Desiccation and starvation resistance in Drosophila: patterns of variationat the species, population and intrapopulation levels

A substantial number of Drosophila studies have investigated variation in d esiccation and starvation resistance, providing an opportunity to test for consistent patterns of direct and correlated responses across studies and a cross the species and population levels. In general, responses to laborator y selection for these traits in D. melanogaster are rapid and indicate abun dant genetic variation in populations. However, slower responses to selecti on for desiccation resistance occur in other species including D. simulans. Clines suggest adaptive divergence although specific selection pressures h ave not been documented empirically. Drosophila species differ markedly in desiccation and starvation resistance and there is also marked variation wi thin species for desiccation resistance that may be linked to local climati c conditions. Laboratory selection experiments on starvation resistance in D. melanogaster suggest that changes in lipid content are largely responsib le for resistance variation but this factor may be less important in explai ning variation among species. For desiccation, lines with increased resista nce show reduced rates of water loss but no changes in the minimum water co ntent that flies can tolerate. Changes in life history traits are sometimes associated with altered levels of stress resistance. Increased starvation resistance is associated with longer development time and reduced early age reproduction in different studies. However, other associations are inconsi stent between studies as in the case of stress resistance changing followin g selection for longevity. Multiple mechanisms may underlie genetic variati on in stress resistance and future studies should address the evolutionary importance of the different mechanisms at the population and species levels .