Assessment of phylogenetic approaches to study the timing of recombination cessation on sex chromosomes

Evolution of Sex-linked Genes and the Role of Pericentromeric Regions in Sex Chromosomes: Insights from Diploid Willows

The evolution of sex chromosomes can involve recombination suppression sometimes involving structural changes, such as inversions, allowing subsequent rearrangements, including inversions and gene transpositions. In the two major genus Salix clades, Salix and Vetrix, almost all species are dioecious, and sex-linked regions have evolved on chromosome 7 and 15, with either male or female heterogamety. We used chromosome conformation capture (Hi-C) and PacBio HiFi (high-fidelity) reads to assemble chromosome-level, gap-free X and Y chromosomes from both clades, S. triandra (15XY system), a basal species in the Vetrix clade, and the Salix clade species S. mesnyi (7XY system). Combining these with other available genome assemblies, we found inversions within the sex-linked regions, which are likely to be pericentromeric and probably recombined rarely in the ancestral species, before sex-linkage evolved. The Y-linked regions in all 15XY and 7XY species include partial duplicates containing exon 1 of an ARR17-like gene similar to male-determining factors in other Salicaceae species. We also found duplicates of a Y-specific gene, which we named MSF. The derived Salix clade 7XY chromosome systems appear to have evolved when these two genes transposed from the 15Y to the 7Y. Additionally, the 7Y chromosomes in S. dunnii and S. chaenomeloides probably evolved from the ancestral 7X of the Salix clade, involving a similar transposition, and loss of the ancestral 7Y. We suggest that pericentromeric regions that recombine infrequently may facilitate the evolution of sex linkage.

The rate of W chromosome degeneration across multiple avian neo-sex chromosomes

When sex chromosomes evolve recombination suppression, the sex-limited chromosome (Y/W) commonly degenerate by losing functional genes. The rate of Y/W degeneration is believed to slow down over time as the most essential genes are maintained by purifying selection, but supporting data are scarce especially for ZW systems. Here, we study W degeneration in Sylvioidea songbirds where multiple autosomal translocations to the sex chromosomes, and multiple recombination suppression events causing separate evolutionary strata, have occurred during the last ~ 28.1-4.5 million years (Myr). We show that the translocated regions have maintained 68.3-97.7% of their original gene content, compared to only 4.2% on the much older ancestral W chromosome. By mapping W gene losses onto a dated phylogeny, we estimate an average gene loss rate of 1.0% per Myr, with only moderate variation between four independent lineages. Consistent with previous studies, evolutionarily constrained and haploinsufficient genes were preferentially maintained on W. However, the gene loss rate did not show any consistent association with strata age or with the number of W genes at strata formation. Our study provides a unique account on the pace of W gene loss and reinforces the significance of purifying selection in maintaining essential genes on sex chromosomes.

Gap-free X and Y chromosome assemblies of Salix arbutifolia reveal an evolutionary change from male to female heterogamety in willows, without a change in the position of the sex-determining locus

In the Vetrix clade of Salix, a genus of woody flowering plants, sex determination involves chromosome 15, but an XY system has changed to a ZW system. We studied the detailed genetic changes involved. We used genome sequencing, with chromosome conformation capture (Hi-C) and PacBio HiFi reads to assemble chromosome level gap-free X and Y of Salix arbutifolia, and distinguished the haplotypes in the 15X- and 15Y-linked regions, to study the evolutionary history of the sex-linked regions (SLRs). Our sequencing revealed heteromorphism of the X and Y haplotypes of the SLR, with the X-linked region being considerably larger than the corresponding Y region, mainly due to accumulated repetitive sequences and gene duplications. The phylogenies of single-copy orthogroups within the SLRs indicate that S. arbutifolia and Salix purpurea share an ancestral SLR within a repeat-rich region near the chromosome 15 centromere. During the change in heterogamety, the X-linked region changed to a W-linked one, while the Z was derived from the Y.

Finding divergent sequences of homomorphic sex chromosomes via diploidized nanopore-based assembly from a single male

Although homomorphic sex chromosomes can have non-recombining regions with elevated sequence divergence between its complements, such divergence signals can be difficult to detect bioinformatically. If found in genomes of e.g. insect pests, these sequences could be targeted by the engineered genetic sexing and control systems. Here, we report an approach that can leverage long-read nanopore sequencing of a single XY male to identify divergent regions of homomorphic sex chromosomes. Long-read data are used for de novo genome assembly that is diploidized in a way that maximizes sex-specific differences between its haploid complements. We show that the correct assembly phasing is supported by the mapping of nanopore reads from the male's haploid Y-bearing sperm cells. The approach revealed a highly divergent region (HDR) near the centromere of the homomorphic sex chromosome of Aedes aegypti, the most important arboviral vector, for which there is a great interest in creating new genetic control tools. HDR is located ~5Mb downstream of the known male-determining locus on chromosome 1 and is significantly enriched for ovary-biased genes. While recombination in HDR ceased relatively recently (~1.4 MYA), HDR gametologs have divergent exons and introns of protein coding genes, and most lncRNA genes became X-specific. Megabases of previously invisible sex-linked sequences provide new putative targets for engineering the genetic systems to control this deadly mosquito. Broadly, our approach expands the toolbox for studying cryptic structure of sex chromosomes.