Adaptation of Drosophila willistoni experimental populations to extreme pH medium

Abstract

Nine population cages with D. Willistoni from the centre of its distribution range (Brasilia) and another nine from the southern margin of its distribution (Eldorado, RS) have been progressively selected for adaptation to three levels of pH of the food medium: low (pH 2), median ( p H 5) and high (pH 10) under a system of discrete generations. The degree of adaptive response was measured by viability tests at the 17th and 49th generations. The high pH populations, both Brasilia and Eldorado, exhibited a faster response to selection starting in the 17th generation whereas the low pH populations showed significant increase of viability from the 49th generation on. However, the developmental rate of high and low populations decreased, and at the 49th generation they were faster on the usual median pH 5 food. Selection for early adult emergence, applied during seven generations, was more effective for the median populations, followed by the high and the low populations. Apparently the elongation of the life cycle was initially a correlated response to selection for survival at extreme pH leading to a partial loss of the ability to respond to selection for faster development.