The evolution of separate sexes is of fundamental importance in biology. Sex determining mechanisms are highly variable and labile and sex chromosomes often degenerate; however, the underlying evolutionary mechanisms are unclear. Theoretical studies predict that sex chromosome evolution is driven by sex ratio selection and/or by different forms of genetic conflict, for example, between males and females or between cytoplasm and nucleus. Empirical evidence for these predictions is scarce, in part because the best-studied systems are animals with ancient sex chromosomes. This project investigates a plant family with separate sexes (Salicaeae, willows and poplars) where a high turnover of recently emerged sex chromosomes has been suggested and sex ratio bias is common. The project has three main objectives: (1) identify the mechanisms for sex determination and sex ratio bias, (2) analyze signs of degeneration in sex-associated region(s), and (3) investigate the evolutionary history and dynamics of genomic region(s) associated with sex-determination and sex-ratio bias. We will combine sex ratio analysis in natural populations and transmission analysis of sex-associated polymorphisms in controlled crosses with whole-genome sequencing, comparative population genomic analyses and theoretical modeling. This comprehensive approach will allow us to gain novel insights into sex chromosome evolution and evolutionary processes at large.