Cellular damage as induced by high temperature is dependent on rate of temperature change – investigating consequences of ramping rates on molecular and organismal phenotypes in Drosophila melanogaster Meigen 1830

Abstract

SUMMARYEcological relevance and repeatability of results obtained in different laboratories are key issues when assessing thermal tolerance of ectotherms. Traditionally, assays have used acute exposures to extreme temperatures. The outcomes of ecologically more relevant ramping experiments, however, are dependent on the rate of temperature change leading to uncertainty of the causal factor for loss of function. Here, we test the physiological consequences of exposing female Drosophila melanogaster to gradually increasing temperatures in so-called ramping assays. We exposed flies to ramping at rates of 0.06 and 0.1°Cmin -1 , respectively. Flies were sampled from the two treatments at 28, 30, 32, 34, 36 and 38°C and tested for heat tolerance and expression levels of the heat shock genes hsp23 and hsp70, as well as Hsp70 protein. Heat shock genes were upregulated more with a slow compared with a faster ramping rate, and heat knock-down tolerance was higher in flies exposed to the faster rate. The fact that slow ramping induces a stronger stress response (Hsp expression) compared with faster ramping suggests that slow ramping induces more heat damage at the cellular level due to longer exposure time. This is supported by the observation that fast ramped flies have higher heat knock-down tolerance. Thus we observed both accumulation of thermal damage at the molecular level and heat hardening at the phenotypic level as a consequence of heat exposure. The balance between these processes is dependent on ramping rate leading to the observed variation in thermal tolerance when using different rates.

Thermal Tolerance Limits of Diamondback Moth in Ramping and Plunging Assays

“…That is, the duration of exposure at higher temperatures had a highly significant impact (P>0.0001) on survival, but only marginally significant (P = 0.0423) at lower temperatures. We speculate that at higher temperatures the severity of tissue injury which could not be repaired was probably higher [5], and cell damage would occur [67]. In contrast, Chapman [53] found that the duration of exposure affected insect survival at low temperatures.…”

Plasticity and cross‐tolerance to heterogeneous environments: divergent stress responses co‐evolved in an African fruit fly

“…The rate of heating has been suggested to influence heat tolerance (Terblanche et al ., ), with slower temperature rate better at improving heat tolerance than fast rates. Slower ramping rates likely give ample time for insects to physiologically adjust (Sørensen et al ., ), for example through expression of Hsps. Therefore, during HKDT experiments, the sudden heat shock may have constrained the insects to mount compensatory adjustments physiologically (e.g.…”

Physiological responses to short-term thermal stress in mayfly (Neocloeon triangulifer) larvae in relation to upper thermal limits

“…We are concerned that studies used to infer the thermal limits of different aquatic insects often focus on short-term acute thermal challenges (critical temperature or CT max type experiments) or thermal ramping rates (see Sorensen et al, 2013) that are not typically experienced by organisms in nature. Despite excellent work showing the importance of temporal aspects of thermal change and exposure durations (Angilletta, 2009;Schulte et al, 2011;Dillon and Frazier, 2013), short-term thermal challenges remain the most common experimental approach used to study thermal physiology.…”