Sex chromosomes in willows : evolutionary studies of the ZW sex chromosome system in Salix viminalis

Sammanfattning: Sexual reproduction is found in most eukaryotes and has in the majority of animal species led to the evolution of separate sexes. In contrast, only 5-6% of all angiospersms are dioecious with female and male flowers on separate individuals. Interestingly, dioecy has evolved hundreds of times independently and at different timepoints in angiosperms. The development of separate sexes requires a sex determination mechanism, which often is located on sex chromosomes. The independent evolution of numerous sex determination and sex chromosome systems in angiosperms allows for studies of processes involved in different stages of their evolution. The ratio between male and female individuals in a dioecious population is expected to be equal due to frequency dependent selection. Distorted sex ratios are however common in many plant and animal species. The overall aim of my thesis was to investigate the sex chromosome system in the dioecious, perennial willow species Salix viminalis that both in natural populations as well as in lab populations often displays female biased sex ratios. Although dioecy evolved from hermaphroditic ancestors before the split between Salix and its sister genus Populus, we found that the two lineages have different sex chromosomes (Populus: Chr. 19, Salix: Chr. 15). As we found no evidence for translocations between these chromosomes, it is most likely that two different sex determination mechanims are present in the two lineages, meaning that sex chromosome turnover has occurred recently. We furthermore determined that S. viminalis is female heterogametic (females Z/W, males Z/Z) and has a single sex determination locus on chromosome 15. The W homolog of the sex determination region contains hemizygous, female specific sequences and the SNP density in this region is increased in females relative to males, witnessing of lost recombination between the Z/W homologs. We did not find a Fast-Z effect or major degeneration of the W chromosome, suggesting a recent evolution. In fact this sex chromosome system is among the youngest observed so far. Based on our data, the insertion of repetitive sequence and sex specific gene expression appear to be among the first processes to happen in sex chromosome evolution. We also determined that female biased sex ratios in S. viminalis are likely caused by an allelic incompatibility between Z homologs which results in the lack of one expected male genotype, reducing the male frequency in the population. My studies thus extended our knowledge on processes involved in sex chromosome evolution and evolution of biased sex ratios in S. viminalis. However, given how common these phenomena are, results from my research can be applied to most organisms with genetic sex determination.