organisms with well-differentiated X and Y chromosomes, such as Drosophila and mammals, consistently detected an excess of genes moving out of the X chromosome and gaining testis-biased expression. Several selective evolutionary mechanisms were shown to be associated with this nonrandom gene traffic, which contributed to the evolution of the X chromosome and autosomes. If selection drives gene traffic, such traffic should also exist in species with Z and W chromosomes, where the females are the heterogametic sex. However, no previous studies on gene traffic in species with female heterogamety have found any nonrandom chromosomal gene movement. Here, we report an excess of retrogenes moving out of the Z chromosome in an organism with the ZW sex determination system, Bombyx mori. In addition, we showed that those "out of Z" retrogenes tended to have ovary-biased expression, which is consistent with the pattern of non-retrogene traffic recently reported in birds and symmetrical to the retrogene movement in mammals and fruit flies out of the X chromosome evolving testis functions. These properties of gene traffic in the ZW system suggest a general role for the heterogamety of sex chromosomes in determining the chromosomal locations and the evolution of sex-biased genes.

Genome-wide analysis of retrogene polymorphisms in Drosophila melanogaster

Gene duplication via retrotransposition has been shown to be an important mechanism in evolution, affecting gene dosage and allowing for the acquisition of new gene functions. Although fixed retrotransposed genes have been found in a variety of species, very little effort has been made to identify retrogene polymorphisms. Here, we examine 37 Illumina-sequenced North American Drosophila melanogaster inbred lines and present the first ever data set and analysis of polymorphic retrogenes in Drosophila. We show that this type of polymorphism is quite common, with any two gametes in the North American population differing in the presence or absence of six retrogenes, accounting for ~13% of gene copy-number heterozygosity. These retrogenes were identified by a straightforward method that can be applied using any type of DNA sequencing data. We also use a variant of this method to conduct a genome-wide scan for intron presence/absence polymorphisms, and show that any two chromosomes in the population likely differ in the presence of multiple introns. We show that these polymorphisms are all in fact deletions rather than intron gain events present in the reference genome. Finally, by leveraging the known location of the parental genes that give rise to the retrogene polymorphisms, we provide direct evidence that natural selection is responsible for the excess of fixations of retrogenes moving off of the X chromosome in Drosophila. Further efforts to identify retrogene and intron presence/absence polymorphisms will undoubtedly improve our understanding of the evolution of gene copy number and gene structure.

Nuclear chromosome dynamics in the Drosophila male germ line contribute to the nonrandom genomic distribution of retrogenes

The origin of RNA-based gene duplicates, that is, retrogenes, involves the reverse transcription of an mRNA derived from a parental gene to generate a cDNA copy, its insertion elsewhere in the genome, and the recruitment of regulatory sequences. Drosophila retrogenes are preferentially expressed in testis and a higher than expected number transpose to autosomal locations from the X chromosome. However, the influence of genomic context on the insertion preference of retrogenes remains poorly understood. We find that the distribution of retrogenes in the Drosophila melanogaster genome can be explained by an insertion bias toward chromosome domains containing testis-biased genes that are located at the nuclear periphery in somatic cells, but at inner positions in the male germ line. The lower fraction of these chromosome domains accessible in the male germ line on the X chromosome as compared with the autosomes also contributes to the scarcity of retrogenes on the X chromosome.

Gene duplication in the evolution of sexual dimorphism

Males and females share most of the same genes, so selection in one sex will typically produce a correlated response in the other sex. Yet, the sexes have evolved to differ in a multitude of behavioral, morphological, and physiological traits. How did this sexual dimorphism evolve despite the presence of a common underlying genome? We investigated the potential role of gene duplication in the evolution of sexual dimorphism. Because duplication events provide extra genetic material, the sexes each might use this redundancy to facilitate sex-specific gene expression, permitting the evolution of dimorphism. We investigated this hypothesis at the genome-wide level in Drosophila melanogaster, using the presence of sex-biased expression as a proxy for the sex-specific specialization of gene function. We expected that if sexually antagonistic selection is a potent force acting upon individual genes, duplication will result in paralog families whose members differ in sex-biased expression. Gene members of the same duplicate family can have different expression patterns in males versus females. In particular, duplicate pairs containing a male-biased gene are found more frequently than expected, in agreement with previous studies. Furthermore, when the singleton ortholog is unbiased, duplication appears to allow one of the paralog copies to acquire male-biased expression. Conversely, female-biased expression is not common among duplicates; fewer duplicate genes are expressed in the female-soma and ovaries than in the male-soma and testes. Expression divergence exists more in older than in younger duplicates pairs, but expression divergence does not correlate with protein sequence divergence. Finally, genomic proximity may have an effect on whether paralogs differ in sex-biased expression. We conclude that the data are consistent with a role of gene duplication in fostering male-biased, but not female-biased, gene expression, thereby aiding the evolution of sexual dimorphism.

Rapid functional divergence of a newly evolved polyubiquitin gene in Drosophila and its role in the trade-off between male fecundity and lifespan

The cost of reproduction is a pivotal trade-off with various biological processes during the evolution of organisms. However, the genes and molecular mechanisms underlying the evolution of balancing reproductive capacity and its cost are still largely unknown. Here, we present a comprehensive study on the evolution, expression, and biological functions of a newly evolved pair of X-linked polyubiquitin tandemly duplicated genes, CG32744 and CG11700, of which the duplication event occurred in Drosophila melanogaster lineage after the split from D. simulans clade. We found that CG32744 retains conserved polyubiquitin-coding sequences across Drosophila species and is ubiquitously expressed, whereas CG11700 has accumulated numerous amino acid changes and shows a male-specific expression pattern. Null mutants of CG11700 have a higher male fecundity but shorter lifespan, whereas its overexpression decreases male fecundity. In contrast, the null mutants of the peptide-conserved CG32744 do not exhibit such phenotypes. These results suggest that CG11700 might have experienced neofunctionalization and evolved important functions in the trade-off between male fecundity and lifespan and that CG32744 likely has retained the ancestral function.