UC San Diego

As the climate warms, the response of organisms to rising temperatures has become an area of increasing interest. Much research is focused on understanding the basis for thermal tolerance, which can help us predict the capacity for adaptation or shifting range limits in response to changes in climate. Of particular interest are intertidal ectotherms, which experience highly variable environmental conditions. One such organism is Tigriopus californicus, a copepod found in high rocky tidepools along the west coast of North America. Its relatively low gene flow contributes to genetic isolation of populations, allowing the study of local adaptation along a latitudinal gradient. Previous studies have shown that southern populations have higher

survivorship following heat stress than northern populations, which is correlated with higher upregulation of important heat shock protein (HSP) genes. However, the physiological mechanisms and gene regulation patterns underlying thermal tolerance are not fully understood. In order to address these questions, thermal performance under acute and chronic thermal stress conditions and at abrupt and gradual ramping rates was assessed in three populations of T. californicus distributed from south to north. Additionally, gene regulation during the heat shock response was examined using knockdown of the heat shock transcription factor (HSF-1) gene. We found that at acute thermal exposures, survivorship and mitochondrial performance follow a latitudinal gradient. Chronic thermal performance is more complex, however, with the mid-latitude population showing decreased performance compared to both the southern and northern populations at lower temperatures. Gene regulation during the heat shock response is similarly crucial to organismal performance, and a knockdown of HSF-1 indicates a complex network of gene interactions. Finally, we found that a slower rate of thermal exposure similar to conditions in the intertidal allows organisms to more highly upregulate important HSPs, conferring protection and minimizing harmful effects of acute thermal stress. These findings allow us to better understand the mechanisms underlying thermal tolerance, determine why certain populations or species outperform others, and predict organismal responses to changing climatic conditions in the future.