Thoracic trident pigmentation in Drosophila melanogaster: Latitudinal and altitudinal clines in Indian populations

Wild populations of Drosophila melanogaster were collected along a latitudinal transect between Ernaculum (10 degrees latitude) and Jammu (32.4 degrees). Altitudes were also highly variable, from sea level up to more than 2000 m. The intensity of dark pigmentation on the thorax (trident) was estimated visually using four phenotypic classes, in flies grown at 17 and 25 degrees C. Significant dines of increasing pigmentation were observed according to latitude and altitude. A multiple regression technique permitted improvement of the relationship between pigmentation and geographic parameters, so that 93% of the pigmentation variations among populations could be predicted by knowing both the latitude and altitude of original populations. These data strongly suggest an adaptive response of thoracic pigmentation to physical factors of the environment, and especially to temperature.