AN INTERACTION BETWEEN ENVIRONMENTAL-TEMPERATURE AND GENETIC-VARIATION FOR BODY-SIZE FOR THE FITNESS OF ADULT FEMALE DROSOPHILA-MELANOGASTER

Drosophila and other ectotherms show geographic genetic variation in b ody size, with larger individuals at higher latitudes and altitudes. T emperature is implicated as an important selective agent because long- term laboratory culture of Drosophila leads to the evolution of larger body size at lower temperatures. In this paper, we tested the hypothe sis that, in Drosophila melanogaster, larger size is favored at lower temperatures in part because of selection on adult females. We used re plicated lines of I). melanogaster artificially selected for increased and decreased wing area with constant cell area. The resulting size d ifferences between the selected lines were due solely to differences i n cell number, and thereby were similar to the cellular basis of clina l variation in body size in nature. We examined Life-history traits of adult females at 18 and 25 degrees C. Rearing for two generations at the two temperatures did not affect the extent of the size differences between Lines from the different selection regimes. There was a stron g interaction between temperature and size selection for both survival and Lifetime reproductive success, with larger females living signifi cantly longer and producing more offspring over their Lifetime only wh en reared and tested in the colder environment. There was also an incr ease in average dairy progeny production in large-line females relativ e to the control and small lines again, only in the colder environment . Thus, the females from the large selection lines were relatively fit ter at the colder temperature. At both experimental temperatures, espe cially the lower one, the small-line females rescheduled their progeny production to later ages. Larger body size may have evolved at higher latitudes and altitudes because of the advantages to the adult female of being larger at lower temperatures.