MYERS, E.M.; JANZEN, F.J.; ADAMS, D.C.; Iowa State University; Iowa State University; Iowa State University: Quantitive Genetics of Shape Variation in the Slider Turtle (Trachemys scripta)
Shape variation is widespread in nature and embodies both a response to and a source for evolution and natural selection. To detect microgeographic patterns of shape evolution, one must assess the quantitative genetic underpinnings of shape variation as well as the selective environment that the organisms have experienced. Here wWe used geometric morphometrics to assess variation in plastron shell shape in neonatal slider turtles (Trachemys scripta) from twenty-two clutches of lab-incubated eggs from two nesting beaches. The covariation between plastron shape and several possible maternal effect variables (yolk hormone levels and egg dimensions) was assessed, and found to be negligible. We subsequently employed two recently proposed methods for estimating heritability from these shape variables, and generalized the univariate approach to accommodate unequal sample sizes. Multivariate analysis of variance indicated that nesting beach has a limited role in describing plastron shape variation among clutches, while differences between individual clutches were highly significant, suggesting a prominent clutch effect. Further analysis to resolve a potential genetic, as opposed to environmental, cause suggested that shape variation may have a heritable component. Both the univariate and multivariate methods of estimating heritability yielded a nearly equivalent value for shape heritability (h2 = 0.6). Heritable shape variation is pronounced in this, and likely other, species of turtles and vertebrates. We discuss possible evolutionary implications of heritable plastron shape.
