Sex chromosomes often show extensive areas of suppressed recombination and cytological differentiation, a well-documented phenomenon in animals and plants. Lack of recombination is expected to limit the efficacy of natural selection, leading to degeneration in gene content. Similarly, fungal mating-type chromosomes, which are responsible for controlling compatibility during mating, can display patterns of suppressed recombination encompassing up to 90% of the chromosome length. The mechanisms responsible for lack of recombination and consequent degeneration remain unclear.Here, I propose to use comparative genomics to investigate the patterns and underlying mechanisms involved in recombination suppression and genomic degeneration in Microbotryum, a model fungal system with a range of dimorphic mating-type chromosomes. I will complement the currently available high-quality genome assemblies for twenty species in the genus with three outgroups, which will allow to polarize all genomic data. I will then use the genomic dataset to: 1) test hypotheses on the origin of recombination suppression in fungal mating-type chromosomes; 2) study the evolution of non-recombining regions in fungal mating-type chromosomes, e.g., their size and age, and the existence of evolutionary strata; and 3) study the patterns and mechanisms of genomic degeneration in non-recombining regions, namely non-synonymous substitution accumulation, transposable elements, disrupted genes, and non-optimal codon usage.Results will not only shed light on the origins and consequences of suppressed recombination and genome degradation in fungal mating-type chromosomes, but will also yield unprecedented insights into the dynamics of sexual reproduction in eukaryotes and contribute for a unified view of the evolution of dimorphic chromosomes.