Convergent evolution of linked mating-type loci in basidiomycetes: an ancient fusion event that has stood the test of time
Sexual development is a key evolutionary innovation of eukaryotes. In many species, mating involves interaction between compatible mating partners that can undergo cell and nuclear fusion and subsequent steps of development including meiosis. Mating compatibility in fungi is governed by mating type determinants, which are localized at mating type (MAT) loci. In basidiomycetes, the ancestral state is hypothesized to be tetrapolar (bifactorial), with two genetically unlinked MAT loci containing homeodomain transcription factor genes (HD locus) and pheromone and pheromone receptor genes (P/R locus), respectively. Alleles at both loci must differ between mating partners for completion of sexual development. However, there are also basidiomycete species with bipolar (unifactorial) mating systems, which can arise through genomic linkage of the HD and P/R loci. In the order Tremellales, which is comprised of mostly yeast-like species, bipolarity is found only in the human pathogenic Cryptococcus species. Here, we describe the analysis of MAT loci from the Trichosporonales, a sister order to the Tremellales. We analyzed genome sequences from 29 strains that belong to 24 species, including two new genome sequences generated in this study. Interestingly, in all of the species analyzed, the MAT loci are fused and a single HD gene is present in each mating type. This is similar to the organization in the pathogenic Cryptococci, which also have linked MAT loci and carry only one HD gene per MAT locus instead of the usual two HD genes found in the vast majority of basidiomycetes. However, the HD and P/R allele combinations in the Trichosporonales are different from those in the pathogenic Cryptococcus species. The differences in allele combinations compared to the bipolar Cryptococci as well as the existence of tetrapolar Tremellales sister species suggest that fusion of the HD and P/R loci and differential loss of one of the two HD genes per MAT allele occurred independently in the Trichosporonales and pathogenic Cryptococci. This finding supports the hypothesis of convergent evolution at the molecular level towards fused mating-type regions in fungi, similar to previous findings in other fungal groups. Unlike the fused MAT loci in several other basidiomycete lineages though, the gene content and gene order within the fused MAT loci are highly conserved in the Trichosporonales, and there is no apparent suppression of recombination extending from the MAT loci to adjacent chromosomal regions, suggesting different mechanisms for the evolution of physically linked MAT loci in these groups.