Rapid Divergence In The Evolution Of Genitalia In Freshwater Crab

Brachyuran genitalia are usually complex and thought to be highly interspecific variables even between closely related species.Morphologies of the male’s first gonopod(G1)and the female vulvae are both treated as basic characteristics of species description of primary freshwater crabs and are important genitalia structures by which to complete internal fertilization.Rapid and divergent evolution of genitalia represents one of the most general evolutionary patterns in animals with internal fertilization,but the causes of this evolutionary trend are poorly understood.Three notable hypotheses,lock-and-key,sexual selection and pleiotropy have been proposed to account for animal genital evolution.However,the lack of relevant in-depth studies of genital morphology quantitative studies,especially the lack of data on female genitalia,has seriously limited our understanding of genital evolution in freshwater crabs.In this study,we quantitatively examined a total of 948 individuals from 69 species of the large Chinese potamid freshwater crab genus Sinopotamon Bott,1967(more than 80% of the described species)and a total of 818 individuals from 18 genera and 64 species of Potamiscinae in the Hengduan Mountains Region which is the highest species diversity area(84.2% of the described species in region),China.Using the geometric morphometric approach,the divergences between male and female genitalia were assessed and compared with non-reproductive structures.The relationship between male and female genitalia of Sinopotamon was assessed by reconstructing the ancestral groundplans of reproductive structures and temporal and spatial variation of the phenotypic divergence.Based on Principal Component Analysis,Canonical Variable Analysis and Cluster Analysis to ananlyze the morphology structure of freshwater crabs in the Hengduan Mountains Region.The main results and conclusions of this study are as follows:(1)The total morphological variance of the structures of the male and female genitalia of the 69 species of Sinopotamon and the three Clades was both significantly greater than that of the non-reproductive structures and the largest total variance was found in the female vulvae.Our comparative evidence uncovered rapid evolutionary divergence of the genitalia of both genders of Sinopotamon,which supports the sexual selection hypothesis of genital evolution.(2)The ancestral groundplans of genital structures from the 60 species in Sinopotamon were reconstructed and it was found that the female vulvae structures were correlated with G1 structures.The plots of the total variance of the structures through time for the lineages indicated that the genital structures underwent a consistent trend in phenotypic divergence during evolutionary time and showed a rapid shift during the Pleistocene(～0.7 Ma),suggesting that the G1 and female vulvae co-evolved.(3)The first two principal components of the dorsal and ventral views of G1,female vulvae,third maxilliped and epistome were respectively 68.122%,69.531%,88.551%,68.001% and 60.522% in 18 genera of freshwater crabs from the Hengduan Mountains Region.The deformation grids show that the variation of the structures mainly occured in the bending direction of the G1’s terminal segment,whole shape of the female vulvae,length of the third maxilliped merus,width of the third maxilliped ischium,as well as the bulge of the upper and lower edges of the epistome.(4)The results of cluster analysis showed that the Tenuipotamon(6 species were sampled,accounting for 86% of the genus)was monophyletic;Aparapotamon(10species were sampled,accounting for 91% of the genus)was a polyphyletic group,supportting A.molarum and A.inflomanum as an independent taxon;Potamiscus(8species were sampled,accounting for 73% of the genus)is also a polyphyletic group,and the taxonomic status of P.montosus,P.yiwuensis need to be further clarified.(5)The total variance of genital structures in most genera(14 genera)of the 18 genera and Montopotamon(10 genera)of freshwater crabs in the Hengduan Mountains Region is greater than that of non-reproductive structures,which supported the sexual selection hypothesis of genital evolution and promoted the rapid divergent evolution of the genitalia of freshwater crabs in this area.The geological history of this region especially the rapid uplift-driven high-altitude environment and sexual selection are critical to explain the species diversity.