Sex Determination in Chelydra

T HE SEX OF AN INDIVIDUAL is a fundamental trait, determining (in animals) whether sperm or eggs are transmitted to form the next generation and thus the pattern of genetic contribution. At the same time, a panoply of behavioral, physiological, and morphological traits intrinsically linked to gonadal sex shape the specific phenotypes of individuals and hence the dynamics of populations. Furthermore, mechanisms of sex determination greatly influence the primary sex ratio and potentially the population sex ratio and effective population size, which are important ecological and evolutionary parameters. Indeed, Fisher (1930) demonstrated that under most circumstances 1:1 primary sex ratios are expected and sexdetermining mechanisms that produce such balanced sex ratios should be favored by selection (see also Bull 1983)

Phylogenetics: Which was first, TSD or GSD?

The basic challenge of evolutionary biology is to explain variation or the lack thereof, be it phenotypic, genetic, phy· logenetic, spatial, temporal, and so on. To illustrate, one gross generalization is that phenotypic traits we think of as being very important to organisms tend to be highly conserved (e.g .. binocular vision in vertebrates). probably because the genomic and developmental underpinnings are essentially fiXed. Thus, one striking feature about sex-determining mechanisms (SDMs), a fundamental aspect of sexual or· ganisms, is the enormous variety (Bull1983)

Temperature-Dependent Sex Determination under Rapid Anthropogenic Environmental Change: Evolution at a Turtle’s Pace?

Organisms become adapted to their environment by evolving through natural selection, a process that generally transpires over many generations. Currently, anthropogenically driven environmental changes are occurring orders of magnitude faster than they did prior to human influence, which could potentially outpace the ability of some organisms to adapt. Here, we focus on traits associated with temperature-dependent sex determination (TSD), a classic polyphenism, in a model turtle species to address the evolutionary potential of species with TSD to respond to rapid climate change. We show, first, that sex-ratio outcomes in species with TSD are sensitive to climatic variation. We then identify the evolutionary potential, in terms of heritability, of TSD and quantify the evolutionary potential of 3 key traits involved in TSD: pivotal temperature, maternal nest-site choice, and nesting phenology. We find that these traits display different patterns of adaptive potential: pivotal temperature exhibits moderate heritable variation, whereas nest-site choice and nesting phenology, with considerable phenotypic plasticity, have only modest evolutionary potential to alter sex ratios. Therefore, the most likely response of species with TSD to anthropogenically induced climate change may be a combination of microevolution in thermal sensitivity of the sex-determining pathway and of plasticity in maternal nesting behavior