Heat-induced effects on longevity and early fecundity reveal differences between the sibling species Drosophila buzzatii and Drosophila koepferae

Sub-lethal exposure to an elevated temperature can improve some traits, including longevity and/or early fecundity (EF), a phenomenon known as hormesis. Here, sub-lethal and repeated exposures to heat were applied in two sibling species, Drosophila buzzatii (Patterson and Wheeler) and Drosophila koepferae (Fontdevila and Wasserman) (both Diptera: Drosophilidae), to assess any possible heat-induced effects on longevity and EF at benign temperature. In addition, heat-knockdown resistance was measured in each species. As in previous studies, D. koepferae was found to be a short-lived species as compared to other Drosophila species, including D. buzzatii. Heat-knockdown resistance in females was higher in D. buzzatii than in D. koepferae, but no significant difference between species was found in males in a mixed-sex environment. Hormesis in longevity was substantial in D. buzzatii, but no hormesis was found in the longevity of D. koepferae. Relative early fecundity (REF) strongly increased due to heat stress in females of D. koepferae, whereas no changes were found for this trait in D. buzzatii. These results show that the hormetic response to repeated exposures to heat can differ between very closely related species that share the same thermal environments in sympatric arid populations where there is no place to hide, differentially affecting traits of the well-known trade-off between longevity and EF. Taken together, the results from this and previous studies suggest that, in contrast to D. buzzatii, the short-lived D. koepferae appears to be the fly that not always can extend its longevity by exposures to an elevated but sub-lethal temperature. Interestingly, in contrast to longevity, EF in D. koepferae and not in D. buzzatii was found to increase due to the exposure to elevated temperature, revealing differences between these sibling species.