Analysis of Interspecific Genital Shape Variation in Watersnakes of the Genus Nerodia and the Potential for Female-driven Species Divergence
Date
2021-07-03
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Abstract
Genital evolution can be driven by a multitude of selective pressures, including the pressure to avoid harm and hybridizations. Recent studies reveal how rapidly evolving female genital structures may drive speciation under selective processes like sexual conflict and reinforcement. This study explores whether the vaginal morphologies of watersnake sister species are diverging more rapidly than male intromittent organs. We used 2D and 3D geometric morphometrics to quantitatively compare the complex shapes of vaginal pouches and hemipenes across the three species N. sipedon, N. fasciata, and N. rhombifer. Previous work examining genital shape differences in female N. sipedon and N. fasciata revealed significant morphological differences. Similar
male genital shape within these two species would support the phenomenon of rapidly evolving female genitalia driving speciation. By also including N. rhombifer in the analysis, we were able to compare samples from a morphologically distinct relative. With the new additions to our sample, our results reinforced previous findings that show significant differences in vaginal pouch shape across species. As we predicted, pairwise
comparisons showed that the vaginal pouch shape of N. rhombifer was more distinct from N. fasciata and N. sipedon than they were from each other. Male shape was also significantly different across
all species, and the same pattern emerged in the pairwise comparisons, which does not support our prediction that hemipene morphology is indistinct across N. fasciata and N. sipedon. Although this result does not provide sufficient data to support our initial hypothesis, it does not rule out female-driven species
divergence. Future steps include using larger samples from Nerodia to explore intraspecific coevolutionary patterns and using 3D geometric morphometrics for female specimens.
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Keywords
Evolution, Geometric Morphometrics, Genitalia, Snakes