Evolution of Photoperiodism in the Threespine Stickleback Gasterosteus aculeatus
O’Brien, Conor Savage
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O’Brien, Conor Savage
In seasonal environments, the ability to take advantage of the favorable seasons and avoid or mitigate the effects of the unfavorable ones is essential for organismal fitness. Many polar and temperate organisms use photoperiod (length of day) to time seasonal life history events because photoperiod's regular annual cycle makes it a very reliable indicator of seasonality. This reliability allows organisms to anticipate and properly prepare for seasonal change. Although photoperiodism is widespread in polar and temperate vertebrates, little is known relative to invertebrates regarding how its use varies with environment and this method's underlying genetic and physiological basis. This dissertation is focused on demonstrating the proper methodology for the study of photoperiodism and establishing the threespine stickleback as a model of vertebrate photoperiodism. Chapter I is an introduction to photoperiodism, how it is influenced by environment, the physiological basis of its output, and a summary of the chapters that follow. Chapter II explains an analytical method to test for causality and applies this method to data that have been interpreted as evidence that the circadian clock is causally involved in photoperiodism. Chapter III describes the photoperiodic response of threespine stickleback Gasterosteus aculeatus populations from two latitudes. These results are used to inform an empirical examination of the previously described assertion that the circadian clock is causally involved in photoperiodism. Chapter IV examines the physiological basis of early photoperiodic response using the threespine stickleback as a model teleost fish. Chapter V summarizes the previous chapters, describes their significance, and suggests future research directions. This dissertation includes both previously published and co-authored material. Supplementary Excel files demonstrating the analyses used in Chapter III are also included in this dissertation.