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.

Key words: ARR17, dioecy, PISTILLATA, Salix integra, sex determination