QUANTITATIVE GENETICS OF PLASTRON SHAPE IN SLIDER TURTLES ( TRACHEMYS SCRIPTA )

Abstract

Shape variation is widespread in nature and embodies both a response to and a source for evolution and natural selection. To detect 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 we used geometric morphometrics to assess variation in plastron shell shape in 1314 neonatal slider turtles (Trachemys scripta) from 162 clutches of laboratory-incubated eggs from two nesting areas. Multivariate analysis of variance indicated that nesting area has a limited role in describing plastron shape variation among clutches, whereas differences between individual clutches were highly significant, suggesting a prominent clutch effect. The covariation between plastron shape and several possible maternal effect variables (yolk hormone levels and egg dimensions) was assessed for a subset of clutches and found to be negligible. We subsequently employed several recently proposed methods for estimating heritability from shape variables, and generalized a univariate approach to accommodate unequal sample sizes. Univariate estimates of shape heritability based on Procrustes distances yielded large values for both nesting populations (h2 approximately 0.86), and multivariate estimates of maximal additive heritability were also large for both nesting populations (h2max approximately 0.57). We also estimated the dominant trend in heritable shape change for each nesting population and found that the direction of shape evolution was not the same for the two sites. Therefore, although the magnitude of shape evolution was similar between nesting populations, the manner in which plastron shape is evolving is not. We conclude that the univariate approach for assessing quantitative genetic parameters from geometric morphometric data has limited utility, because it is unable to accurately describe how shape is evolving.

Pivotal temperature and sexual dimorphism of Podocnemis expansa hatchlings (Testudines: Podocnemididae) from Bananal Island, Brazil

“…Nonetheless, these differences enrich our results as they provide one more indication that GM may be suitable for population differentiation as well as sex differentiation. A similar conclusion had been reached by MYERS et al (2006) who were able to discriminate between two populations of Trachemys scripta (Schoepff, 1792), separated by less than 5 km, based on the shape of the plastron using GM methods. In conclusion, our results suggest that, in addition to metapopulation structure (SITES JR et al 1999, BOCK et al 2001, the carapace shape variation of P. expansa and this study) may provide a feasible, non-invasive and cheap method to differentiate between populations, what can be very useful in ecological studies.…”

“…I order to ascertain the relationship between sex dimorphism and carapace shape we used GM methods previously employed by MYERS et al (2006). For our analyses we quantified carapace shape in the following manner.…”

Geometric morphometrics in herpetology: modern tools for enhancing the study of morphological variation in amphibians and reptiles

“…Also trying to characterise the direction of phenotypic change, STAyTon (2006) proposed a new method for quantifying data disparity patterns and used it to test for convergence in skull shape among herbivorous lizards. In the field of quantitative genetics of shape, MyErS et al (2006) generalized the univariate approximation of shape heritability based on procrustes distance (MonTEIro et al, 2002) for application with unequal sample sizes among families, thus complementing other existing multivariate methods (i.e. kLInGEnBErG, 2003;kLInGEnBErG & MonTEIro, 2005), and used it to study patterns of plastron shape heritability in slider turtles.…”

Geometry and self-righting of turtles

“…temperature (Elnitsky & Claussen 2006). Both righting behaviour (Ashe 1970;Wassersug & Izumi-Kurotani 1993;Rivera et al 2004;Stancher et al 2006) and the evolution of shell morphology (Rouault & Blanc 1978;Claude et al 2003;Myers et al 2006) of turtles have been studied recently. An example of their interaction is the sexual dimorphism of species where males are often overturned during combats (Bonnet et al 2001;Willemsen & Hailey 2003;Mann et al 2006), and their shell has adapted to facilitate righting.…”