Adaptive differentiation of thermotolerance in Drosophila along a microclimatic gradient

We examined whether a remarkable occurrence - the physiological evolution of two Drosophila melanogaster populations, despite a spatial separation of only 100-400 m, was idiosyncratic and temporary, or persisted over multiple years. We ascertained the high-temperature tolerance of Drosophila descended from populations on the north-facing slope (NFS) and south-facing slope (SFS) of 'Evolution Canyon' (Lower Nahal Oren, Mt Carmel, Israel), which were collected in 1997, 1999, and 2000. Results for these Drosophila uniformly resembled other studies in many respects: an inverse relationship between survival and heat-shock temperature, male-female differences in thermotolerance, and inducible thermotolerance. Importantly, for all years of collection, SFS flies consistently exceeded NFS flies in basal and inducible thermotolerance after diverse heat shocks, with and without thermal pretreatment, and whether isofemale lines, synthetic populations, or inbred lines were compared. Inbred lines, however, had lower thermotolerance than outbred lines. Several nonexclusive processes may explain the evolution of such physiological differentiation.