How to assess Drosophila cold tolerance: chill coma temperature and lower lethal temperature are the best predictors of cold distribution limits

Abstract

Summary 1. Thermal tolerance may limit and therefore predict ectotherm geographic distributions. However, which of the many metrics of thermal tolerance best predict distribution is often unclear, even for drosophilids, which constitute a popular and well-described animal model. 2. Five metrics of cold tolerance were measured for 14 Drosophila species to determine which metrics most strongly correlate with geographic distribution. The species represent tropical to temperate regions but all were reared under similar (common garden) conditions (20°C). The traits measured were: chill coma temperature (CT min ), lethal temperature (LTe 50 ), lethal time at low temperature (LTi 50 ), chill coma recovery time (CCRT) and supercooling point (SCP). 3. Measures of CT min , LTe 50 and LTi 50 proved to be the best predictors to describe the variation in realized latitudinal distributions (R 2 = 0Á699, R 2 = 0Á741 and 0Á550, respectively) and estimated environmental cold exposure (R 2 = 0Á633, R 2 = 0Á641 and 0Á511, respectively).Measures of CCRT also correlated significantly with estimated minimum temperature (R 2 = 0Á373), while the SCP did not. These results remained consistent after phylogenetically independent analysis or when applying nonlinear regression. Moreover, our findings were supported by a similar analysis based on existing data compiled from the Drosophila cold tolerance literature. 4. Trait correlations were strong between LTe 50 , LTi 50 and CT min , respectively (0Á83 > R 2 > 0Á55). However, surprisingly, there was only a weak correlation between the entrance into coma (CT min ) and the recovery from chill coma (CCRT) (R 2 = 0Á256). 5.Considering the findings of the present study, data from previous studies and the logistical constraints of each measure of cold tolerance, we conclude that CT min and LTe 50 are superior measures when estimating the ecologically relevant cold tolerance of drosophilids. Of these two traits, CT min requires less equipment, time and animals and thereby presents a relatively fast, simple and dynamic measure of cold tolerance.

An invitation to measure insect cold tolerance: Methods, approaches, and workflow

“…(Hazell and Bale, 2011). Because insects cannot move, feed, reproduce, or evade predators while in chill coma, the CT min provides a useful lower limit to insect function (Andersen et al, 2015b;David et al, 1998;MacMillan and Sinclair, 2011a), and is often used to approximate insect cold tolerance because it is broadly correlated to lethal limits (Andersen et al, 2015b). Resumption of movement after rewarming (chill coma recovery, CCR) is usually measured as the amount of time it takes insects to recover from a standardised period in chill coma (David et al, 1998;MacMillan and Sinclair, 2011a).…”

“…Additionally, because chill coma is usually non-lethal, it is possible to use insects for further experiments, or even breed from them, allowing for selection experiments (e.g. Bertoli et al, 2010;Franke et al, 2012;Mori and Kimura, 2008 and Sinclair, 2011a), CT min and CCR are not measures of cold tolerance, but rather of resistance to the effects of cold (although it provides a useful estimate of thermal biology for comparative purposes; Andersen et al, 2015b). CT min is also limited as a measurement of resistance to cold, because it can only be measured in mobile life stages; specifically, monitoring when an insect enters chill coma requires identifying and timing the loss or recovery of some form of movement.…”

“…Andersen et al, 2015b;David et al, 2003;Findsen et al, 2013;Macdonald et al, 2004;Ransberry et al, 2011). has a lower bound (an insect can't take a negative time to recover), survival analyses or generalized linear models might be more appropriate than linear parametric statistics such as ANOVA.…”

“…In Drosophila, for example, the CT min provides a reasonably good proxy for cold tolerance (including LT 50 ), and is thus a parsimonious choice for comparison among treatments (Andersen et al, 2015b). If this information is not available, then choosing an appropriate metric is going to be based partially on the life history of the organism, and striking a useful measure may be somewhat dependent on luck.…”

“…Similarly, insects in deserts and tropical high mountains can also be regularly exposed to potentially-lethal freezing conditions (Sømme, 1995;Sømme et al, 1996;Sømme and Zachariassen, 1981). Thus, low temperature biology is a key component of insect fitness, and one of the best determinants of insect distribution (Andersen et al, 2015b). …”