Reviewers ' comments : Reviewer # 2 ( Remarks to the Author )

Branco et al. present an analysis of recombination on the mating-type chromosomes of anthersmut fungi. Mating-type chromosomes are analogous to sex chromosomes, except that the fungi lack the clear hallmarks of sexual dimorphism that typically are associated with sex chromosomes. The authors use gene order, evolutionary divergence and phylogenetic approaches to show that regions of recombination suppression have emerged around the mating type loci independently in multiple lineages. They go on to show that these regions bear the clear hallmarks of nonrecombining regions (gene loss, transposon accumulation, etc) that have been shown on other nascent sex chromosome systems (including Ahmed et al. 2014 Current Biology 24: 1945-1957).