Breeding structure of Drosophila buzzatii in relation to competition in prickly pears (Opuntia ficus-indica)

Rotting Opuntia ficus-indica fruits (prickly pears) are used as breeding sites for up to four Drosophila species (D melanogaster, D simulans, D buzzatii and D hydei) in southern Spain. A field experiment showed that the larvae of D buzzatii are resource limited in Opuntia fruits available for oviposition for 108 h. Experimental fruits infested with D larvae were divided into two halves; the larvae in one half were allowed to develop normally, while those in the other half were provided with extra food. Approximately five times as many D buzzatii emerged from the supplemented as from the control halves, and the flies emerging from the supplemented halves were, on average, larger than those emerging from the control halves. F- statistics were estimated from allozyme data for the D buzzatii files. The values obtained from the supplemented halves, coupled with computer simulations to compare these estimates with the expected values generated by a limited number of mating pairs contributing progeny to a fruit, suggest an effective size of about 30 individuals. Even though 95% bootstrap confidence intervals for F(IS) estimates comparing the supplemented and control halves do not overlap, computer simulations suggest that we cannot support the hypothesis that selection is acting on allozyme variation.