Wen-Juan Ma

Sex chromosome evolution is traditionally viewed as a unidirectional process of recombination suppression followed by progressive Y degeneration, yet many vertebrates deviate from this canonical trajectory. Frogs represent a striking exception: they retain homomorphic sex chromosomes, exhibit extreme heterochiasmy, and undergo X-Y recombination in sex-reversed females, providing a powerful system to explore alternative evolutionary pathways. Here, we investigate a Swiss Alpine population of the European common frog ( Rana temporaria ) with homomorphic XY chromosomes and three coexisting male genotypes, using whole genome sequencing of pooled samples carrying distinct Y haplotypes, whole genome sequencing of doubled haploid YY individuals, and transcriptomic (RNA-seq) data. We uncover extreme heterogeneity in X–Y divergence: two fully differentiated Y haplotypes span nearly 90% of the sex chromosome (~618–625 Mb), forming the largest non-recombining region (large NRR) described in a vertebrate to date, whereas a semi-differentiated Y harbors only a NRR of 4.64 Mb (small NRR), and XX males display no detectable X-Y divergence. Despite extensive recombination suppression, Y degeneration is minimal even within the large NRR: we detect elevated transposable element insertions but no gene loss, Y-linked gene copy decay, faster-X effects, or enrichment of sex-biased genes. All Y haplotypes share a small NRR encompassing the candidate master sex-determining gene Dmrt1 , defining a common sex-determining locus. Our data support a dynamic model in which extreme heterochiasmy and recurrent X–Y recombination via sex reversal repeatedly reset X-Y divergence, generating and potentially maintaining multiple Y haplotypes with distinct evolutionary histories. The long-term coexistence of these haplotypes across the species range is compatible with a dynamic equilibrium between drift-driven loss and mutation- and recombination-driven renewal of Y chromosome variation. Together, our findings reveal a non-canonical, reversible pathway of sex chromosome evolution shaped by sex-specific recombination patterns and sex reversal, challenging the universality of classical models and highlighting the value of non-model vertebrates for understanding sex chromosome diversity.

Sex chromosome evolution is traditionally viewed as a unidirectional process of recombination suppression followed by progressive Y degeneration, yet many vertebrates deviate from this canonical trajectory. Frogs represent a striking exception: they retain homomorphic sex chromosomes, exhibit extreme heterochiasmy, and undergo X-Y recombination in sex-reversed females, providing a powerful system to explore alternative evolutionary pathways. Here, we investigate a Swiss Alpine population of the European common frog ( Rana temporaria ) with homomorphic XY chromosomes and three coexisting male genotypes, using whole genome sequencing of pooled samples carrying distinct Y haplotypes, whole genome sequencing of doubled haploid YY individuals, and transcriptomic (RNA-seq) data. We uncover extreme heterogeneity in X–Y divergence: two fully differentiated Y haplotypes span nearly 90% of the sex chromosome (∼618–625 Mb), forming the largest non-recombining region (large NRR) described in a vertebrate to date, whereas a semi-differentiated Y harbors only a NRR of 4.64 Mb (small NRR), and XX males display no detectable X-Y divergence. Despite extensive recombination suppression, Y degeneration is minimal even within the large NRR: we detect elevated transposable element insertions but no gene loss, Y-linked gene copy decay, faster-X effects, or enrichment of sex-biased genes. All Y haplotypes share a small NRR encompassing the candidate master sex-determining gene Dmrt1 , defining a common sex-determining locus. Our data support a dynamic model in which extreme heterochiasmy and recurrent X–Y recombination via sex reversal repeatedly reset X-Y divergence, generating and potentially maintaining multiple Y haplotypes with distinct evolutionary histories. The long-term coexistence of these haplotypes across the species range is compatible with a dynamic equilibrium between drift-driven loss and mutation- and recombination-driven renewal of Y chromosome variation. Together, our findings reveal a non-canonical, reversible pathway of sex chromosome evolution shaped by sex-specific recombination patterns and sex reversal, challenging the universality of classical models and highlighting the value of non-model vertebrates for understanding sex chromosome diversity.

A few species have evolved multiple sex chromosome systems with more than two Xs or Ys due to sex chromosome-autosome translocations. Among vertebrates, frogs (Anura) have the highest known number of such neo-sex chromosome systems, making them interesting for studying how such systems evolve. In this work, we investigated two Leptodactylus species, L. pentadactylus (LPE) and L. paraensis (LPA), with large ring multivalents in male meiosis, using genomic and cytogenetic investigation of repetitive DNA sequences, including satellite DNAs (satDNAs), and transposable elements (TEs). SatDNA mapping identify individual chromosomes in the LPE ring, and morphologies suggest that all chromosomes are shared with the LPA ring although a common ring origin is not firmly supported. In situ mapping suggests recent satDNA accumulation in subtelomeric regions since the split from the outgroups, likely unrelated to the translocations that created sex-linkage, which probably involved breaks in the pericentromeric regions.

