Genome introgressions drive evolution across the animal1, plant2 and fungal3 kingdoms. Introgressions initiate from archaic admixtures followed by repeated backcrossing to one parental species. However, how introgressions arise in reproductively isolated species, such as yeast4, has remained unclear. Here we identify a clonal descendant of the ancestral yeast hybrid that founded the extant Saccharomyces cerevisiae Alpechin lineage5, which carries abundant Saccharomyces paradoxus introgressions. We show that this clonal descendant, hereafter defined as a 'living ancestor', retained the ancestral genome structure of the first-generation hybrid with contiguous S. cerevisiae and S. paradoxus subgenomes. The ancestral first-generation hybrid underwent catastrophic genomic instability through more than a hundred mitotic recombination events, mainly manifesting as homozygous genome blocks generated by loss of heterozygosity. These homozygous sequence blocks rescue hybrid fertility by restoring meiotic recombination and are the direct origins of the introgressions present in the Alpechin lineage. We suggest a plausible route for introgression evolution through the reconstruction of extinct stages and propose that genome instability allows hybrids to overcome reproductive isolation and enables introgressions to emerge.

中文翻译：

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酵母活的祖先揭示了基因组渗入的起源

基因组渗入推动了动物 1、植物 2 和真菌 3 王国的进化。基因渗入从古老的混合物开始，然后反复回交到一个亲本物种。然而，基因渗入是如何在生殖分离的物种（如酵母4）中产生的，仍不清楚。在这里，我们确定了祖先酵母杂种的克隆后代，该杂种建立了现存的酿酒酵母 Alpechin 谱系5，该谱系携带丰富的悖论酵母渗入。我们表明，这种克隆后代，此后被定义为“活着的祖先”，保留了第一代杂交种的祖先基因组结构，具有连续的酿酒酵母和悖论酵母亚基因组。祖先的第一代杂种通过一百多个有丝分裂重组事件经历了灾难性的基因组不稳定性，主要表现为杂合性缺失产生的纯合基因组块。这些纯合序列通过恢复减数分裂重组来阻止挽救杂交生育力，并且是 Alpechin 谱系中存在的基因渗入的直接起源。我们建议通过重建灭绝阶段进行基因渗入进化的合理途径，并提出基因组不稳定性允许杂种克服生殖隔离并使基因渗入出现。