Key Laboratory of Aquatic Biodiversity and Conservation, Institute of Hydrobiology, Chinese Academy of Sciences, Wuhan, Hubei, China.
University of Chinese Academy of Sciences, Beijing, China.
Genome Biol Evol. 2020 Dec 6;12(12):2328-2343. doi: 10.1093/gbe/evaa197.
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.
性甚至在主要的无性物种中也有优势。一些纤毛虫似乎利用了这种具有多种交配类型的生殖策略。然而，单细胞纤毛虫 Tetrahymena thermophila 中决定交配类型组成的因素还知之甚少，而且这一过程还受到后代中不符合孟德尔遗传的交配类型决定的影响。因此，我们开发了一种新的群体遗传学模型，以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，无论是否存在自然选择，种群中要么七种交配类型共存，要么只有一种交配类型固定。为了了解哪些因素可能更有影响力，并验证理论预测的有效性，我们在实验室中维持了五个重复种群，以便可以控制或测量几个因素。我们利用全基因组测序来识别新出现的突变，并确定交配类型组成。惊人的是，所有种群都受到新出现的有益突变的强烈选择，这些突变携带的交配类型被固定下来，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 T. thermophila 可以利用哪些进化策略来优化种群适应性。
