The detrimental acclimation hypothesis

Abstract

Response from Wilson and FranklinLoeschcke and Hoffmann [1xSee all References][1] highlight some important research that reveals several of the molecular mechanisms underlying acclimatory responses for heat hardening in Drosophila. However, we disagree with their specific conclusion that ‘for long-term acclimation, the optimal developmental temperature hypothesis…will usually apply and the effects of acclimation will usually be detrimental’. In this context, we believe that Loeschcke and Hoffmann are equating acclimation with exposure of the organisms to the stressful environment rather than strictly to the physiological response. We suspect our differences in opinion are symptomatic of a larger problem within evolutionary and comparative physiology, which is the absence of a consistent and precise definition of acclimation.Our recent TREE article [2xTesting the beneficial acclimation hypothesis. Wilson, R.S. and Franklin, C.E. Trends Ecol. Evol. 2002; 17: 66–70Abstract | Full Text | Full Text PDF | Scopus (185)See all References][2] was motivated by the confusion surrounding interpretations of tests of the beneficial acclimation hypothesis (BAH), which probably stems from the inconsistent and confusing use of the term acclimation. Throughout the literature, there are many definitions of acclimation [3xThe influence of temperature and thermal acclimation on physiological function. Rome, L.C. et al. : 183–205See all References][3]; interestingly, none of which has taken advantage of recent advances in molecular biology. If acclimation is to remain a useful concept within biology, and not just equated with the more generic term phenotypic plasticity, then we need a precise definition.The concept of acclimation has a rich tradition within comparative physiology and has most commonly been considered as a facultative physiological response to environmental change. However, recent tests of the BAH have seen it used interchangeably with the term phenotypic plasticity. The expression ‘facultative response’ is an important consideration, because it emphasizes that an organism utilizes sensory mechanisms to perceive the state of the environment to activate effectors aimed at preventing the new environment from producing a certain undesirable effect [4xWilliams, G.C. See all References][4]. This is in sharp contrast to direct environmental effects or susceptibilities that result from environmental factors getting through and producing a change in spite of any responses that might be activated (4). It is these facultative physiological responses, and not the entire suite of phenotypic changes resulting from environmental variation, that are salient to the study of acclimation.Given our increased understanding of the molecular mechanisms underlying acclimatory responses, as highlighted by Loeschcke and Hoffmann, we feel that it is possible to formulate a more restrictive and useful contemporary definition. We advocate defining acclimation as ‘a physiological response involving sensory mechanisms detecting an environmental change and effecting a gene-regulated change in phenotypic expression’. This would effectively restrict acclimation to gene-regulated physiological responses and exclude direct environmental susceptibilities. Tightening of the current definition of acclimation is not purely a semantic issue if biologists want to maintain it as a more specific case of physiological phenotypic plasticity.