Data from: Coevolution of female and male genital components to avoid genital size mismatches in sexually dimorphic spiders

Background: In most animal groups, it is unclear how body size variation relates to genital size differences between the sexes. While most morphological features tend to scale with total somatic size, this does not necessarily hold for genitalia because divergent evolution in somatic size between the sexes would cause genital size mismatches. Theory predicts that the interplay of female-biased sexual size dimorphism (SSD) and sexual genital size dimorphism (SGD) should adhere to the ‘positive genital divergence’, the ‘constant genital divergence’, or the ‘negative genital divergence’ model, but these models remain largely untested. We test their validity in the spider family Nephilidae known for the highest degrees of SSD among terrestrial animals. Results: Through comparative analyses of sex-specific somatic and genital sizes, we first demonstrate that 99 of the 351 pairs of traits are phylogenetically correlated. Through factor analyses we then group these traits for MCMCglmm analyses that test broader correlation patterns, and these reveal significant correlations in 10 out of the 36 pairwise comparisons. Both types of analyses agree that female somatic and internal genital sizes evolve independently. While sizes of non-intromittent male genital parts coevolve with male body size, the size of the intromittent male genital parts is independent of the male somatic size. Instead, male intromittent genital size coevolves with female (external and, in part, internal) genital size. All analyses also agree that SGD and SSD evolve independently. Conclusions: Internal dimensions of female genitalia evolve independently of female body size in nephilid spiders, and similarly, male intromittent genital size evolves independently of the male body size. The size of the male intromittent organ (the embolus) and the sizes of female internal and external genital components thus seem to respond to selection against genital size mismatches. In accord with these interpretations, we reject the validity of the existing theoretical models of genital and somatic size dimorphism in spiders.

研究背景：在绝大多数动物类群中，躯体大小的变异如何与两性间的生殖器大小差异相关联，目前仍不甚明晰。尽管多数形态特征往往随整体躯体大小呈比例变化，但生殖器未必遵循这一规律——因为两性间躯体大小的演化分化，可能会引发生殖器大小匹配失衡。理论预测，雌性偏向的性大小二态性（sexual size dimorphism, SSD）与生殖器大小性二态性（sexual genital size dimorphism, SGD）之间的相互作用，应符合“正生殖器分化”、“恒定生殖器分化”或“负生殖器分化”模型，但此类模型迄今仍未得到充分验证。本研究以陆生动物中性大小二态性程度最高的络新妇科（Nephilidae）蜘蛛为研究对象，对上述模型的有效性进行检验。

研究结果：通过对两性躯体与生殖器大小的比较分析，我们首先证实351个性状对中的99对存在系统发育相关性。随后通过因子分析对这些性状进行分组，开展MCMCglmm分析以检验更宽泛的相关性模式，该分析在36组两两比较中，有10组呈现出显著相关性。两类分析均表明，雌性躯体大小与雌性内生殖器大小的演化彼此独立。非插入式雄性生殖器结构的大小随雄性躯体大小协同演化，而雄性插入式生殖器结构的大小则与雄性躯体大小无关。与之相反，雄性插入式生殖器的大小与雌性（外生殖器及部分内生殖器）的大小协同演化。所有分析亦一致表明，生殖器大小性二态性与性大小二态性的演化彼此独立。

研究结论：络新妇科蜘蛛的雌性生殖器内部维度演化与雌性躯体大小无关，同理，雄性插入式生殖器的大小演化亦与雄性躯体大小无关。由此可见，雄性插入式器官（插入器，embolus）的大小，以及雌性内外生殖器各组分的大小，似乎均朝向避免生殖器大小匹配失衡的方向经受选择。基于上述研究结果，我们推翻了现有关于蜘蛛躯体与生殖器大小二态性的理论模型的有效性。