Department of Molecular Genetics and Microbiology, Duke University Medical Center, Durham, North Carolina, United States of America.
Lehrstuhl für Allgemeine und Molekulare Botanik, Ruhr-Universität Bochum, Bochum, Germany.
PLoS Genet. 2019 Sep 6;15(9):e1008365. doi: 10.1371/journal.pgen.1008365. eCollection 2019 Sep.
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 (MAT) loci. In basidiomycetes, the ancestral state is hypothesized to be tetrapolar, 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 basidiomycetes with bipolar mating systems, which can arise through genomic linkage of the HD and P/R loci. In the order Tremellales, bipolarity is found only in the pathogenic Cryptococcus species. Here, we describe the analysis of MAT loci from 24 species of the Trichosporonales, a sister order to the Tremellales. In all of the species analyzed, the MAT loci are fused and a single HD gene is present in each mating type, similar to the organization in the pathogenic Cryptococci. However, the HD and P/R allele combinations in the Trichosporonales are different from those in the pathogenic Cryptococci. This and the existence of tetrapolar species in the Tremellales suggest that fusion of the HD and P/R loci occurred independently in the Trichosporonales and pathogenic Cryptococci, supporting the hypothesis of convergent evolution towards fused MAT regions, 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.
性发育是真核生物的一个关键进化创新。在许多物种中，交配涉及到相容的交配伙伴之间的相互作用，这些伙伴可以经历细胞和核融合以及随后的发育步骤，包括减数分裂。真菌的交配相容性受交配型（MAT）基因座控制。在担子菌中，假设原始状态是四极的，有两个遗传上不相关的 MAT 基因座，分别包含同源域转录因子基因（HD 基因座）和信息素和信息素受体基因（P/R 基因座）。为了完成性发育，两个交配伙伴的两个基因座的等位基因必须不同。然而，也有一些担子菌具有二极交配系统，这可能是由于 HD 和 P/R 基因座的基因组连锁。在 Tremellales 目中，二极性仅在致病性隐球菌物种中发现。在这里，我们描述了对 Trichosporonales 目 24 种物种的 MAT 基因座的分析，该目是 Tremellales 目的姐妹目。在所分析的所有物种中，MAT 基因座融合，每个交配型中只有一个 HD 基因存在，类似于致病性隐球菌的组织。然而， Trichosporonales 中的 HD 和 P/R 等位基因组合与致病性隐球菌不同。这以及 Tremellales 目中存在四极物种表明，HD 和 P/R 基因座的融合在 Trichosporonales 和致病性隐球菌中是独立发生的，支持了向融合 MAT 区域趋同进化的假说，类似于之前在其他真菌群中发现的情况。然而，与其他几个担子菌谱系中的融合 MAT 基因座不同， Trichosporonales 中的融合 MAT 基因座的基因内容和基因顺序高度保守，并且没有明显的重组抑制从 MAT 基因座延伸到相邻的染色体区域，这表明这些群体中物理连锁的 MAT 基因座的进化机制不同。
