Sex determination system turnovers play important roles in the willows speciation

Almost all species in the genus Salix (willow) are dioecious, but some have male and some female heterogamety, and the chromosomal location of the sex-linked regions (termed SDSs) differs between different species. We first analyzed the SDSs of two species, Salix cardiophylla and S. interior, whose positions in the Salix phylogeny make them important species for understanding a sex chromosome turnover that has been detected in their relatives, and that changed the system from male to female heterogamety. We show that both species have male heterogamety, with XY-linked regions on chromosome 15 (termed a 15XY system). The sex-linked regions occupy 21.3% and 22.8% of the entire reference chromosome, respectively. By constructing phylogenetic trees of species with known SDSs, we determined the phylogenetic positions of all the species. Reconstruction of SDSs revealed that 15XY system is likely the ancestral of willows. Finally, we tested for both current and ancestral gene flow between different species with the same or different sex-determining systems, as the sex chromosomes can play important roles in reproductive isolation between species. We inferred lower gene flow between species with XY on chromosome 7 (7XY) and ZW on chromosome 15 (15ZW) systems, compared with gene flow either between species with XY on chromosome 15 (15XY) and 15ZW systems or between species with 7XY and 15XY systems. We argue that, although sex chromosomes turnovers in willows may not create complete reproductive barriers, gene flow may be reduced between species with different SDSs. ### Competing Interest Statement The authors have declared no competing interest.