Sex ratio variation and the evolution of sex chromosomes in willows

Sexual reproduction is common and plays a fundamental role in evolutionary processes. Many organisms have separate sexes that are determined by sex chromosomes. However, processes driving the evolution of sex chromosomes, in particular recombination arrest, remain under debate and include sexually antagonistic selection and sex ratio selection. Based on theoretical studies, balanced sex ratios at birth or seed production are expected, while natural populations often have biased sex ratios among reproductive individuals. Uncovering the ecological and genetic mechanisms generating sex ratio bias is key to understanding the evolutionary significance of sex ratio selection. We investigated sex determination and sex ratio bias in willows (Salix), a dioecious genus with well-documented sex ratio bias and turnover of sex determination systems. We generated a haplotype-resolved reference genome of the arctic-alpine dwarf willow S. herbacea and identified ZW sex-determination on chromosome 15 using whole-genome resequencing. Our results suggest that the sex-linked region with suppressed recombination recently expanded nearly threefold due to a large inversion on the W-chromosome. We further identified a new candidate sex-determining gene (PISTILLATA) in Salix, that may be associated with sex chromosome turnover. In a natural population and controlled crosses of S. herbacea, we detected strong and consistent female-biased population sex ratios, despite highly variable sex ratios among families, ranging from male-biased to female-only. Sex ratio bias among reproductive individuals increased slightly with elevation, but was not associated with intrinsic differences between sexes in germination, mortality or flowering. This suggests that sex ratio bias arises mainly through genetic mechanisms. In a three-generation pedigree, sex ratio bias was maternally inherited. We further investigated allele transmission in controlled crosses using Pool-Seq and found a dominant pattern of maternal autosomal alleles with biased transmission in the rare sex only, consistent with W-drive and autosomal restorers as a mechanism generating sex ratio bias. In one family, however, we detected evidence for flowering suppression underlying a male-biased sex ratio, as well as for structural variation on the sex chromosomes. Together, this thesis provides insights into mechanisms generating sex ratio bias and how these might contribute to the highly dynamic evolution of sex chromosomes.