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

1. 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 1h) 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.10 page(s