Genetic characterization of geographic populations using morphometrical traits in Drosophila melanogaster: isogroups versus isofemale lines.

Studies of short or medium range geographic variations play an increasing role in ecological genetics, and sensitive techniques are required to detect them. In this respect, two sampling techniques were compared in D. melanogaster. The biological data were provided by the analysis of four natural populations from the same geographic area, Spain (one) and Southern France (three), for four morphometrical traits: abdomen and thoracic pigmentation, and wing and thorax lengths. Traits were measured on wild living females and on their progeny reared in the laboratory at 25 degrees C. For progeny analyses, two techniques were compared: the usual isofemale line technique, sib families issued from a single female, and a new isogroup technique, the progeny produced by a group of 20 wild-collected parents. Large phenotypic variations were observed in wild living flies, corresponding to the unstability of natural environmental conditions during their development. Among laboratory grown flies, variations were much smaller. Between isogroups, differences were small, due to sampling error and some common environment effects. Variations between lines were much greater, thus demonstrating a strong genetic component. When different populations have to be compared, the isogroup technique should be preferred since, for the same amount of work, the lesser variability between groups provides a more precise characterization of the population means.