Sources of variation for heat resistance in Drosophila hydei: developmental rearing and hardening acclimation, cross generational effects, (sex) and laboratory adaptation

© 2016 Michele Kenneth JohnstoneTemperature is often seen as being the most important intrinsic variable which shapes how successfully an ectotherm persists in its environment. How ectotherms respond to increases in temperature, a likely occurrence given current climate change trends, will therefore often shape their future survival and distribution patterns. Drosophila is a widely used model to study adaptation to changing thermal conditions, with studies focussing on both genetic and non-genetic factors associated with adaptation. Plastic effects on thermal resistance have been studied in Drosophila with short term (thermal shock hardening) and long term (acclimation) exposure to temperature eliciting non-genetic responses. The evolution of these plastic effects are not currently well understood, but the field remains important in determining short term responses in nature. Selection experiments measuring the underlying genetic potential of populations to modulate their upper and lower thermal limits have been carried out on several species, with both the thermal assay used as well as the environmental conditions being used for testing considered important in dictating the selection response. In this thesis, the main theme which permeates throughout involves the sources of variation which influence heat resistance in Drosophila hydei. In the second Chapter of this thesis, the presence of cross generational effects was investigated for heat resistance in D. hydei. A thermal rearing regime was developed to investigate whether these short term effects were detectable at different stages of introduction in the laboratory environment and for how long they persist. Weak evidence for cross generational effects were detected in populations newly introduced into the laboratory, however these effects were not consistent in direction. When populations were re-tested, no evidence for cross generational effects was found. As a by-product of the re-testing process, a uniform increase for both populations and experimental condition was detected for increased heat resistance. The possibility for laboratory adaptation was posed and laid the groundwork for the third Chapter of this thesis. To investigate potential laboratory adaptation, an experimental protocol involving multiple populations of D. hydei with different times spent in the laboratory but tested for their heat resistance at the same time was used. As well as using two developmental rearing temperatures, the effects of sex, and of heat hardening were incorporated into the experiment. Two experimental timepoints were used with six months separating them. Most populations were re-tested at the second timepoint which allowed direct comparison for the evolution of those populations. Evidence for laboratory adaptation was found, with the oldest population displaying significantly lower levels of heat resistance compared to all other populations. Unusually high heat resistance levels were recorded for populations newly introduced into the laboratory, while populations showed an increase in resistance between the two experimental timepoints. The effects of developmental rearing temperature were substantial, and were matched only by population differences. The influence of hardening was non-significant as a standalone variable, but it did interact significantly with the sex of an individual. This thesis has added knowledge to the field of thermal biology and has supplemented other studies showing the ability of Drosophila to modulate its thermal resistance in response to differing environmental variables. The effects of laboratory adaptation on heat resistance have rarely been considered in the literature, and this study, whilst not definitive, suggests the importance of this factor in adaptation