Graduation Term

2022

Degree Name

Doctor of Philosophy (PhD)

Department

School of Biological Sciences

Committee Chair

Rachel M Bowden

Committee Member

Ryan T Paitz

Abstract

In turtles and numerous other reptilian species, sex is determined by the environmental temperatures during embryonic development, which is referred to temperature-dependent sex determination (TSD). In the type of TSD common in turtles, cool temperatures (below 29 °C) during embryonic development produce mainly males while warm temperatures (above 29 °C) produce mainly females. Gonadal differentiation in TSD species involves numerous genes that are also necessary to produce a testis or ovary in species with genotypic sex determination. However, most of what we know about TSD comes from studies performed in the laboratory using constant temperature incubation conditions, which are not reflective of natural temperatures that fluctuate daily and/or seasonally. As climate change models predict that heat waves will increase in length, frequency, and intensity as climate change progresses, it is of utmost importance to understand how more ecologically relevant incubation conditions affect thermally sensitive species, such as species with TSD. My research with the red-eared slider turtle (Trachemys scripta) focuses on how more variable incubation conditions, such as heat waves varying in timing or continuity, affect sex determination and the expression of underlying genes. Results show that when heat waves, coupled with fluctuating temperature regimes, were applied during the middle third of development it resulted in female-biased sex ratios, while heat waves applied before or after this period resulted in male-biased sex ratios. Results also show that continuous heat waves produce more females compared to the same amount of heat exposure broken up in a discontinuous fashion. More continuous heat waves also induce expression of Cyp19A1, a gene involved in ovarian development while discontinuous heat waves do not. My research also shows that expression of Kdm6b, a gene involved in testicular development, is significantly lower under fluctuating, cool temperatures compared to constant, cool temperatures. Lastly, results show that exposing embryos to conflicting thermal cues delays commitment to gonadal fate, in that early heat exposure causes embryos to stay sensitive to heat exposure longer into developmental stages that are later than those traditionally considered the thermosensitive period. These results highlight the importance of studying how temperature affects biological processes using ecologically relevant temperatures; using constant temperatures ignores natural variability in temperature. Understanding of how biological processes operate under natural conditions is critically needed, particularly in the face of a rapidly changing climate.

Access Type

Dissertation-Open Access

DOI

https://doi.org/10.30707/ETD2022.20220606094359295965.999993

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