Abstract 摘要
Balancing selection, an evolutionary force that retains genetic diversity, has been detected in multiple genes and organisms, such as the sexual mating loci in fungi. However, to quantify the strength of balancing selection and define the mating-related genes require a large number of strains. In tetrapolar basidiomycete fungi, sexual type is determined by two unlinked loci, MATA and MATB. Genes in both loci define mating type identity, control successful mating and completion of the life cycle. These loci are usually highly diverse. Previous studies have speculated, based on culture crosses, that species of the non-model genus Trichaptum (Hymenochaetales, Basidiomycota) possess a tetrapolar mating system, with multiple alleles. Here, we sequenced a hundred and eighty strains of three Trichaptum species. We characterized the chromosomal location of MATA and MATB, the molecular structure of MAT regions and their allelic richness. The sequencing effort was sufficient to molecularly characterize multiple MAT alleles segregating before the speciation event of Trichaptum species. Analyses suggested that long-term balancing selection has generated trans-species polymorphisms. Mating sequences were classified in different allelic classes based on an amino acid identity (AAI) threshold supported by phylogenetics. 17,550 mating types were predicted based on the allelic classes. In vitro crosses allowed us to support the degree of allelic divergence needed for successful mating. Even with the high amount of divergence, key amino acids in functional domains are conserved. We conclude that the genetic diversity of mating loci in Trichaptum is due to long-term balancing selection, with limited recombination and duplication activity. The large number of sequenced strains highlighted the importance of sequencing multiple individuals from different species to detect the mating-related genes, the mechanisms generating diversity and the evolutionary forces maintaining them. 平衡选择（balancing selection）是一种保留遗传多样性的进化力量，已经在多种基因和生物体中检测到，例如真菌中的有性交配位点。然而，要量化平衡选择的强度并定义交配相关基因需要大量的菌株（strains）。在 tetrapolar basidiomycete 菌真菌中，有性类型由两个不连锁的基因座 MATA 和 MATB 决定。两个基因座中的基因定义了交配类型身份，控制成功交配和生命周期的完成。这些基因座通常是高度多样化的。基于真菌培养物的杂交，先前的研究推测，non-model 属 Trichaptum（Hymenochaetales，Basidiomycota）的物种具有多个等位基因的 tetrapolar 交配系统。在这里，我们对3个 Trichaptum 物种的180株进行了测序。我们确定了 MATA 和 MATB 的染色体位置、MAT 区域的分子结构及其等位基因的丰富度。测序工作足以在分子上确定在 Trichaptum 物种形成之前分离的多个 MAT 等位基因。分析表明，长期平衡选择（long-term balancing selection）产生了跨物种多态性。根据系统发育学支持的氨基酸同一性 (AAI， amino acid identity) 阈值，可以将交配序列分为不同的等位基因类别（allelic classes）。根据等位基因类别预测了17,550种交配类型。体外杂交使我们能够支持成功交配所需的等位基因分歧程度(degree of allelic divergence)。即使存在大量分歧，功能域中的关键氨基酸也是保守的。我们得出的结论是， Trichaptum 中交配位点的遗传多样性是由于带有有限的重组和复制活动的长期平衡选择的结果。大量测序的菌株凸显了对来自不同物种的多个个体进行测序以检测交配相关的基因、产生多样性的机制和维持它们的进化力量的重要性。