The male-heterogametic sex determination system on chromosome 15 of Salix triandra and Salix arbutifolia reveals ancestral male heterogamety and subsequent turnover events in the genus Salix

Abstract

Dioecious Salix evolved more than 45 million years ago, but have homomorphic sex chromosomes, suggesting that turnover event(s) prevented major differentiation. Sex chromosome turnover events have been inferred in the sister genus, Populus. The genus Salix includes two main clades, Salix and Vetrix, with several previously studied Vetrix clade species having female-heterogametic (ZW) sex-determining systems (SDSs) on chromosome 15, while two Salix clade species have male-heterogametic (XY) SDSs on chromosome 7. We here studied two basal taxa of the Vetrix clade, S. arbutifolia and S. triandra. Analyses of whole genome resequencing data for genome-wide associations (GWAS) with the sexes and genetic differentiation between the sexes (F ST values) showed that both species have male heterogamety with a sexdetermining locus on chromosome 15, suggesting an early turnover event within the Vetrix clade, perhaps promoted by sexually antagonistic or (and) sex-ratio selection. Changepoint analysis based on F ST values identi ed small sex-linked regions of ~3.33 Mb and ~2.80 Mb in S. arbutifolia and S. triandra, respectively. The molecular mechanism of sex-determination remains unknown. Ancestral state reconstruction of SDS and stamen numbers suggests that at least two turnover events occurred in Salix. Willows with XY SDS usually have multiple stamens, while ZW species have two stamens.

Gap-free X and Y chromosomes ofSalix arbutifoliareveal an evolutionary change from male to female heterogamety in willows, without a change in the sex-determining region

“…(a) Phylogenetic relationships of species with known sex determination systems in Salix and turnover events in the Vetrix clade, adapted from Wang, et al, (2023). (b) Microsynteny between the Salix dunnii autosome 15 and the Salix arbutifolia sex chromosomes 15X and 15Y, showing the micro-heteromorphism due to a large region present only in the 15X sex-linked region.…”

“…We also downloaded re-sequencing reads from 21 individual S. arbutifolia females and 20 males obtained by (Wang et al, 2023) to apply the chromosome quotient (CQ) method (Hall et al, 2013) to distinguish the 15X and 15Y haplotypes. CQ is the ratio of female to male alignments to a given reference sequence, using the stringent criterion that the entire read must align with zero mismatches.…”

“…A change to a different region within the same chromosome is also possible (Ieda et al, 2018; Kabir et al, 2022). The Salicaceae display all these types (Yang et al, 2021; Li et al, 2022; Wang et al, 2023). It is hypothesized that, in heterogametic transitions, W-linked regions are likely to derive from ancestral X-linked regions (or vice versa), because these carry feminizing genes, while the Z and Y may often carry masculinizing genes (Miura, 2007; Ma et al, 2018; Ogata et al, 2018; Furman et al, 2020).…”

“…Salix includes two main clades: the Salix and Vetrix (Gulyaev et al, 2022). In the Vetrix clade, an XY to ZW transition has been found, with both having physically small sex-linked regions being on chromosome 15 (Wang et al, 2022; Wang et al, 2023). The extant species with female heterogamety, which we abbreviate to 15ZW (e.g., S. purpurea , S. viminalis , and S. polyclona ) and S. arbutifolia (with male heterogamety, or 15XY) share an ancestral 15XY system, but high-quality X and Y reference assemblies of S. arbutifolia are currently lacking, which hinders understanding of these evolutionary changes.…”

Evolutionary origin and establishment of a dioecious diploid‐tetraploid complex

“…The genus Salix includes ~450 species, of which most are dioecious (Fang et al, 1999; He, Wagner, et al, 2021). The genus is divided into two main groups based on sequence divergence, and these have different sex determination systems: the Salix group has XY system on chromosome 7, and the Vetrix group has ZW or XY systems on chromosome 15 (Gulyaev et al, 2022; Wang et al, 2023), though the sex chromosomes are not heteromorphic in either case. The Vetrix clade includes ~85% species of the genus and radiated in the Qinghai‐Tibetan Plateau (QTP) and adjacent regions (He, Wagner, et al, 2021).…”