Data from: Condition dependence of male and female genital structures in the seed beetle Callosobruchus maculatus (Coleoptera: Bruchidae)

Theory predicts that costly secondary sexual traits will evolve heightened condition dependence, and many studies have reported strong condition dependence of signal and weapon traits in a variety of species. However, although genital structures often play key roles in intersexual interactions and appear to be subject to sexual or sexually antagonistic selection, few studies have examined the condition dependence of genital structures, especially in both sexes simultaneously. We investigated the responses of male and female genital structures to manipulation of larval diet quality (new versus once-used mung beans) in the bruchid seed beetle Callosobruchus maculatus. We quantified effects on mean relative size and static allometry of the male aedeagus, aedeagal spines, flap and paramere and the female reproductive tract and bursal spines. None of the male traits showed a significant effect of diet quality. In females, we found that longer bursal spines (relative to body size) were expressed on low-quality diet. Although the function of bursal spines is poorly understood, we suggest that greater bursal spine length in low-condition females may represent a sexually antagonistic adaptation. Overall, we found no evidence that genital traits in C. maculatus are expressed to a greater extent when nutrients are more abundant. This suggests that, even though some genital traits appear to function as secondary sexual traits, genital traits do not exhibit heightened condition dependence in this species. We discuss possible reasons for this finding.

理论研究表明，携带演化成本的次级性征（secondary sexual traits）会演化出更强的条件依赖性（condition dependence），且已有大量研究报道，诸多物类的信号性状与武器性状均存在显著的条件依赖性。然而，尽管生殖器结构在两性交互过程中通常发挥关键作用，且似乎受到性选择或性拮抗选择（sexually antagonistic selection）的作用，但鲜有研究探讨生殖器结构的条件依赖性，针对雌雄两性同时开展的相关研究更是匮乏。本研究以豆象（bruchid seed beetle）绿豆象（Callosobruchus maculatus）为实验对象，探究其雌雄生殖器结构对幼虫日粮质量（新鲜绿豆与已重复使用过的绿豆）操控处理的响应。我们量化分析了雄虫阳茎（aedeagus）、阳茎棘突、阳基瓣与阳基侧突（paramere），以及雌虫生殖道与囊突（bursal spines）的平均相对大小与静态异速生长（static allometry）效应。雄虫的所有检测性状均未表现出受日粮质量影响的显著效应。在雌虫群体中，我们观察到，饲喂低质量日粮的个体其囊突长度（相对于体型大小）更长。尽管目前对囊突的功能认知仍较为有限，但我们推测，低生理条件雌虫的囊突更长这一现象，可能代表了一种性拮抗适应（sexually antagonistic adaptation）。总体而言，我们未发现证据表明，在营养供给更充足的情况下，C. maculatus的生殖器性状会呈现出更强的表达水平。这提示，尽管部分生殖器性状似乎承担了次级性征的功能，但该物种的生殖器性状并未表现出增强的条件依赖性。最后我们对本研究结果的潜在成因展开了讨论。