Sex chromosome-to-autosome transposition events counter Y-chromosome gene loss in mammals

Background: Although the mammalian X and Y chromosomes evolved from a single pair of autosomes, they are highly differentiated: the Y chromosome is dramatically smaller than the X and has lost most of its genes. The surviving genes are a specialized set with extraordinary evolutionary longevity. Most mammalian lineages have experienced delayed, or relatively recent, loss of at least one conserved Y-linked gene. An extreme example of this phenomenon is in the Japanese spiny rat, where the Y chromosome has disappeared altogether. In this species, many Y-linked genes were rescued by transposition to new genomic locations, but until our work presented here, this has been considered an isolated case. Results: We describe eight cases of genes that have relocated to autosomes in mammalian lineages where the corresponding Y-linked gene has been lost. These gene transpositions originated from either the X or Y chromosomes, and are observed in diverse mammalian lineages: occurring at least once in marsupials, apes, and cattle, and at least twice in rodents and marmoset. For two genes - EIF1AX/Y and RPS4X/Y - transposition to autosomes occurred independently in three distinct lineages. Conclusions: Rescue of Y-linked gene loss through transposition to autosomes has previously been reported for a single isolated rodent species. However, our findings indicate that this compensatory mechanism is widespread among mammalian species. Thus, Y-linked gene loss emerges as an additional driver of gene transposition from the sex chromosomes, a phenomenon thought to be driven primarily by meiotic sex chromosome inactivation.National Institutes of Health (U.S.) (Grant HG000257

Intrachromosomal homologous recombination between inverted amplicons on opposing Y-chromosome arms

Amplicons – large, nearly identical repeats in direct or inverted orientation – are abundant in the male-specific region of the human Y chromosome (MSY) and provide targets for intrachromosomal non-allelic homologous recombination (NAHR). Thus far, NAHR events resulting in deletions, duplications, inversions, or isodicentric chromosomes have been reported only for amplicon pairs located exclusively on the short arm (Yp) or the long arm (Yq). Here we report our finding of four men with Y chromosomes that evidently formed by intrachromosomal NAHR between inverted repeat pairs comprising one amplicon on Yp and one amplicon on Yq. In two men with spermatogenic failure, sister-chromatid crossing-over resulted in pseudoisoYp chromosome formation and loss of distal Yq. In two men with normal spermatogenesis, intrachromatid crossing-over generated pericentric inversions. These findings highlight the recombinogenic nature of the MSY, as intrachromosomal NAHR occurs for nearly all Y-chromosome amplicon pairs, even those located on opposing chromosome arms.National Institutes of Health (U.S.)Netherlands Organization for Scientific ResearchAcademic Medical Center (University of Amsterdam

Isodicentric Y Chromosomes and Sex Disorders as Byproducts of Homologous Recombination that Maintains Palindromes

Massive palindromes in the human Y chromosome harbor mirror-image gene pairs essential for spermatogenesis. During evolution, these gene pairs have been maintained by intrapalindrome, arm-to-arm recombination. The mechanism of intrapalindrome recombination and risk of harmful effects are unknown. We report 51 patients with isodicentric Y (idicY) chromosomes formed by homologous crossing over between opposing arms of palindromes on sister chromatids. These ectopic recombination events occur at nearly all Y-linked palindromes. Based on our findings, we propose that intrapalindrome sequence identity is maintained via noncrossover pathways of homologous recombination. DNA double-strand breaks that initiate these pathways can be alternatively resolved by crossing over between sister chromatids to form idicY chromosomes, with clinical consequences ranging from spermatogenic failure to sex reversal and Turner syndrome. Our observations imply that crossover and noncrossover pathways are active in nearly all Y-linked palindromes, exposing an Achilles' heel in the mechanism that preserves palindrome-borne genes.National Institutes of Health (U.S.)Howard Hughes Medical InstituteNetherlands Organization for Scientific ResearchUniversity of Amsterdam. Academic Medical CenterBoehringer Ingelheim (Fellowship

TEX11 is mutated in infertile men with azoospermia and regulates genome-wide recombination rates in mouse

