Multivariate stabilizing sexual selection and the evolution of male and female genital morphology in the red flour beetle

Male genitals are highly divergent in animals with internal fertilization.
Most studies attempting to explain this diversity have focussed on testing
the major hypotheses of genital evolution (the lock-and-key, pleiotropy
and sexual selection hypotheses) and quantifying the form of selection
targeting male genitals has played an important role in this endeavour.
However, we currently know far less about selection targeting female
genitals or how male and female genitals interact during mating. Here we
use formal selection analysis to show that genital size and shape is
subject to strong multivariate stabilizing sexual selection in both sexes
of the red flour beetle, Tribolium castaneum. Moreover, we show
significant sexual selection on the covariance between the sexes for
specific aspects of genital shape suggesting that male and female
genitalia also interact to determine the successful transfer of a
spermatophore during mating. Our work therefore highlights the important
role that both male and female genital morphology plays in determining
mating success and that these effects can occur independently, as well as
through their interaction. Moreover, it cautions against the
overly-simplistic view that the sexual selection targeting genital
morphology will always be directional in form and restricted primarily to
males.

在实行体内受精的动物类群中，雄性生殖器形态具有极高的分化程度。绝大多数旨在解释这一分化现象的研究，均聚焦于验证生殖器演化的三大核心假说——锁钥假说（lock-and-key）、基因多效性假说（pleiotropy）与性选择假说——而量化针对雄性生殖器的选择压力形式，也成为此类研究的重要组成部分。
然而，目前学界对雌性生殖器所受选择压力，以及交配过程中雌雄生殖器的互作机制，仍所知有限。本研究通过正式的选择分析方法证实，赤拟谷盗（Tribolium castaneum）雌雄两性的生殖器大小与形态，均受到强烈的多变量稳定型性选择作用。此外，本研究还发现，针对雌雄生殖器特定形态特征间协方差的性选择效应显著，这表明雌雄生殖器的互作同样是决定交配过程中精荚（spermatophore）成功转移的关键因素。
综上，本研究凸显了雌雄生殖器形态在决定交配成功率中的重要作用：这类效应既可以独立发挥作用，也可通过两性间的互作实现。此外，本研究也提醒学界，不应过度简化地认为针对生殖器形态的性选择总是呈定向形式，且仅局限于雄性个体。