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Sheng Sun, Marco A. Coelho, Joseph Heitman, Minou Nowrousian

• 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 the mating type \(\textit {(MAT)}\) loci. In basidiomycetes, the ancestral state is hypothesized to be tetrapolar, with two genetically unlinked \(\it MAT\) loci containing homeodomain transcription factor genes (\(\it {HD}\) locus) and pheromone and pheromone receptor genes (\(\it {P/R}\) locus), respectively. Alleles at both loci must differ between mating partners for completion of sexual development. However, there are also basidiomycetes with bipolar mating systems, which can arise through genomic linkage of the \(\it HD\) and \(\textit {P/R}\) loci. In the order \(\it Tremellales\), bipolarity is found only in the pathogenic \(\it Cryptococcus\) species. Here, we describe the analysis of \(\it MAT\) loci from 24 species of the \(\it Trichosporonales\), a sister order to the \(\it Tremellales\). In all of the species analyzed, the \(\it MAT\) loci are fused and a single \(\it HD\) gene is present in each mating type, similar to the organization in the pathogenic Cryptococci. However, the \(\it HD\) and \(\it P/R\) allele combinations in the \(\it Trichosporonales\) are different from those in the pathogenic Cryptococci. This and the existence of tetrapolar species in the \(\it Tremellales\) suggest that fusion of the \(\it HD\) and \(\textit {P/R}\) loci occurred independently in the \(\it Trichosporonales\) and pathogenic Cryptococci, supporting the hypothesis of convergent evolution towards fused \(\it MAT\) regions, similar to previous findings in other fungal groups. Unlike the fused \(\it MAT\) loci in several other basidiomycete lineages though, the gene content and gene order within the fused \(\it MAT\) loci are highly conserved in the \(\it Trichosporonales\), and there is no apparent suppression of recombination extending from the \(\it MAT\) loci to adjacent chromosomal regions, suggesting different mechanisms for the evolution of physically linked \(\it MAT\) loci in these groups.