Phenotypic plasticity in upper thermal limits is weakly related to Drosophila species distributions

Abstract

Summary1. Acclimation and hardening represent examples of phenotypic plasticity, the extent to which phenotypes produced by the same genotype vary under different environments. Widespread species are expected to differ in thermal plasticity from narrowly distributed tropical species, but this has rarely been tested particularly when species are reared under the same conditions. 2. We investigated acclimation and hardening responses of 11 widespread or tropically restricted Drosophila species from Australia using estimates of heat resistance where temperatures were increased suddenly (static measure) or slowly (ramping measure), and after controlling for phylogenetic relatedness. We predicted that restricted species would show little acclimation regardless of the method used, whilst widespread species would respond well after a hardening treatment (35°C for 1 h) particularly under ramping. 3. These predictions were partially supported. There was a tendency for the tropically restricted species to be less plastic than the widespread species, although variation among species within the two groups was generally greater than between the groups. For acclimation and stress resistance measured under ramping acclimation, there was an association between the southernmost latitude at which species were found (reflecting variability in climatic conditions they encountered) and knockdown resistance after controlling for phylogeny. There was also evidence of significant divergence from the ancestral state in the ramping trait, likely reflecting a history of direct or indirect selection for ramping knockdown resistance in Drosophila. 4. There was a significant negative association between basal resistance and hardening capacity for static acclimation in the widespread species, suggesting a limit to the extent that plastic responses vary independently of basal resistance. 5. The reduced plastic response in tropically restricted species and negative association between hardening and basal resistance suggest a limit to the effectiveness of plastic responses in changing upper thermal limits for countering increases in thermal stress under future climate change.

Evidence for lower plasticity in CTMAX at warmer developmental temperatures

“…At the intraspecific level, a weak negative association between plasticity (measured as an adult hardening responses) and latitude was detected in D. melanogaster , whereby tropical populations were more plastic than their subtropical/temperate counterparts (Sgro et al ., ), which is inconsistent with the climatic variability hypothesis. At the interspecific level, significant adult hardening responses in heat knockdown have been shown in a number of Drosophila species (Kellett et al ., ; Mitchell et al ., ); although plasticity was only weakly associated with the environment (southern latitude used as a proxy for environment) in one study (Mitchell et al ., ), such that tropical species tended to be less plastic than their widespread counterparts. Significant developmental acclimation effects have also been detected at the interspecific level in Drosophila for CT MAX , but plasticity was unrelated to the species’ environment or latitude of origin (Overgaard et al ., ; Schou et al ., ).…”

“…The inconsistencies in the above studies could be driven by a number of factors. Primarily, the number of treatments used to assess plasticity is often fewer than four (Mitchell et al ., ; Overgaard et al ., ; Gunderson & Stillman, ; Seebacher et al ., ), meaning that plastic responses are often assumed to be linear, even though plastic responses often take on a quadratic shape (Murren et al ., ; but see Schou et al ., ). Failure to capture the range of the plastic response may result in vastly different conclusions being made, particularly if plasticity increases/decreases beyond the thermal window examined.…”

Tolerance to High Temperature Extremes in an Invasive Lace Bug, Corythucha ciliata (Hemiptera: Tingidae), in Subtropical China

“…Insects maintain their physiological functions only within a specific range of temperature, and temperature extremes may reduce their survival and reproduction [13]. Temperature can limit geographic range of herbivorous insects by causing direct mortality or by limiting the distribution of host plants [14], [15]. The increasingly abrupt heat waves caused by climate change might increase the frequency of insects' exposure to high temperatures that may alter physiological tolerance to environmental stress and may, therefore, cause the changes in biology and distribution of insects [16].…”

Ecologically relevant measures of tolerance to potentially lethal temperatures

“…Sexes were separated without use of CO 2 to avoid additional stress (Champion de Crespigny and Wedell, 2008). The experimental procedures closely followed those described in Mitchell et al (Mitchell et al, 2011) for both ramping and static temperature stress experiments. Briefly, flies were put into small vials with or without 2ml of fly medium added.…”

“…To test the potential association between thermal stress and desiccation/energy expenditure more generally in D. melanogaster, we used flies from a culture originating in Gordonvale, North Queensland, previously described in Mitchell et al (Mitchell et al, 2011). The culture was maintained as a massbred population at a density of 400 individuals per 250ml bottle.…”