Schausberger Peter, Walzer Andreas, Murata Yasumasa, Osakabe Masahiro
Department of Behavioural Biology, University of Vienna, Vienna, Austria.
Group of Arthropod Ecology and Behavior, Department of Crop Sciences, Vienna, Austria.
PLoS One. 2017 Nov 30;12(11):e0188924. doi: 10.1371/journal.pone.0188924. eCollection 2017.
Polyandry, i.e. females mating with multiple males, is more common than previously anticipated and potentially provides both direct and indirect fitness benefits to females. The level of polyandry (defined by the lifetime number of male mates of a female) is an important determinant of the occurrence and intensity of sexual selection acting on male phenotypes. While the forces of sexual selection acting on phenotypic male traits such as body size are relatively well understood, sexual selection acting on phenotypic plasticity of these traits is unexplored. We tackled this issue by scrutinizing the link between polyandry and phenotypic plasticity of male body size in two sympatric plant-inhabiting predatory mite species, Phytoseiulus persimilis and Neoseiulus californicus. These two species are similar in life history, ecological niche requirements, mating behavior, polygyny and female body size plasticity but strikingly differ in the level of both polyandry and phenotypic plasticity of male body size (both lower in P. persimilis). We hypothesized that deviations from standard body size, i.e. the size achieved under favorable conditions, incur higher costs for males in the less polyandrous P. persimilis. To test our hypotheses, we conducted two experiments on (i) the effects of male body size on spermatophore transfer in singly mating females and (ii) the effects of mate sequence (switching the order of standard-sized and small males) on mating behavior and paternity success in doubly mating females. In P. persimilis but not N. californicus, small males transferred fewer but larger spermatophores to the females; in both species, females re-mated more likely with standard-sized following small than small following standard-sized males; in P. persimilis, first standard-sized males sired a higher proportion of offspring produced after re-mating by the female than first small males, whereas in N. californicus the paternity success of small and standard-sized males was independent of the mating sequence. Based on our results and pertinent previous studies, which showed that females of P. persimilis, but not N. californicus, prefer mating with standard-sized over small males and allow them fertilizing more eggs, the lack of interspecific difference in female body size plasticity, and the absence of any clue pointing at a role of natural selection, we suggest that the interspecific difference in male body size plasticity is sexually selected. Our study provides an indication of sexual selection constraining plasticity of male phenotypes, suggesting that the level of polyandry may be an important co-determinant of the level of phenotypic plasticity of male body size.
一妻多夫制，即雌性与多个雄性交配，比之前预期的更为常见，并且可能为雌性带来直接和间接的适应性益处。一妻多夫制的程度（由雌性一生中的雄性配偶数量定义）是影响雄性表型的性选择发生与否及强度的一个重要决定因素。虽然作用于雄性表型特征（如体型大小）的性选择力量已得到较好理解，但作用于这些特征表型可塑性的性选择尚未得到探索。我们通过研究两种同域栖息于植物上的捕食螨物种——智利小植绥螨（Phytoseiulus persimilis）和加州新小绥螨（Neoseiulus californicus）——的一妻多夫制与雄性体型表型可塑性之间的联系来解决这个问题。这两个物种在生活史、生态位需求、交配行为、一夫多妻制和雌性体型可塑性方面相似，但在一妻多夫制程度和雄性体型表型可塑性方面存在显著差异（智利小植绥螨的这两个方面都较低）。我们假设，偏离标准体型，即在有利条件下达到的体型，对于一妻多夫制程度较低的智利小植绥螨的雄性来说成本更高。为了验证我们的假设，我们进行了两项实验：（i）雄性体型对单配雌性中精包传递的影响；（ii）配偶顺序（改变标准体型和小体型雄性的交配顺序）对双配雌性的交配行为和父权成功的影响。在智利小植绥螨中，而非加州新小绥螨中，小体型雄性向雌性传递的精包数量较少但体积较大；在这两个物种中，雌性在与小体型雄性交配后更有可能与标准体型雄性再次交配，而非与标准体型雄性交配后再与小体型雄性交配；在智利小植绥螨中，首次与标准体型雄性交配的雄性在雌性再次交配后所产生的后代中所占比例高于首次与小体型雄性交配的雄性，而在加州新小绥螨中，则不存在体型大小的差异，小体型和标准体型雄性的父权成功与否与交配顺序无关。基于我们的研究结果以及之前的相关研究，这些研究表明智利小植绥螨的雌性（而非加州新小绥螨的雌性）更喜欢与标准体型而非小体型雄性交配，并允许它们使更多卵子受精，雌性体型可塑性不存在种间差异，且没有任何线索表明自然选择发挥了作用，我们认为雄性体型可塑性的种间差异是由性选择导致的。我们的研究表明性选择限制了雄性表型的可塑性，这表明一妻多夫制程度可能是雄性体型表型可塑性程度的一个重要共同决定因素。
