Short Term Adult Plasticity in Drosophila Melanogaster and its Role in Climatic Adaptation

Adaptation to environmental heterogeneity is a fundamental aspect in evolutionary biology. A constantly changing environment puts continuous stress on organisms, and causes spatially and temporally varying selection regimes where survival depends on responsiveness. Phenotypic plasticity is an important mechanism enabling this responsiveness, which manifests upon exposure to an environmental stressor and facilitates a more resistant phenotype. Environmental heterogeneity exposure at the adult life stage of an organism produces a plastic response that is important for local adaptation and persistence. Hence, adaptive plasticity is an important mechanism of adaptation to localized environmental variation. To study short term exposure plasticity, we sampled Northern and Southern populations of Drosophila melanogaster, originating from distinct geographic regions and habitats in eastern North America. To elicit a plastic response these populations were exposed to two environmental variables, temperature and photoperiod, for a short-term (five-day) treatment. Flies that had been exposed to this treatment were then tested for phenotypic stress response using chill coma tolerance, heat shock and starvation resistance assays, all of which act as proxies for fitness. To test their response to the natural environment, the same populations were exposed to outdoor field conditions for a treatment equivalent to that in the lab, after which their stress response to heat and cold tolerance was recorded. Whole genome level plasticity was observed by sequencing the transcriptome of lab flies exposed to the same treatment of crossed temperature and photoperiod regimes as the phenotyped flies; thus, allowing for a complimentary gene expression plasticity study. Geographic origin and temperature treatment determined the phenotypic stress response for the three stress assays. Photoperiod showed significant interaction with temperature, indicating that D. melanogaster is responding to both cues in order to modify its life-history strategies. The field results showed the Northern population had a faster chill coma recovery time when exposed to extreme cold temperatures relative to the Southern population, where this was not observed, suggesting adaptive cold response plasticity is important in the Northern population’s fitness. Lastly, the Northern and Southern populations showed a differential expression plasticity response, which is consistent with the expected patterns based on spatially varying selection