Adaptations to environmental stress in altitudinal populations of two Drosophila species

Opposite clinal variation for desiccation and starvation tolerance are observed in four altitudinal populations (219-2202 m), each of two sympatric and cold adapted species: Drosophila takahashii and Drosophila nepalensis from northern India. The high-altitude populations are more tolerant to desiccation than those from lower altitudes, whereas the reverse trend occurs for starvation tolerance. The magnitude of tolerances are significantly high in D. nepalensis, which is better adapted to cold conditions. During winter months (November to February), there are significant decreases in T(max), T(min) and relative humidity along the altitudinal transect. Higher desiccation resistance can develop under cold conditions over short-range, altitudinally varying, geographical areas (250 km) compared with our previously reported long-range (>2000 km), latitudinal variations under tropical climatic conditions. However, significant starvation tolerances are favoured by small body size, higher dispersal rate and higher ambient temperature of the site of origin of populations. Significant correlations of two climatic factors (the mean monthly coefficients of variation of temperature and relative humidity) with these two physiological traits can best explain the observed altitudinal clinal variations under natural conditions.