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Abstract
Interactions between developing embryos and their environment have broad ecological and evolutionary implications. This is clearly exemplified in species with temperature-dependent sex determination (TSD), in which thermal cues experienced during a discrete window of development irreversibly determine offspring sex. This dissertation uses TSD in the American alligator (Alligator mississippiensis) as a model to address fundamental questions regarding the processes governing developmental plasticity. We investigate the ecological drivers of variation in nest thermal environments, the proximate mechanisms by which thermal cues establish sexually dimorphic phenotypes, and the adaptive consequences of links between incubation temperature and hatchling survival. In Chapter 2, using a multi-year dataset of alligator nest temperatures, we show that while intra-annual nest temperature variation is associated with maternal nest site choice, trends in maximum air temperatures drive variation in mean nest temperatures across years. Consequently, future climatic scenarios are predicted to yield substantial shifts in mean nest temperatures and resulting offspring sex ratios across the species’ range, though the specific outcomes vary widely between emission scenarios. In Chapter 3, we explore the mechanistic underpinnings of TSD and, through a series of temperature and exogenous hormone treatments, clarify the relative contributions of thermal and endocrine cues in establishing sexually dimorphic phenotypic trajectories. Upstream chromatin modifiers are among a subset of genes for which expression is regulated solely by incubation temperature and not endocrine signals. In Chapter 4, we examine how the transcriptional dynamics of chromatin modifiers, KDM6B and JARID2, and testis-associated genes, SOX9 and DMRT1, respond to thermal fluctuations between female- and male-promoting temperatures that are frequently observed in wild nests. Findings suggest embryos integrate opposing environmental signals during TSD via a temporal hierarchy of responses wherein post-transcriptional regulation and expression of chromatin modifiers respond relatively rapidly to transient thermal cues while downstream effector genes lag behind. In Chapter 5, we test how incubation temperature and phenotypic sex influence alligator hatchling survival. We find that male-promoting temperatures confer higher survival regardless of sex and when considered in the context of alligator reproductive ecology, our results provide evidence for differential survival-to-maturity as a driver of the adaptive value of TSD.