Phenotypic plasticity in response to thermal variability within and across generations

Abstract

In their natural habitats, organisms experience environmental variability throughhout their lives at multiple timescales. Yet, their ability to respond to realistic environmental challenges in the long-term is underappreciated in the field of experimental biology. This thesis seeks to address existing knowledge gaps in our understanding of organismal responses to ecologically relevant thermal variability through long-term experiments using zebrafish (Danio rerio), a model organism. Here, I examine multiple forms of phenotypic plasticity, or genotype-environment interactions that ultimately lead to phenotypic variation, both within and across generations in response to long-term thermal variability experienced from hatching through sexual maturity. Using an experimental framework that describes the contributions of both early and late life thermal environments to variation in within-generation physiology and reproduction, as well as offspring metabolism, I show that there are fundamental differences between organismal phenotypic responses to constant temperatures and thermal variability, even when thermal means are equal. I first show that zebrafish experience concurrent changes in multiple physiological traits when reared under thermal variability, including improved thermal and hypoxia tolerance, modifications to metabolic rate, and enhanced aerobic performance; these changes are largely mediated by developmental plasticity in response to early life environments. Next, I show that life-history is modified by both early and late life exposure to thermal variability, resulting in changes to spawning success, the relationship between egg quality and number, and body size in both males and females. Last, through investigating offspring metabolism of parents held under thermal variability, I show that offspring metabolism is modified based on parental early life experiences, such that early parental rearing under thermal variability reduces offspring metabolism independently of impacts on egg size. These changes may ultimately serve to enhance organismal performance in thermally variable environments, and my results suggest that phenotypic plasticity can mediate deleterious consequences of ongoing environmental change. Overall, my thesis illustrates the underappreciated capacity of organisms to plastically respond to environmental changes, given ecologically realistic conditions that span their full ontogeny.