Modularity and integration of copulatory structures in male Ratfish, Hydrolagus colliei



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Genitalia are one of the most diverse structures in nature. Different evolutionary processes, such as natural and sexual selection, influence their diverse and complex morphology. A combination of these processes may operate simultaneously in complex genital structures on a single system because they can have multiple modules and different genes that are responsible for the phenotype development. Male spotted ratfish, Hydralogus colliei, have modular structures that function together to achieve copulation, including two grasping structures, a frontal tenaculum paired pre-pelvic tenacula, and one intromittent structure, paired claspers. We produced 3D models of structures of adult and juvenile males and used a 3D geometric morphometric approach to study their allometric patterns, and their integration. We asked what the roles of ontogeny and function are in shaping the modular patterns and the integration among the components of these complex genital traits because they are unlikely to share the same development. In adults, we found that there is no significant relationship between body size and the pelvic tenacula and claspers, but the relationship between body size and frontal tenaculum size was significant. In juveniles, we found different significant relationships between body size and the copulatory structures. Interestingly, we found that the structures’ centroid size and shape change suddenly and significantly when the juveniles become adults. This discrete rather than continuous growth pattern for genitalia has not been previously described in vertebrate groups.



Biological Sciences, Evolution, Genital Morphology, Sex, Ratfish, Chimera