Genome‐wide recombination is essential for genome stability, evolution, and speciation. Mouse Tex11, an X‐linked meiosis‐specific gene, promotes meiotic recombination and chromosomal synapsis. Here, we report that TEX11 is mutated in infertile men with non‐obstructive azoospermia and that an analogous mutation in the mouse impairs meiosis. Genetic screening of a large cohort of idiopathic infertile men reveals that TEX11 mutations, including frameshift and splicing acceptor site mutations, cause infertility in 1% of azoospermic men. Functional evaluation of three analogous human TEX11 missense mutations in transgenic mouse models identified one mutation (V748A) as a potential infertility allele and found two mutations non‐causative. In the mouse model, an intronless autosomal Tex11 transgene functionally substitutes for the X‐linked Tex11 gene, providing genetic evidence for the X‐to‐autosomal retrotransposition evolution phenomenon. Furthermore, we find that TEX11 protein levels modulate genome‐wide recombination rates in both sexes. These studies indicate that TEX11 alleles affecting expression level or substituting single amino acids may contribute to variations in recombination rates between sexes and among individuals in humans.Howard Hughes Medical Institute (Award)National Institutes of Health (U.S.) (NIH/NIGMS grant R01GM076327

Convergent evolution of chicken Z and human X chromosomes by expansion and gene acquisition

In birds, as in mammals, one pair of chromosomes differs between the sexes. In birds, males are ZZ and females ZW. In mammals, males are XY and females XX. Like the mammalian XY pair, the avian ZW pair is believed to have evolved from autosomes, with most change occurring in the chromosomes found in only one sex—the W and Y chromosomes1, 2, 3, 4, 5. By contrast, the sex chromosomes found in both sexes—the Z and X chromosomes—are assumed to have diverged little from their autosomal progenitors2. Here we report findings that challenge this assumption for both the chicken Z chromosome and the human X chromosome. The chicken Z chromosome, which we sequenced essentially to completion, is less gene-dense than chicken autosomes but contains a massive tandem array containing hundreds of duplicated genes expressed in testes. A comprehensive comparison of the chicken Z chromosome with the finished sequence of the human X chromosome demonstrates that each evolved independently from different portions of the ancestral genome. Despite this independence, the chicken Z and human X chromosomes share features that distinguish them from autosomes: the acquisition and amplification of testis-expressed genes, and a low gene density resulting from an expansion of intergenic regions. These features were not present on the autosomes from which the Z and X chromosomes originated but were instead acquired during the evolution of Z and X as sex chromosomes. We conclude that the avian Z and mammalian X chromosomes followed convergent evolutionary trajectories, despite their evolving with opposite (female versus male) systems of heterogamety. More broadly, in birds and mammals, sex chromosome evolution involved not only gene loss in sex-specific chromosomes, but also marked expansion and gene acquisition in sex chromosomes common to males and females.National Science Foundation (U.S.)Howard Hughes Medical Institut

Chimpanzee and Human Y Chromosomes Are Remarkably Divergent in Structure and Gene Content

LetterThe human Y chromosome began to evolve from an autosome hundreds of millions of years ago, acquiring a sex-determining function and undergoing a series of inversions that suppressed crossing over with the X chromosome[1, 2]. Little is known about the recent evolution of the Y chromosome because only the human Y chromosome has been fully sequenced. Prevailing theories hold that Y chromosomes evolve by gene loss, the pace of which slows over time, eventually leading to a paucity of genes, and stasis [3, 4]. These theories have been buttressed by partial sequence data from newly emergent plant and animal Y chromosomes [5, 6, 7, 8], but they have not been tested in older, highly evolved Y chromosomes such as that of humans. Here we finished sequencing of the male-specific region of the Y chromosome (MSY) in our closest living relative, the chimpanzee, achieving levels of accuracy and completion previously reached for the human MSY. By comparing the MSYs of the two species we show that they differ radically in sequence structure and gene content, indicating rapid evolution during the past 6 million years. The chimpanzee MSY contains twice as many massive palindromes as the human MSY, yet it has lost large fractions of the MSY protein-coding genes and gene families present in the last common ancestor. We suggest that the extraordinary divergence of the chimpanzee and human MSYs was driven by four synergistic factors: the prominent role of the MSY in sperm production, ‘genetic hitchhiking’ effects in the absence of meiotic crossing over, frequent ectopic recombination within the MSY, and species differences in mating behaviour. Although genetic decay may be the principal dynamic in the evolution of newly emergent Y chromosomes, wholesale renovation is the paramount theme in the continuing evolution of chimpanzee, human and perhaps other older MSYs.National Institutes of Health (U.S.)Howard Hughes Medical Institut

