Format: ORAL

Xiaomeng Mao1, Pr Ingvarsson2, Vinciane Mossion1, Sophie Karrenberg1

1 Department of Ecology and Genetics, Uppsala University 2 Department of Plant Biology, Swedish University of Agricultural Sciences

Sex chromosomes have been hypothesized to be drivers of speciation, however, they are difficult to study due to their complex and dynamic evolutionary trajectories. The species-rich, dioecious genus Salix (willows) has homomorphic sex chromosomes and is an ideal model system for the evolution of sex chromosomes. We de-novo assembled a chromosome-level genome of the dwarf willow S. herbacea using Pacbio HiFi and HiC sequencing. We detected a female heterogametic sex determination system and resolved both Z and W haplotypes on chromosome 15. Two sex-associated regions were located at both ends of a ~5Mb inversion: a 1.8Mb female-specific region (missing from males) and a 0.5Mb region with high sequence divergence between sexes. This suggests stepwise evolution of the sex chromosomes. The sex-associated regions of S. herbaeca show only partial synteny with the closely related S. purpurea indicating that the region expanded and evolved recently. We currently explore the evolutionary mechanisms that gave rise to the sex-associated regions in S. herbacea using whole genome re-sequencing of two S.herbacea populations and populations of eight further Salix species from the Vetrix section. We analyze signatures of selection as well as differentiation between populations and species in ZW homologous regions. With this study, we contribute to the understanding of fast-evolving sex chromosomes with possible consequences for lineage divergence and speciation.