Thoracic trident pigmentation in Drosophila melanogaster : Differentiation of geographical populations

Abstract

A phenotypic classification of trident pigmentation allowed the characterization ol any natural population by a pigmentation score, ranging from 0 to 3. After some training, independent observers could produce very similar score values. Growth temperature influences pigmentation intensity and the response curves exhibit a U-shape, with a minimum at about 25 "C. For the description of natural populations, 2 different growth temperatures, 17 °C and 25 °C were chosen. Crosses between a dark French strain and a light Afrotropical strain produced intermediate offspring, but a clear maternal effect differentiated the reciprocal Fl’s. Numerous populations from various part of the world were inves- tigated and results arranged according to the latitude. For temperate populations collected between 34 and 48° of latitude a steep cline was observed (pigmentation being much more darker in high latitude) suggesting an adaptive pressure on this phenotype : environmental factors which may explain this cline being temperature, insolation and desiccation. In tropical populations on the other hand a large variability was observed but without any relation to latitude.