GENETIC-STRUCTURE IS DETERMINED BY STOCHASTIC FACTORS IN A NATURAL-POPULATION OF DROSOPHILA-BUZZATII IN ARGENTINA

D. buzzatii is a cactophilic species associated with several cactaceae in Argentina. This particular ecological niche implies that this spec ies is faced with a non-uniform environment constituted by discrete an d ephemeral breeding sites, which are colonized by a finite number of inseminated females. The genetic consequences of this population struc ture upon the second chromosome polymorphism were investigated by mean s of F-statistics in a natural endemic population of Argentina. The pr esent study suggests that differentiation of inversion frequencies in third instar larvae among breeding sites has taken place mainly at ran dom and selection is not operating to determine the structure of this population. The average number of parents breeding on a single pad see ms to be similar to the number colonizing Opuntia ficus indica rotting cladodes in Carboneras, a derived population from Spain, There is no significant excess of heterokaryotypes within pads or in the populatio n as a whole. The results obtained in the present study suggest that t he potential role of selective versus stochastic factors relative to t he among pad heterogeneity in the population here studied is different from that of the Spanish population previously reported. Potential me chanisms responsible for these differences are discussed.