BACKGROUND Opportunistic yeast pathogens of the genus Candida are an important medical problem. Candida albicans, the most prevalent Candida species, is a natural commensal of humans that can adopt a pathogenic behavior. This species is highly heterozygous and cannot undergo meiosis, adopting instead a parasexual cycle that increases genetic variability and potentially leads to advantages under stress conditions. However, the origin of C. albicans heterozygosity is unknown, and we hypothesize that it could result from ancestral hybridization. We tested this idea by analyzing available genomes of C. albicans isolates and comparing them to those of hybrid and non-hybrid strains of other Candida species. RESULTS Our results show compelling evidence that C. albicans is an evolved hybrid. The genomic patterns observed in C. albicans are similar to those of other hybrids such as Candida orthopsilosis MCO456 and Candida inconspicua, suggesting that it also descends from a hybrid of two divergent lineages. Our analysis indicates that most of the divergence between haplotypes in C. albicans heterozygous blocks was already present in a putative heterozygous ancestor, with an estimated 2.8% divergence between homeologous chromosomes. The levels and patterns of ancestral heterozygosity found cannot be fully explained under the paradigm of vertical evolution and are not consistent with continuous gene flux arising from lineage-specific events of admixture. CONCLUSIONS Although the inferred level of sequence divergence between the putative parental lineages (2.8%) is not clearly beyond current species boundaries in Saccharomycotina, we show here that all analyzed C. albicans strains derive from a single hybrid ancestor and diverged by extensive loss of heterozygosity. This finding has important implications for our understanding of C. albicans evolution, including the loss of the sexual cycle, the origin of the association with humans, and the evolution of virulence traits.

中文翻译：

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酵母机会病原体白色念珠菌的杂种起源的基因组证据。

背景技术念珠菌属的机会酵母病原体是重要的医学问题。白色念珠菌（Candida albicans）是最普遍的念珠菌物种，是人类的自然代名词，可以采取致病性行为。该物种高度杂合，不能进行减数分裂，取而代之的是一个副性循环，该循环增加了遗传变异性，并可能在胁迫条件下带来优势。然而，白色念珠菌杂合性的起源是未知的，我们假设它可能是由祖先杂交产生的。我们通过分析白色念珠菌分离物的可用基因组并将其与其他念珠菌物种的杂种和非杂种菌株的基因组进行比较，测试了该想法。结果我们的结果显示出令人信服的证据，即白色念珠菌是进化的杂种。在C中观察到的基因组模式 白色念珠菌与其他杂种相似，如假丝酵母念珠菌MCO456和不透明念珠菌，表明它也来自两个不同谱系的杂种。我们的分析表明，在白色念珠菌杂合块中单倍型之间的大多数差异已经存在于推定的杂合祖先中，估计同源染色体之间的差异为2.8％。发现的祖先杂合性的水平和模式不能在垂直进化的范式下得到充分的解释，并且与由混合谱系特异性事件引起的连续基因通量不一致。结论尽管推定的亲代谱系之间的序列差异的推断水平（2.8％）并未明显超出酿酒酵母的当前物种边界，但我们在此表明​​所有分析的C。白色念珠菌菌株源自单个杂种祖先，并因杂合性的广泛丧失而分化。这一发现对我们对白色念珠菌进化的理解具有重要意义，包括性循环的丧失，与人的联系的起源以及毒力特性的进化。