Variations in body melanisation, ovariole number and fecundity in highland and lowland populations of Drosophila melanogaster from the Indian subcontinent

We investigated geographical variations in three fitness-related traits (body melanisation, ovariole number and fecundity) in laboratory-reared offspring of 10 populations of Drosophila melanogaster. The populations were collected from adjacent lowland and highland localities (similar to 80-100 km apart) in the tropical as well as subtropical regions (11.15-31.06 degrees N) covering a linear distance about 3 000 kilometers from south to north on the Indian subcontinent. Persistence of within-as well as between-population differences at 21 degrees C suggest that observed variations in fitness-related traits have a genetic basis. Populations from higher altitudes showed consistently higher trait values (1.4-fold increase) as compared with their corresponding lowland populations. By contrast, latitudinal variations were about two-fold higher across the entire continent. Along latitude as well as altitude, population means showed higher correlation values (r > 0.98) between all the three fitness traits. However, on the basis of within-population analysis (assorted darker and lighter flies), changes in body melanisation were significantly correlated with fecundity but not with ovariole number. Thus, analysis of within-population trait variability should be preferred as compared with data on population means for adaptive significance of fitness-related traits. In the present study, the role of climatic selection is evident from regression analysis with changes in annual average temperature of the sites of origin of populations along latitude as well as altitude.