Sex Chromosome Evolution in Haploid Dioecy

In diploid dioecious diploid organisms, where sex determination is governed by dimorphic sex chromosomes, sex chromosomes are expected to differ in genetic variability, size, gene content and gene expression due to asymmetric heterozygosity and suppression of recombination. By contrast, in haploid dioecy, sex chromosomes are equally heterozygous and show suppressed recombination in a similar extent. Therefore, if sex-specific evolutionary forces are negligible, sex chromosomes under haploid dioecy are expected to be influenced by similar evolutionary forces and should follow similar evolutionary trajectories. Nevertheless, experimental evidence contradicts this hypothesis in several aspects and suggests that U and V chromosomes differ in size, gene content and level of degeneration. Therefore, we aim at understanding in what extent and why evolutionary trajectories of sex chromosomes under haploid dieocy deviate from theoretical predictions. We investigate this question using the dioicous Marchantia polymorpha and its monoicous sister species as a model to gain detailed insights into the evolution of sex chromosomes under haploid dioecy. In particular, I will present results of a genetic cross to identify sex chromosome-linked genes in M. polymorpha. Then I will also highlight our comparative genomics effort to understand how sex chromosomes evolved from a putatively monoicous or dioecious ancestor.