THE EVOLUTIONARY HISTORY OF DROSOPHILA-BUZZATII .33. ARE OPUNTIA HOSTS A SELECTIVE FACTOR FOR THE INVERSION POLYMORPHISM

Previous work has shown fitness differences among chromosomal arrangem ents by means of selection component analysis in two Drosophila buzzat ii natural populations, one of which is native to Argentina and the ot her a colonized population from Carboneras, Spain. Founder effects or niche shifts were proposed to explain the differences observed in the pattern of pleiotropic effects of inversions on fitness components. In this paper, we address the possible role of niche shifts by determini ng whether differential attraction to, oviposition on, or utilization of the rotting cladodes of two different Opuntia species (O. quimilo a nd O. ficus-indica) occurred among individuals carrying different seco nd chromosome karyotypes in a natural Argentinian population. Through the analysis of more than 2500 individuals comprising five different l ife cycle stages associated with the necroses of these two cactus spec ies, we found that the distributions of inversion frequencies in sampl es of adult flies, third instar larvae and emerging adults collected o n both Opuntia species were not significantly different. Likewise, no evidence of differential oviposition was observed. These findings sugg est that niche shifts cannot, solely, account for the changes observed in the Carboneras population. In addition, the selection component an alysis did not reveal any significant relationship between chromosomal arrangements and the fitness components tested. These results suggest either that fitness differences might be too small to be detected or that the assumptions of the model concerning the mode of selection may not be tenable in the studied population.