Data from: Hidden genetic variation in the germline genome of Tetrahymena thermophila

Genome architecture varies greatly among eukaryotes. This diversity may profoundly affect the origin and maintenance of genetic variation within a population. Ciliates are microbial eukaryotes with unusual genome features, such as separation of germline and somatic genomes within a single cell and amitotic division. These features have previously been proposed to increase the rate of molecular evolution in these species. Here, we assessed the fitness effects of genetic variation in the two genomes of natural isolates of the ciliate Tetrahymena thermophila. We find more extensive genetic variation in fitness in the transcriptionally-silent germline genome than in the expressed somatic genome. Surprisingly, this variation is not primarily deleterious, but has both beneficial and deleterious effects. We conclude that Tetrahymena genome architecture allows for the maintenance of genetic variation that would otherwise be eliminated by selection. We consider the effect of selection on the two genomes and the impacts of reproductive strategies and the mechanism of sex determination on the structure of this variation.

真核生物的基因组架构（genome architecture）差异极大。这种多样性可对种群内遗传变异的起源与维持产生深远影响。纤毛虫（ciliate）是一类具备特殊基因组特征的微生物真核生物，例如单个细胞内同时存在种系基因组与体细胞基因组的分离，以及无丝分裂（amitotic division）。此前已有研究提出，这类特征会提升这些物种的分子进化速率。本研究对嗜热四膜虫（Tetrahymena thermophila）自然分离株的两套基因组中的遗传变异的适合度效应进行了评估。我们发现，相较于表达型体细胞基因组，转录沉默的种系基因组中存在更为广泛的适合度相关遗传变异。令人意外的是，这类变异并非主要呈有害性，而是同时兼具有益与有害效应。我们据此得出结论：嗜热四膜虫的基因组架构允许维持那些本应被自然选择清除的遗传变异。我们还探讨了自然选择对两套基因组的影响，以及繁殖策略与性别决定机制对这类变异结构的作用。