Multiple independent evolution of non-recombining mating-type chromosomes in anther-smut fungi

Sex chromosomes often display striking features, with recombination suppression and size dimorphism. The proximal and evolutionary mechanisms driving recombination cessation are however still not completely elucidated. Here we used a unique dataset with chromosome-length assemblies of closely related species of anther-smut fungi. We show that multiple species evolved independently large regions of recombination suppression on their mating-type chromosomes, involving different chromosomal rearrangements for linking the two mating-type loci. At least five independent events of linkage of the mating-type loci occurred, at different times, and with four different types of chromosomal rearrangements. This represents the first known case of recent and frequent transitions among closely related species, which suggests the existence of a very strong and convergent selection for linking mating type loci.