Adaptations to environmental stress in altitudinal populations of two Drosophila species RAVI PARKASH, PANKAJ K TYAGI, INDU SHARMA and S. RAJPUROHIT Department of Biosciences, Maharshi Dayanand University, Rohtak, India Abstract. 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 north- ern India. The high-altitude populations are more tolerant to desiccation than those from lower altitudes, whereas the reverse trend occurs for starvation toler- ance. 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), latitu- dinal variations under tropical climatic conditions. However, significant starva- tion 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. Key words. Adaptations, altitudinal populations, climatic stress, desiccation and starvation tolerance, Drosophila takahashii, D. nepalensis, environmental stress. Introduction In all living organisms, physiological traits play a major role in conferring adaptations under varying environmental conditions (Hoffmann & Parsons, 1991; Bijlsma & Loeschecke, 1997). Wild and laboratory populations of Drosophila constitute one of the model systems for such investigations. Desiccation resistance and starvation toler- ance are complex physiological traits that may involve diverse mechanisms and have received much attention (Edney, 1977; Hoffmann & Parsons, 1989, 1991, 1993; Hoffmann & Harshman, 1999). Drosophila adults are dependent on humidity and water resources and several investigations have considered their capacity to resist heat-desiccation stress and their survival capacity in the absence of food (Parsons, 1980; Parkash & Munjal, 1999; Harshman & Hoffmann, 2000). Numerous laboratory experiments show that desiccation tolerance and starvation tolerance are genetically variable, respond rapidly to direc- tional selection and demonstrate high heritability (Hoffmann & Parsons, 1993; Blows & Hoffmann, 1993; Chippindale et al., 1996, 1998; Harshman & Hoffmann, 2000). A simultaneous increase in starvation and desicca- tion tolerance is also observed in strains selected for longer life span (Service et al., 1985; Service, 1987; Rose et al., 1992). All these observations suggest a lack of trade-off between the two traits, which may possess a common genetic basis. Patterns of geographical variation in desiccation and starvation resistance at the species, population and intra- population levels have been reviewed (Hoffmann & Harshman, 1999). Most of the earlier investigations involved analysis of desiccation resistance only and detailed studies on variation within populations under Correspondence: Ravi Parkash, U.H.-9, Medical Enclave, Rohtak-124 001, India. Tel.: þ91 1262 212577; fax: þ91 1262 243791; e-mail: rpgenetics@rediffmail.com Physiological Entomology (2005) 30, 353–361 DOI: 10.1111/j.1365-3032.2005.00470.x # 2005 The Royal Entomological Society 353