Cellular basis of wing size variation in Drosophila melanogaster: a comparison of latitudinal clines on two continents

We investigated the cellular basis of two extensive, continuous, latitudina l, genetic, body size clines of Drosophila melanogaster by measuring wing a rea and cell size in the wing blade of adult flies reared under standard, l aboratory conditions. We report that the contribution of cell size to an Au stralian cline is much smaller than that to a South American cline. The dat a suggest that neither cell size nor cell number were the targets of select ion, but rather wing area itself, or a trait closely related to it. We hypo thesize that the differences between the continents were caused by differen ces in the initial pattern of genetic variation for the cell traits and/or by the direction of selection on the source populations of the clines. Desp ite large differences between continents in the cellular basis of the latit udinal variation, multiple regression analysis, using the individual variat ion within populations, showed that the relationship between cell size and cell number was changed with latitude in the same way in the two clines. Th e relative contribution of cell number to wing area variation increased wit h latitude, probably because of compensatory interactions with cell size as a consequence of the latitudinal increase in cell number. Our findings are discussed in relation to the cellular basis of evolutionary change in labo ratory thermal selection lines and natural populations along latitudinal cl ines.