How does the phenotype respond to environmental inputs?
Most of evolutionary biology relies on the assumption that genetic variation (i.e. differences in genomic composition) underlies the diversity of the phenotypes that are exposed to natural selection and allows its evolution. But a great proportion of the phenotypic variation we observe in nature derives from environmental sensitivity of the genome which influences its expression during development (regulatory differences). Discrete traits such as alternative morphs (e.g. threshold traits) may result from environmental modulation of the expression of major genes, rather than from the added expression of quantitative genes.
TSD represents a form of phenotypic plasticity (a thermal sexual polyphenism) where identical genomes can permanently differentiate into either sex depending on the environmental conditions. Our work on sex determination helps us understand the basis of phenotypic plasticity and the recurrent evolution of developmental canalization (GSD) in a group of closely related taxa.
The role of growth plasticity on sexual dimorphism
We found that sex-specific plasticity, the differential response of the genome of males and females to different environments, mediates the degree of sexual dimorphism in the snapping turtle (Chelydra serpentina), a species with male-larger sexual size dimorphism (Ceballos and Valenzuela 2011).
We are currently studying if the same is true in species with female-larger dimorphism and whether sex-specific plasticity plays a role in shaping macroevolutionary patterns such as Rensch's rule in turtles (Ceballos et al. 2012). For this, we investigated the plasticity of body growth in Podocnemis expansa, a species we have studied intensely from a sex determination and ecological genetics perspective (Valenzuela 2000, Valenzuela 2001a,b,c). his project was funded in part by the National Science Foundation DEB 0808047 and the Turtle Conservation Fund.