Sex offers advantages even in primarily asexual species. Some ciliates appear to utilize such reproductive strategy with many mating types. However, the factors determining the composition of mating types in the unicellular ciliate Tetrahymena thermophila are poorly understood, and this is further complicated by non-Mendelian determination of mating type in the offspring. We therefore developed a novel population genetics model to predict how various factors influence the dynamics of mating type composition, including natural selection. The model predicted either the coexistence of all seven mating types or fixation of a single mating type in a population, depending on parameter combinations, irrespective of natural selection. To understand what factor(s) may be more influential and to test the validity of theoretical prediction, five replicate populations were maintained in laboratory such that several factors could be controlled or measured. Whole-genome sequencing was used to identify newly arising mutations and determine mating type composition. Strikingly, all populations were found to be driven by strong selection on newly arising beneficial mutations to fixation of their carrying mating types, and the trajectories of speed to fixation agreed well with our theoretical predictions. This study illustrates the evolutionary strategies that T. thermophila can utilize to optimize population fitness.

中文翻译：

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嗜热四膜虫交配型组成的驱动因素。

即使在主要是无性繁殖的物种中，性也有优势。一些纤毛虫似乎在许多交配类型中使用这种繁殖策略。然而，决定单细胞纤毛虫嗜热四膜虫交配类型组成的因素对后代的交配类型的非孟德尔确定进一步复杂化。因此，我们开发了一种新的种群遗传学模型来预测各种因素如何影响交配类型组成的动态，包括自然选择。该模型预测了所有七种交配类型的共存或种群中单一交配类型的固定，这取决于参数组合，而与自然选择无关。为了了解哪些因素可能更具影响力并测试理论预测的有效性，在实验室中保留了五个重复种群，以便可以控制或测量几个因素。全基因组测序用于识别新出现的突变并确定交配类型组成。引人注目的是，发现所有种群都受到对新出现的有益突变的强烈选择以固定其携带交配类型的驱动，并且固定速度的轨迹与我们的理论预测非常吻合。这项研究说明了进化策略T. thermophila可用于优化种群适应度。