The genetic analysis of large numbers of Saccharomyces cerevisiae × S. uvarum ("cevarum") and S. kudriavzevii × S. uvarum ("kudvarum") hybrids in our previous studies revealed that these species are isolated by a postzygotic double-sterility barrier. We proposed a model in which the first barrier is due to the abruption of the meiotic process by the failure of the chromosomes of the subgenomes to pair (and recombine) in meiosis and the second barrier is assumed to be the result of the suppression of mating by allospecific MAT heterozygosity. While the former is analogous to the major mechanism of postzygotic reproductive isolation in plants and animals, the latter seems to be Saccharomyces specific. To bolster the assumed involvement of MAT in the second sterility barrier, we produced synthetic alloploid two-species cevarum and kudvarum hybrids with homo- and heterothallic backgrounds as well as three-species S. cerevisiae × S. kudvarum × S. uvarum ("cekudvarum") hybrids by mass-mating and examined their MAT loci using species- and cassette-specific primer pairs. We found that the allospecific MAT heterozygosity repressed MAT switching and mating in the hybrids and in the viable but sterile spores produced by the cevarum hybrids that had increased (allotetraploid) genomes. The loss of heterozygosity by meiotic malsegregation of MAT-carrying chromosomes in the latter hybrids broke down the sterility barrier. The resulting spores nullisomic for the S. uvarum chromosome produced vegetative cells capable of MAT switching and conjugation, opening the way for GARMe (Genome Autoreduction in Meiosis), the process that leads to chimeric genomes.

中文翻译：

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MAT杂合性和酵母菌生殖分离中的第二个不育障碍。

在我们先前的研究中，对大量酿酒酵母×葡萄链霉菌（“ cevarum”）和库氏葡萄球菌×葡萄孢菌（“ kudvarum”）进行的遗传分析表明，这些菌种是通过合子后双重无菌屏障分离的。我们提出了一个模型，其中第一个障碍是由于亚基因组染色体在减数分裂中配对（和重组）失败而导致减数分裂过程的中断，而第二个障碍被认为是抑制交配的结果通过同种异体MAT杂合性。前者类似于动植物后合子生殖分离的主要机制，而后者似乎是酿酒酵母特有的。为了支持MAT参与第二个无菌障碍，我们通过大量交配生产了具有同种和异种背景的人工异源二种杂种牛杂种和库尔德杂种，以及啤酒异种×酿酒酵母×乌贼种（“ cekudvarum”）三种杂种，并使用它们检查了MAT基因座物种和盒特异性引物对。我们发现同种异体MAT杂合性抑制杂种和由增加的（异源四倍体）基因组的cevarum杂种产生的可行但无菌孢子中的MAT切换和交配。在后面的杂种中，携带MAT的染色体通过减数分裂错误的分离而失去了杂合性，打破了不育障碍。由此产生的孢子对葡萄球菌染色体无效，产生了能够进行MAT转换和结合的营养细胞，为GARMe（减数分裂中的基因组自动还原）开辟了道路，