Date of Award
Doctor of Philosophy
Zoology and Genetics
Fredric J. Janzen
Many reptiles exhibit temperature-dependent sex determination (TSD), a sex-determining mechanism in which the incubation environment permanently determines offspring sex. This research had two main objectives: (1) to evaluate the roles of two traits (nesting behavior by females and offspring sex ratios in response to thermal incubation conditions) thought to be important for maintaining sex ratios in this system, using the painted turtle ( Chrysemys picta), and (2) evaluate the adaptive significance of TSD in a genotypic sex determining system (GSD).;Observations on the nesting behavior of painted turtles suggest that either females in this population do not use soil surface temperature as a cue for selecting nest sites, or select sites with intermediate soil surface temperatures that may be less likely to bias sex ratios.;Geographic comparisons from two populations of C. picta inhabiting differing climates (Illinois and New Mexico) demonstrated that the New Mexico population exhibited a significantly higher pivotal temperature (temperature producing a 1:1 sex ratio) than the Illinois population. However, this difference was small compared to differences in climatic conditions experienced by the populations.;Chrysemys picta nests from Illinois and New Mexico experienced similar nest temperatures despite a relatively hot year in New Mexico. Nests in New Mexico were not laid in sites most likely to reduce nest temperatures, but instead at sites experiencing high soil moisture, which indirectly reduced nest temperatures.;In simulation models, pivotal temperatures evolved more rapidly than did nest-site choice by females in response to perturbed sex ratios. Natal philopatry to nest sites also caused maladaptive nesting behavior in terms of Fisherian sex ratio selection.;Simulation models demonstrated that TSD invades populations exhibiting GSD and reaches fixation through several avenues that do not include a widely accepted adaptive function for TSD (the Charnov-Bull model).;Results from these studies suggest that the likelihood of TSD being relatively neutral compared to GSD in reptiles deserves more attention. Consequently, selection for female behavior and offspring thermal sensitivity to adaptively adjust sex ratios may be fairly weak. This conclusion is supported by the small observed differences in pivotal temperatures and lack of strong patterns of thermally-based nest-site selection.
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu
Carrie Lynne Morjan
Morjan, Carrie Lynne, "Temperature-dependent sex determination and the evolutionary potential for sex ratio in the painted turtle, Chrysemys picta " (2002). Retrospective Theses and Dissertations. 1012.