Department of Biology and Biochemistry, University of Houston, Houston, Texas 77204-5001, USA.
Evolution. 2011 Jul;65(7):2050-60. doi: 10.1111/j.1558-5646.2011.01266.x. Epub 2011 Mar 19.
Sex allocation theory has been remarkably successful at explaining the prevalence of even sex ratios in natural populations and at identifying specific conditions that can result in biased sex ratios. Much of this theory focuses on parental sex determination (SD) strategies. Here, we consider instead the evolutionary causes and consequences of mixed offspring SD strategies, in which the genotype of an individual determines not its sex, but the probability of developing one of multiple sexes. We find that alleles specifying mixed offspring SD strategies can generally outcompete alleles that specify pure strategies, but generate constraints that may prevent a population from reaching an even sex ratio. We use our model to analyze sex ratios in natural populations of Tetrahymena thermophila, a ciliate with seven sexes determined by mixed SD alleles. We show that probabilistic SD is sufficient to account for the occurrence of skewed sex ratios in natural populations of T. thermophila, provided that their effective population sizes are small. Our results highlight the importance of genetic drift in sex ratio evolution and suggest that mixed offspring SD strategies should be more common than currently thought.
性分配理论在解释自然种群中两性比例的普遍性以及确定导致偏性比的具体条件方面取得了显著成功。该理论的很大一部分集中于父母的性别决定 (SD) 策略。在这里，我们转而考虑混合后代 SD 策略的进化原因和后果，其中个体的基因型不是决定其性别，而是决定其发展为多种性别之一的概率。我们发现，指定混合后代 SD 策略的等位基因通常可以胜过指定纯策略的等位基因，但会产生可能阻止种群达到均等性别比的约束。我们使用我们的模型来分析具有由混合 SD 等位基因决定的七种性别的草履虫 Tetrahymena thermophila 的自然种群中的性别比例。我们表明，概率性 SD 足以解释自然种群中 T. thermophila 偏性比的发生，前提是它们的有效种群规模较小。我们的结果强调了遗传漂变在性别比进化中的重要性，并表明混合后代 SD 策略应该比目前认为的更为普遍。
