Data from: Sex ratio and gamete size across eastern North America in Dictyostelium discoideum, a social amoeba with three sexes

Theory indicates that numbers of mating types should tend towards infinity or remain at two. The social amoeba, Dictyostelium discoideum, however, has three mating types. It is therefore a mystery how this species has broken the threshold of two mating types, but has not increased towards a much higher number. Frequency dependent selection on rare types in combination with isogamy, a form of reproduction involving gametes similar in size, could explain the evolution of multiple mating types in this system. Other factors, such as drift, may be preventing the evolution of more than three. We first looked for evidence of isogamy by measuring gamete size associated with each type. We found no evidence of size dissimilarities between gametes. We then looked for evidence of balancing selection, by examining mating type distributions in natural populations and comparing genetic differentiation at the mating type locus to that at more neutral loci. We found that mating type frequency varied among the three populations we examined, with only one of the three showing an even sex ratio, which does not support balancing selection. However, we found more population structure at neutral loci than the mating type locus, suggesting that the three mating types are indeed maintained at intermediate frequencies by balancing selection. Overall, the data are consistent with balancing selection acting on D. discoideum mating types, but with a sufficiently weak rare sex advantage to allow for drift, a potential explanation for why these amoebae have only three mating types.

理论研究表明，交配型的数量应当趋向于无穷多，或稳定维持在两种。然而，社会性变形虫盘基网柄菌（Dictyostelium discoideum）却拥有三种交配型。因此，该物种如何突破两种交配型的阈值，却未进一步演化出更多交配型，仍是一个未解之谜。结合同配生殖（即生殖过程中配子大小相似的繁殖方式）对稀有交配型的频率依赖选择，或可解释该物种多交配型的演化机制。其他因素（如遗传漂变）或阻碍了该物种演化出三种以上的交配型。本研究首先通过测定各交配型对应的配子大小以寻找同配生殖的相关证据，结果未发现配子间存在大小差异的相关证据。随后，我们通过分析自然种群中的交配型分布，并比较交配型基因座与更多中性基因座的遗传分化程度，寻找平衡选择的相关证据。研究发现，我们所检测的三个种群的交配型频率存在显著差异，其中仅一个种群呈现出均等的性比，这一结果并不支持平衡选择假说。但我们发现中性基因座的群体结构程度高于交配型基因座，这表明三种交配型确实是通过平衡选择维持在中等频率的。总体而言，本研究数据与盘基网柄菌交配型受平衡选择作用的假说相符，但稀有交配型的选择优势足够微弱，足以允许遗传漂变的发生——这或可解释为何该类变形虫仅拥有三种交配型。