Sex determination, the developmental process that directs embryos toward male or female fates, is controlled by master sex-determining genes whose origins and evolutionary dynamics remain poorly understood outside of a few model systems. In contrast to the highly differentiated sex chromosomes of mammals, birds, and Drosophila, most anurans (frogs and toads) maintain homomorphic sex chromosomes that exhibit a rapid turnover, even among closely related species. Master sex-determining genes evolve via gene duplication or via allelic diversification, and sex chromosome turnover is driven by gene translocation or novel mutations in the existing genes involved in the sexual developmental pathway. To uncover the mechanisms underlying the emergence of master sex-determining genes and sex chromosome turnover, we analyzed 53 published anuran genomes and one caecilian genome (>200 Mya divergence) and available transcriptomes. We asked how often master sex-determining genes arise by gene duplication, whether and how often gene translocation associates with sex chromosome turnover, and if master sex-determining genes evolve under positive selection. We find that chromosome-level synteny is remarkably conserved, with only a few fusions or fissions and no evidence for translocation of four candidate master sex-determining genes (Dmrt1, Foxl2, Bod1l, and Sox3). Only Dmrt1 duplicated in 3 out of 50 species (excluding tetraploid Xenopus), and it showed strong testis-biased expression in all 8 species with available gonadal expression data. While Dmrt1 has evolved under purifying selection, Dmrt1 duplicates exhibit elevated nonsynonymous substitution rates and tendency towards positive selection. Lineage-specific amino acid changes were observed in the conserved DM domain of Dmrt1. These results demonstrate that, in anurans, master sex-determining genes rarely arise via gene duplication, and more likely evolve via allelic diversification. Sex chromosome turnover is not associated with gene translocation and is more likely driven by mutations on genes involved in sexual developmental pathways. All candidate sex-determining genes were under strong purifying selection, with the exception of duplications which are linked to positive selection. Our results suggest future research on anuran sex determination and sex chromosome evolution should focus on identifying allelic diversification and novel mutations on genes involved in sexual developmental pathways.

Sex determination, the developmental process that directs embryos toward male or female fates, is controlled by master sex-determining genes whose origins and evolutionary dynamics remain poorly understood outside of a few model systems. In contrast to the highly differentiated sex chromosomes of mammals, birds, and Drosophila , most anurans (frogs and toads) maintain homomorphic sex chromosomes that exhibit a rapid turnover, even among closely related species. To uncover the mechanisms underlying the emergence of new master sex-determining genes and sex chromosome turnover, we analysed 53 published anurans and one caecilian genome (>200 Ma divergence) and available transcriptomes. We asked how often new master sex-determining genes arise by gene duplication, whether and how often gene translocation associates with sex chromosome turnover, and if new master sex-determining genes evolve under positive selection. We find that chromosome-level synteny is remarkably conserved, with only a few fusions or fissions and no evidence for translocation of four candidate master sex-determining genes ( Dmrt1, Foxl2, Bod1l, Sox3 ). Only Dmrt1 duplicated in 3 out of 50 species (excluding tetraploid Xenopus ), and it showed strong testis-biased expression in all 8 species with available gonadal expression data. While Dmrt1 has evolved under purifying selection, Dmrt1 duplicates exhibit elevated nonsynonymous substitution rates and a tendency towards positive selection. Lineage-specific amino acid changes were observed in the conserved DM domain of Dmrt1 . These results demonstrate that, in anurans, master sex-determining genes arise rarely via gene duplication, and more likely evolve via allelic diversification. Sex chromosome turnover is not associated with gene translocation, and is more likely driven by mutations on genes involved in sexual developmental pathway. All candidate sex-determining genes were under strong purifying selection, with the exception of duplications which are linked to positive selection. Our results suggest future research on anuran sex determination and sex chromosome evolution should focus on identifying allelic diversification and novel mutations on genes involved in sexual developmental pathway.

