Invertebrate Responses to Stressors: Physiology and Life-History, Phenotypic Plasticity and Microevolution
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Authors
Lee, Ting Yat
Date
2025-09-04
Type
thesis
Language
eng
Keyword
Entomology , Life history , Insecticide resistance , Hypoxia , Adoxophyes honmai , Daphnia
Alternative Title
Abstract
Environmental stressors may negatively affect individual fitness. To survive and reproduce under stress, individuals allocate resources to mechanisms that buffer against stress. Over time, divergent environments can drive adaptation, although the evolution of energetically costly traits may carry trade-offs. In this thesis, I examined how organisms demonstrate phenotypic plasticity in transcription and life history to cope with stressors, and how these responses vary across populations with different evolutionary histories. I also investigated how life-history traits covary under stress, and whether traits that promote survival impose fitness trade-offs.
I began by experimentally exposing individuals from field populations that may have evolved under different selection pressure to the same stressor. Using two model systems, Daphnia under hypoxia and Adoxophyes honmai with tebufenozide, I found that stress triggered upregulation of specific transcripts related to oxygen transport and anaerobic metabolism or detoxification. Populations also differed in both baseline and inducible transcription, indicating evolutionary divergence in transcriptional regulation.
Next, I investigated whether the evolution of insecticide resistance was associated with life-history trade-offs. Despite evidence that the resistant A. honmai population had acquired multiple resistance mechanisms, resistant individuals did not experience detectable fitness costs under a range of conditions. Instead, they often outperformed susceptible counterparts. My transcriptomic and food-limitation experiments suggest that enhanced digestive capacity may have offset potential energetic costs.
Finally, I assessed the potential for transgenerational plasticity in response to chlorfenapyr. Although sublethal chlorfenapyr exposure in both generations reduced performance, parental exposure had no measurable effects on offspring life history. Importantly, life-history traits were positively associated, and these correlations strengthened with insecticide exposure, suggesting a common mechanism that allows for higher fitness in more demanding environments.
Together, these findings reveal how gene expression and life-history traits are shaped by evolutionary history and environmental conditions. I quantified sources of trait variation across generations and found that resistance traits may not incur fitness penalties. Rather, resistance was aligned with increased performance. These insights underscore the importance of understanding how resistance traits interact with life history, as trait covariation plays a central role in shaping the trajectory of resistance evolution and its management in pest populations.