Mammalian Y chromosomes retain widely expressed dosage-sensitive regulators

The human X and Y chromosomes evolved from an ordinary pair of autosomes, but millions of years ago genetic decay ravaged the Y chromosome, and only three per cent of its ancestral genes survived. We reconstructed the evolution of the Y chromosome across eight mammals to identify biases in gene content and the selective pressures that preserved the surviving ancestral genes. Our findings indicate that survival was nonrandom, and in two cases, convergent across placental and marsupial mammals. We conclude that the gene content of the Y chromosome became specialized through selection to maintain the ancestral dosage of homologous X-Y gene pairs that function as broadly expressed regulators of transcription, translation and protein stability. We propose that beyond its roles in testis determination and spermatogenesis, the Y chromosome is essential for male viability, and has unappreciated roles in Turner (tm) s syndrome and in phenotypic differences between the sexes in health and disease

Strict evolutionary conservation followed rapid gene loss on human and rhesus Y chromosomes

The human X and Y chromosomes evolved from an ordinary pair of autosomes during the past 200–300 million years[superscript 1, 2, 3]. The human MSY (male-specific region of Y chromosome) retains only three percent of the ancestral autosomes’ genes owing to genetic decay[superscript 4, 5]. This evolutionary decay was driven by a series of five ‘stratification’ events. Each event suppressed X–Y crossing over within a chromosome segment or ‘stratum’, incorporated that segment into the MSY and subjected its genes to the erosive forces that attend the absence of crossing over[superscript 2, 6]. The last of these events occurred 30 million years ago, 5 million years before the human and Old World monkey lineages diverged. Although speculation abounds regarding ongoing decay and looming extinction of the human Y chromosome[superscript 7, 8, 9, 10], remarkably little is known about how many MSY genes were lost in the human lineage in the 25 million years that have followed its separation from the Old World monkey lineage. To investigate this question, we sequenced the MSY of the rhesus macaque, an Old World monkey, and compared it to the human MSY. We discovered that during the last 25 million years MSY gene loss in the human lineage was limited to the youngest stratum (stratum 5), which comprises three percent of the human MSY. In the older strata, which collectively comprise the bulk of the human MSY, gene loss evidently ceased more than 25 million years ago. Likewise, the rhesus MSY has not lost any older genes (from strata 1–4) during the past 25 million years, despite its major structural differences to the human MSY. The rhesus MSY is simpler, with few amplified gene families or palindromes that might enable intrachromosomal recombination and repair. We present an empirical reconstruction of human MSY evolution in which each stratum transitioned from rapid, exponential loss of ancestral genes to strict conservation through purifying selection

Avian W and mammalian Y chromosomes convergently retained dosage-sensitive regulators

After birds diverged from mammals, different ancestral autosomes evolved into sex chromosomes in each lineage. In birds, females are ZW and males are ZZ, but in mammals females are XX and males are XY. We sequenced the chicken W chromosome, compared its gene content with our reconstruction of the ancestral autosomes, and followed the evolutionary trajectory of ancestral W-linked genes across birds. Avian W chromosomes evolved in parallel with mammalian Y chromosomes, preserving ancestral genes through selection to maintain the dosage of broadly expressed regulators of key cellular processes. We propose that, like the human Y chromosome, the chicken W chromosome is essential for embryonic viability of the heterogametic sex. Unlike other sequenced sex chromosomes, the chicken W chromosome did not acquire and amplify genes specifically expressed in reproductive tissues. We speculate that the pressures that drive the acquisition of reproduction-related genes on sex chromosomes may be specific to the male germ line