Date of Completion

2021

Thesis Type

College of Arts and Science Honors

Department

Department of Biology

First Advisor

Dr. Sara Helms Cahan

Keywords

Biology, thermal stress, fruit fly, drosophila melanogaster, development, heat shock

Abstract

Animal development is a complex process that requires successful completion of multiple steps at different developmental stages to produce adult organs and systems. Environmental stress experienced during crucial developmental stages could therefore disrupt the proper functioning and survival of individuals as adults long after the stressor has passed. Early embryonic stages may be particularly susceptible to long-lasting effects because cellular mechanisms of stress resistance are relatively underdeveloped. In chronically cold or hot environments, such stress may impose significant natural selection on early embryonic developmental systems to improve developmental resilience in the face of temperature extremes.

In this study, I tested the impact of thermal stress on development in the fruit fly Drosophila melanogaster, for which the sequence of development has been well-described and is known to experience significant heat stress during embryogenesis in the field. I asked two main questions: 1) Are early embryonic stages of development more sensitive to thermal stress than later developmental stages, and 2) have hotter climates resulted in adaptive resilience to thermal stress during early embryonic development? To test whether early embryos are more sensitive to thermal stress, I compared survival, morphological and performance metrics of flies of the Canton-S strain exposed to cold or heat stress at 1, 24, or 60 hours in development. To test whether high temperatures result in adaptive resilience, two tropical and two temperate populations of D. melanogaster from around the globe were tested. The tropical populations originated from Chiapas, Mexico and Guam, and the temperate populations both originated from northern Vermont. The eggs of these populations were reared at 25°C for 1 hour before being transferred to 18°C, 25°C, 30°C, and 32°C incubators and tested to see if they also showed defects seen in Canton-S flies under thermal stress. I found that later developmental stages acclimated better to moderate thermal stress and incurred fewer lasting phenotypic consequences because of that thermal stress compared to early developmental stages. Early embryos experienced a high proportion of deformed wings and many of the pupae failed to eclose into adults. Twenty four-hour flies were found to have a greater proportion of properly developed wings, eclosed from pupae into adults at a higher proportion, and displayed superior upper and lower thermal limits than 1-hour flies.

When testing for thermal adaptation during early development between tropical and temperate populations, I found substantial variation between the tropical populations, with only the Chiapas population displaying evidence for thermal adaptation. Chiapas routinely performed better in eclosion success, climbing success, and CTmax. The Guam population, however, frequently performed equally or worse than the temperate populations. Thus, thermal adaptation during development may not have acted equally or even similarly on populations from the same climate.

Both parts of this research have important implications for the future of D. melanogaster populations as climate change will continue to affect daily and seasonal temperatures for many D. melanogaster populations. Because flies in early embryonic development are highly sensitive to moderate thermal stress, D. melanogaster populations need to have sufficient adaptive potential to adapt to changing climates during early development. Results from the Chiapas population illustrate how thermal adaptation during early development can buffer populations against moderate thermal stress, possibly allowing populations of D. melanogaster around the globe to adapt to hotter temperatures that arise from climate change.

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