Data from: Polyandrous females provide sons with more competitive sperm: support for the sexy-sperm hypothesis in the rattlebox moth (Utetheisa ornatrix)

Given the costs of multiple-mating, why has female polyandry evolved? Utetheisa ornatrix moths are well-suited for studying multiple mating in females because females are highly polyandrous over their lifespan, with each male mate transferring a substantial spermatophore with both genetic and non-genetic material. The accumulation of resources might explain the prevalence of polyandry in this species, but another, not mutually-exclusive, possibility is that females mate multiply to increase the probability that their sons will inherit more-competitive sperm. This latter “sexy-sperm” hypothesis posits that female multiple mating and male sperm competitiveness co-evolve via a Fisherian runaway process. We tested the sexy-sperm hypothesis by using competitive double matings to compare the sperm competition success of sons of polyandrous versus monandrous females. In accordance with sexy-sperm theory, we found that in 511 offspring across 17 families, the male whose polyandrous mother mated once with each of three different males sired significantly more of all total offspring (81%) than did the male whose monandrous mother was mated thrice to a single male. Interestingly, sons of polyandrous mothers had a significantly biased sex ratio of their brood toward sons, also in support of the hypothesis.

鉴于多次交配存在演化代价，雌性多配（polyandry）为何能够得以演化？饰黄丽夜蛾（Utetheisa ornatrix）是研究雌性多次交配的理想模型物种，因其雌性个体在整个生命周期中表现出极强的多配性，且每只交配雄虫都会传递包含遗传与非遗传物质的大型精包（spermatophore）。资源积累假说或许可以解释该物种中多配性的普遍存在，但另一种并非与之互斥的可能性是：雌性通过多次交配，提升其子代雄性继承更具竞争力精子的概率。这一被称为‘性感精子假说（sexy-sperm hypothesis）’的理论提出，雌性多次交配与雄性精子竞争力会通过费舍尔失控演化过程（Fisherian runaway）实现协同演化。本研究通过竞争性双交配实验，对比了多配雌性与单配（monandrous）雌性所产雄性后代的精子竞争成功率，以此检验性感精子假说。基于该假说的理论预期，我们在涵盖17个家系的511个子代个体中发现：母亲为多配型（先后与3只不同雄虫各交配1次）的雄性，其后代总占比（81%）显著高于母亲为单配型（仅与同一只雄虫交配3次）的雄性。值得注意的是，多配雌性所产雄性后代的子代性比显著偏向雄性，这一结果同样支持性感精子假说。