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.

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