Latitudinal clines for morphometrical traits in Drosophila kikkawai: a study of natural populations from the Indian subcontinent

Abstract

Natural populations of Drosophila kikkawai were collected in India and Sri Lanka, along a latitudinal transect ranging from 6·8° to 31·8° N latitude. Six morphometrical traits were analysed: wing and thorax length, body weight, ovariole number, and abdominal and sternopleural bristle numbers. Significant clines were observed for the three size-related traits and for ovariole number, corresponding to a regular increase in the mean value with latitude, but not for bristle numbers. Due to the utilization of two types of laboratory food, data were distributed into two separate data sets. A low-nutrient food produced smaller flies on average because of more intense crowding. The two rearing conditions produced significant clines but with significantly different slopes. The wing/thorax ratio, which is inversely related to wing loading, also increased with latitude. The analysis of Indian climatic conditions suggested that winter temperature, decreasing from south to north, could be more efficient than summer temperature, which varies in an opposite way, as a selective factor for inducing the clinal 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 should help to identify the two species when found in sympatry.