Chromosome‐scale assembly of Salix integra reveals a conserved sex‐determination mechanism in 15ZW clade I willows

Abstract Sex chromosomes frequently undergo turnover through the recruitment of new sex‐determining genes or translocation of ancestral genes. This phenomenon is particularly evident in Salix (Salicaceae). In Populus , the sister genus of Salix , a partial‐ ARR17 ‐based sex‐determination mechanism has been reported, and is also found in several Salix species. In Vetrix 15ZW clade I of Salix , species share a female heterogamety system on chromosome 15. A partial‐ PI ( PISTILLATA )‐based mechanism has been proposed for several species in this clade, except for Salix purpurea , which appears to use a two‐gene model involving ARR17 and GATA15 . To further investigate the evolution of sex‐determining factors in this clade, we assembled a high‐quality, haplotype‐resolved genome of Salix integra , a close relative of S. purpurea . Based on resequencing data from males and females, we identified W and Z‐linked regions located in pericentromeric regions on chromosome 15, consistent with those reported in other Salix species. Comparative analyses showed that S. integra possesses a partial‐ PI ‐based mechanism, supporting the hypothesis that PI sequences were recruited in the ancestor of 15ZW clade I, whereas the ancestor of S. purpurea recruited a two‐gene model later. Together, these results advance our understanding of the relationship between sex‐determining plasticity and sex chromosome evolution in plants.