Abstract
Phenotypic plasticity can lead to phenotypic changes in response to novel environmental conditions within a single generation. Plastic responses may depend on the time point during the life cycle when environmental changes are experienced, because each life stage typically has unique physiological requirements and susceptibilities that lead to differences in sensitivity or lasting carry-over effects later in life. Additionally, sensitivity can differ between sexes, especially when exposure to environmental stressors occurs during reproduction. Therefore, studies on plasticity in reproduction need to account for potential life stage and sex differences in response in order to better understand the biological impacts of rapid environmental changes, particularly warming climates. Epigenetic modifications may play a key role in regulating phenotypic plasticity, allowing for rapid and potentially reversible changes in gene expression without altering DNA sequence. Current evidence suggests epigenetic changes such as DNA methylation occur in response to elevated temperature but epigenomic studies are rare in many groups of insects, particularly Lepidoptera. Therefore, epigenetic studies of molecular responses to heat among different sexes, tissues, and life stages are needed to broaden our understanding of molecular mechanisms mediating plasticity. This dissertation examines how the timing of heat exposure throughout the lifespan influences reproduction and patterns of DNA methylation in the European corn borer moth (Ostrinia nubilalis). To evaluate the stage-specific and carry-over effects of elevated temperature on reproduction, European corn borers were exposed to heat at different life stages and had multiple exposures to heat to test for potential additive effects. Heat exposure could influence reproduction positively (shorter development times, increased egg production) or negatively (longer development times, reduced egg production). Results indicated that exposure during pupation and continuing through adulthood led to reduced egg cluster production in mating pairs, while single-stage exposures had limited effects on reproduction. Enzymatic methyl-seq (EMseq) was then used to quantify DNA methylation changes in heat-exposed moths in two tissue types. We measured DNA methylation in abdominal tissue from individuals exposed to heat or exposed to ambient temperature. Heat exposure during pupal and adult stages led to a small but significant increase in DNA methylation genome-wide with higher methylation levels in female abdominal tissue. Differentially methylated regions localized to distinct genes in each sex and to pathways such as Hippo signaling, ubiquitin mediated proteolysis, and histone modification. Differential methylation was also found in genes that contribute to spermatogenesis and oogenesis. Patterns of DNA methylation in head segments of males and females exposed to heat were also measured. In contrast to abdominal tissue, there was higher DNA methylation in male compared to female heads, suggesting sex differences in methylation vary by the tissue type sampled. Additionally, we observed methylation shifts among sexes in genes involved in Wnt signaling and neurogenesis, which may contribute to the development of sex-specific traits in the heads of Lepidoptera. These findings highlight temperature-induced changes in reproduction, the epigenetic shifts that correspond to these changes, and how these responses may differ among life stages, between sexes, and across tissues. This work provides a basis for future comparisons of epigenetic responses to heat across species, allowing for the identification of shared or unique molecular pathways underlying responses to heat across Lepidoptera and other insect groups. This research contributes to a deeper understanding of how insects may cope with thermal stress at the molecular level, with implications for predicting species resilience in the face of climate change.