Adaptation of Drosophila to temperature extremes: bringing together quantitative and molecular approaches

Abstract

Although adaptation of Drosophila to thermal extremes has been investigated for many years, only recently has much progress been made in identifying the genetic and physiological basis of evolutionary shifts in thermoresistance. Here we examine the way the Drosophila research has been used to understand the evolution of plastic responses, tradeoffs and limits to selection, and to develop links between laboratory studies and adaptive shifts leading to population and species differences. Several methods have been devised to rapidly measure heat and cold resistance, but the relevance of these measures to selection pressures in nature remains largely unknown. Plastic responses to thermal extremes are usually divided into short-term exposures to sub-lethal conditions or into longer-term exposures (often referred to as hardening and acclimation respectively). Hardening responses appear to have costs associated with the expression of a heat shock protein (Hsp70). Costs of acclimation are more difficult to identify because exposing Drosophila to suboptimal conditions for a long time can have deleterious effects unrelated to the acclimation response. Quantitative genetic analyses have revealed genetic variation for thermoresistance under laboratory conditions, but variation under natural conditions has rarely been identified. In a few cases selection responses within laboratory populations have been linked to specific candidate genes and physiological mechanisms. Population comparisons have provided evidence for clinal variation in thermoresistance traits, although many studies lack power because only a few populations have been considered. Clinal patterns in candidate genes have also been demonstrated. However evidence for direct selection for thermoresistance and for the involvement of specific genes under natural conditions is mostly lacking. Clinal responses to cold extremes can involve changes in diapause strategies and altered patterns of reproduction. Inbreeding influences thermoresistance and acclimation responses, but inbreeding effects may be environment-specific. Species differences in heat or in cold resistance commonly match the geographical (climatic) distributions of species. Interspecific differences for heat resistance are usually smaller than for cold resistance. Drosophila species from the same location can differ markedly for stress resistance, and this may allow species to occupy different niches. Rapid progress is likely in the next few years in identifying genes and traits underlying variation in stress resistance among populations and species of Drosophila, and in comparing these findings to those from other taxa.