Basidiomycete fungi typically have two mating-type loci controlling mating compatibility, HD and PR, residing on different chromosomes. Loss-of-function in mating compatibility has been reported at the PR genes in a few heterothallic basidiomycetes, but not for the HD genes. In Microbotryum anther-smut fungi, there have been repeated linkage events between the HD and PR loci through chromosome fusions, leading to non-recombining regions. Here, we found that two sister Microbotryum species parasitizing Dianthus plants, M. superbum and M. shykoffianum, as well as the distantly related M. scorzonerae, have their HD and PR loci on different chromosomes, but with the PR chromosome fused with a part of the ancestral HD chromosome. In addition, recombination suppression has extended stepwise, generating evolutionary strata. In all three species, the HD genes lost their function in mating compatibility, natural diploid strains being often homozygous at the HD locus. Strains could be homozygous for a disrupted HD2 gene, that was hardly expressed during mating. Mating tests confirmed that a single genetic factor controlled mating compatibility and that haploid strains with identical HD alleles could mate and produce hyphae. This study shows that a unifactorial mating-type determinism can evolve, repeatedly, from a bifactorial system, by different mechanisms.

A few species have evolved multiple sex chromosome systems with more than two Xs or Ys. These involve sex chromosome-autosome translocations (sometimes called fusions as very small heterochromatic arms may be deleted), creating neo-sex chromosome systems. Among vertebrates, frogs (Anura) have the highest known number of such translocation systems. This study within the genus Leptodactylus, investigated the two species L. pentadactylus (LPE) and L. paraensis (LPA), in which large ring multivalents are seen in male meiosis, indicating translocations involving the sex chromosomes. Four other species studied do not have such rings, but they share characteristics making rearrangements less likely to be eliminated. To start understanding the formation of multivalents, we used genomic and cytogenetic methods to investigate repetitive DNA sequences, including satellite DNAs, rDNAs, and telomeric sequences, and conducted comparative genomic hybridization (CGH). The LPE genome includes a large number of satDNA families, and in situ mapping of several satDNAs individually identified eight of the ten chromosomes in its multivalent. In LPA, morphological similarities indicate that several chromosomes are shared by the multivalents of both species, and a candidate ancestral sex chromosome pair could be identified. In situ mapping in LPE suggests recent satDNA accumulation in the subtelomeric regions, which differ from those in the outgroup species, contrary to the expectation that the translocations create sex-linkage in the pericentromeric regions.

The Dmrt gene family is characterized by a conserved DM domain, and is crucial to sex determination and sexual differentiation. Dmrt1 is pivotal in testis formation and function by interacting with genes crucial for Sertoli cell differentiation, such as Sox9. Across 128 vertebrate species, Dmrt1 exhibits sexually dimorphic expression, prior to gonadal sex differentiation, implicating its role in master regulation of sex determination. Dmrt1 emerges as a master sex-determining gene in one fish, frog, chicken, and turtle, with candidacy in 12 other vertebrate species. Recent studies suggest epigenetic regulation of Dmrt1 in its promoter methylation, and transposable element insertion introducing epigenetic modification to cis-regulatory elements of Dmrt1, alongside noncoding RNA involvement, in sex-determining mechanisms. Additionally, alternative splicing of Dmrt1 was found in all major vertebrate groups except amphibians. Dmrt1 has evolved many lineage-specific isoforms (ranging from 2 to 10), and various isoforms showed sex, tissue, or development-specific expression, which is in contrast to the sex-specific splicing of its homolog Dsx across insects. Future research should focus on understanding the molecular basis of environmental sex determination from a broader taxon, and that of epigenetic regulation. It is also essential to understand why and how alternative splicing variants of Dmrt1 evolve in vertebrates, the specific roles each isoform plays, as well as the significant differences in the molecular mechanisms and functions of alternative splicing between Dmrt1 in vertebrates and Dsx in insects. Understanding the differences could provide deeper insights into the evolution of sex-determining mechanisms between vertebrates and insects.

Sex chromosomes have independently evolved in species with separate sexes in most lineages across the tree of life. However, the well-accepted canonical model of sex chromosome evolution is not universally supported. There is no single trajectory for sex chromosome formation and evolution across the tree of life, suggesting the underlying mechanisms and evolutionary forces are diverse and lineage specific. We review the diversity of sex chromosome systems, describe the canonical model of sex chromosome evolution, and summarize studies challenging various aspects of this model. They include evidence that many lineages experience frequent sex chromosome turnovers or maintain homomorphic sex chromosomes over long periods of time, suggesting sex chromosome degeneration is not inevitable. Sometimes the sex-limited Y/W chromosomes expand before they contract in size. Both transposable elements and gene gains could contribute to this size expansion, which further challenges gene loss being the hallmark of sex chromosome degeneration. Finally, empirical support for the role of sexually antagonistic selection as a driver of recombination suppression on sex chromosomes remains elusive. We summarize models that result in loss of recombination without invoking sexually antagonistic selection, which have not been empirically verified yet, and suggest future avenues for sex chromosome research.