LATITUDINAL CLINES FOR MORPHOMETRICAL TRAITS IN DROSOPHILA-KIKKAWAI -A STUDY OF NATURAL-POPULATIONS FROM THE INDIAN SUBCONTINENT

Natural populations of Drosophila kikkawai were collected in India and Sri Lanka, along a latitudinal transect ranging from 6.8 degrees to 3 1.8 degrees N latitude. Six morphometrical traits were analysed: wing and thorax length, body weight, ovariole number, and abdominal and ste rnopleural bristle numbers. Significant dines were observed for the th ree size-related traits and for ovariole number, corresponding to a re gular increase in the mean value with latitude, but not for bristle nu mbers. Due to the utilization of two types of laboratory food, data we re distributed into two separate data sets. A low-nutrient food produc ed smaller flies on average because of more intense crowding. The two rearing conditions produced significant dines but with significantly d ifferent slopes. The wing/thorax ratio, which is inversely related to wing loading, also increased with latitude. The analysis of Indian cli matic conditions suggested that winter temperature, decreasing from so uth to north, could be more efficient than summer temperature, which v aries in an opposite way, as a selective factor for inducing the clina l variations. The sibling species D. leontia, which is known only from the humid tropics, was found to be much smaller than D, kikkawai and did not fit the clinal regressions. Such morphological differences sho uld help to identify the two species when found in sympatry.