Department of Evolutionary Biology, Bielefeld University, Morgenbreede 45, 33615, Bielefeld, Germany.
BMC Evol Biol. 2019 Oct 11;19(1):185. doi: 10.1186/s12862-019-1514-0.
Studying reproductive trait allometries can help to understand optimal male investment strategies under sexual selection. In promiscuous mating systems, studies across several taxa suggest that testes allometry is usually positive, presumably due to strong selection on sperm numbers through intense sperm competition. Here, we investigated testes allometry in a bush-cricket species, Metaplastes ornatus, in which females mate promiscuously, but where sperm removal behaviour by males likely drastically reduces realised sperm competition level.
As hypothesised, we found evidence for negative testes allometry and hence a fundamentally different male investment strategy compared to species under intense sperm competition. In addition, the mean relative testes size of M. ornatus was small compared to other species of bush-crickets. Surprisingly, the spermatophore gland, a potential alternative trait that males could invest in instead of testes, also did not show positive allometry, but was approximately isometric. We further observed the expected pattern of negative allometry for the male morphological structure responsible for sperm removal in this species, the subgenital plate, supporting the one-size-fits-all hypothesis for intromittent genitalia.
Our findings suggest that the evolution of sperm removal behaviour in M. ornatus was a key adaptation for avoiding sperm competition, with important consequences for reproductive trait allometries. Nevertheless, they also imply that it does not pay for larger males to invest disproportionately in nuptial gift production in this species.
研究生殖特征的异速生长可以帮助理解在性选择下雄性的最佳投资策略。在滥交的交配系统中，多个分类群的研究表明，睾丸的异速生长通常是正相关的，这可能是由于通过强烈的精子竞争对精子数量的强烈选择。在这里，我们研究了 bush-cricket 物种 Metaplastes ornatus 的睾丸异速生长，其中雌性滥交交配，但雄性的精子去除行为可能会大大降低实际的精子竞争水平。
正如假设的那样，我们发现了睾丸异速生长的证据，因此与处于强烈精子竞争下的物种相比，雄性的投资策略有根本的不同。此外，与其他 bush-crickets 物种相比，M. ornatus 的平均相对睾丸大小较小。令人惊讶的是，精子腺，一种雄性可以投资的潜在替代特征，也没有表现出正异速生长，而是近似等比生长。我们还观察到，在这种物种中负责精子去除的雄性形态结构的亚生殖盘，与内生殖器的一刀切假说一致，表现出预期的负异速生长模式。
我们的研究结果表明，M. ornatus 中精子去除行为的进化是避免精子竞争的关键适应，对生殖特征的异速生长有重要影响。然而，这也意味着在这个物种中，体型较大的雄性不会不成比例地投资于婚礼物的生产。
