Speed of exposure to rapid cold hardening and genotype drive the level of acclimation response in Drosophila melanogaster

Authors: Gerken, Alison; ELLER-SMITH, OLIVIA - University Of Kansas; MORGAN, THEODORE - Kansas State University

Submitted to: Journal of Thermal Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/25/2018
Publication Date: 6/26/2018
Citation: Gerken, A.R., Eller-Smith, O.C., Morgan, T.J. 2018. Speed of exposure to rapid cold hardening and genotype drive the level of acclimation response in Drosophila melanogaster. Journal of Thermal Biology. 76:21-28. https://doi.org/10.1016/j.jtherbio.2018.06.011.
DOI: https://doi.org/10.1016/j.jtherbio.2018.06.011

Interpretive Summary: Laboratory experiments may not reproduce thermal environments as accurately as what insects experience in the wild. However, we can compare differences in standard protocol and more environmentally realistic protocol to understand if there are differences in insect survivorship to cold stress under these two conditions. Most insects show an increase in survivorship if they are exposed to a mild cold temperature before exposure to an extreme cold temperature. This is known as rapid cold hardening and has been shown to induce physiological changes that prepare the insect for the extreme cold. Here, we find that for the fruit fly Drosophila melanogaster slowly decreasing temperature before exposure to extreme cold leads to lower survivorship compared to both a faster decrease in temperature and a direct exposure from room temperature to mild cold temperature to extreme cold temperature. However, among our populations of insects tested, each population has a specific rank in survivorship regardless of preparatory exposure. In addition, if insects are rewarmed to room temperature following exposure to extreme cold temperatures, they may be able to adjust their physiology and increase their survival. We find that this is not the case; insects exposed to both a preparatory treatment and a rewarming treatment have the lowest survival of any experimental exposures tested here. This study suggests that there is an innate level of cold tolerance in a given population, regardless of preparatory exposure, and rewarming following a pre-treatment is detrimental to the survival of these insects.

Technical Abstract: Mimicking ecologically relevant conditions is a goal when evaluating thermal tolerances of ectotherms since exposing animals to artificial thermal regimes may not provide an accurate picture of a thermal profile. Rapid cold hardening (RCH) occurs in ectotherms and generally increases survivorship through a pre-exposure to non-lethal temperature. Here we assess survivorship across a subset of the Drosophila melanogaster Reference Panel for direct RCH and ramping RCH pre-exposures at semi-natural cooling rates (0.1 deg C/min and 0.5 deg C/min) as well as post-treatment warming. We find that all three pre-treatment exposures significantly increase survivorship. Even though the magnitude of survivorship across each treatment was significantly different, we find significant correlations in survivorship across genotypes, suggesting that regardless of the pre-treatment, genotypes have an innate ability to undergo RCH. When rewarming is introduced, survivorship significantly decreased compared to pretreatment alone. Our results suggest rewarming is detrimental to survival while a slower ramping RCH has lower survivorship than direct or fast ramping RCH most likely due to the increased duration at 4 deg C which could allow the flies to more successfully modulate their physiology rather than having to continuously adjust as ramping occurs.