Multiple stress resistance traits were investigated in the cactophilic fly Drosophila buzzatii. Adults from seven populations derived from North‐Western Argentina were compared with respect to traits relevant for thermal stress resistance and for resistance to other forms of environmental stress. The populations were collected along an altitudinal gradient spanning more than 2000 m in height, showing large climatic differences. The results suggest that knock‐down resistance to heat stress, desiccation resistance and Hsp70 expression at a relatively severe stressful temperature best reflect thermal adaptation in this species. Furthermore, cold resistance seemed to be of less importance than heat resistance, at least for the adult life stage, in these populations. Clinal variation in thermal resistance traits over short geographical distances suggests relatively strong adaptive differentiation of the populations. This study provides the first evidence for altitudinal differentiation in stress‐related traits, and suggests that Hsp70 expression level can be related to altitudinal clines of heat‐stress resistance.

We tested for variation in longevity, senescence rate and early fecundity of Drosophila buzzatii along an elevational transect in Argentina, using laboratory-reared flies in laboratory tests performed to avoid extrinsic mortality. At 25 degrees C, females from lowland populations lived longer and had a lower demographic rate of senescence than females from highland populations. Minimal instead of maximal temperature at the sites of origin of population best predicted this cline. A very different pattern was found at higher test temperature. At 29.5 degrees C, longevity of males increased with altitude of origin of population. No clinal trend was apparent for longevity of females at 29.5 degrees C. There was evidence for a trade-off between early fecundity and longevity at non-stressful temperature (25 degrees C) along the altitudinal gradient. This trait association is consistent with evolutionary theories of aging. Population-by-temperature and sex-by-temperature interactions indicate that senescence patterns are expressed in environment specific ways.

Previous work showed that mild-heat stress induces longevity hormesis in a model organism, D. melanogaster. Here we compared the possible heat-induced hormesis in longevity of other species of Drosophila, D. buzzatii and its sibling species D. koepferae, in a single-sex environment. Possible correlations between longevity and heat-stress resistance were also tested by measuring longevity, heat-knockdown resistance and the heat-induced Hsp70 expression for each species in a common environment. D. buzzatii was longer lived than D. koepferae at benign temperature. Knockdown resistance to heat stress was positively correlated to longevity within species. However, the shorter-lived species was more resistant to knockdown by heat stress than the longer-lived species. The heat-induced Hsp70 expression was similar between species. A heat-shock treatment (37 degrees C for 1 h at 4 days of age) extended mean longevity in the longer lived species but not in the shorter lived species. In D. koepferae, the demographic rate of senescence decreased but the baseline mortality rate increased by heat-shock, resulting in no extension of mean longevity. Sympatric populations of closely related species can be differentially sensitive to temperature and exhibit different patterns of 37 degrees C-induced hormesis in demographic senescence and longevity. The results also show that positive correlations between stress resistance and life span within species can shift in sign across closely related species. Finally, this study shows that heat-induced hormesis in longevity can be found across different Drosophila species, as hormetic effects are not limited to the previously studied D. melanogaster.