Genetic variation in the shape of cold-survival curves in a single fly population suggests potential for selection from climate variability.

Temperature variation is one of the primary challenges facing ectotherms, and the ability to tolerate a range of thermal environments is critical for setting current and future species distributions. Low temperature is particularly challenging for ectotherms because winter conditions have strong latitudinal and temporal variation. Lower lethal temperature (LLT) is a common metric of cold tolerance used in studies of local adaptation and plasticity. Comparisons of LLT across groups typically assume parallel S-shaped survival curves, but genetic variation in the shape of survival vs. temperature curves has not been assessed. Here, we measured the ability of 36 lines of the Drosophila Genetic Reference Panel (DGRP) to survive a 1-h cold shock at seven ecologically relevant low temperatures (-1 to -7 °C) to create a high-resolution response curve for each genotype. We observed surprising variation both in the magnitude of survival and in the shapes of the response curves, with the curves clustering into four distinct shapes. To encompass variation in the shapes of these survival curves, we developed a new cold tolerance metric, cumulative cold tolerance (CCT). By comparing our survival data with climatological data, we propose that variation in the shapes of cold-survival curves arose from weak selection pressure to survive intermediate subzero temperatures in this mid-latitude population of flies. Using publicly available genome sequence and transcript expression data for these lines, we identified several candidate genes associated with CCT, and using transgenic RNAi, we confirmed a functional role for many of these genes.

Responses of the bed bug, Cimex lectularius, to temperature extremes and dehydration: levels of tolerance, rapid cold hardening and expression of heat shock proteins.

This study of the bed bug, Cimex lectularius, examines tolerance of adult females to extremes in temperature and loss of body water. Although the supercooling point (SCP) of the bed bugs was approximately -20 degrees C, all were killed by a direct 1 h exposure to -16 degrees C. Thus, this species cannot tolerate freezing and is killed at temperatures well above its SCP. Neither cold acclimation at 4 degrees C for 2 weeks nor dehydration (15% loss of water content) enhanced cold tolerance. However, bed bugs have the capacity for rapid cold hardening, i.e. a 1-h exposure to 0 degrees C improved their subsequent tolerance of -14 and -16 degrees C. In response to heat stress, fewer than 20% of the bugs survived a 1-h exposure to 46 degrees C, and nearly all were killed at 48 degrees C. Dehydration, heat acclimation at 30 degrees C for 2 weeks and rapid heat hardening at 37 degrees C for 1 h all failed to improve heat tolerance. Expression of the mRNAs encoding two heat shock proteins (Hsps), Hsp70 and Hsp90, was elevated in response to heat stress, cold stress and during dehydration and rehydration. The response of Hsp90 was more pronounced than that of Hsp70 during dehydration and rehydration. Our results define the tolerance limits for bed bugs to these commonly encountered stresses of temperature and low humidity and indicate a role for Hsps in responding to these stresses.