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 breedi ng 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 al lowed 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 emergi ng from the supplemented halves were, on average, larger than those em erging from the control halves. F-statistics were estimated from alloz yme data for the D buzzatii flies. The values obtained from the supple mented halves, coupled with computer simulations to compare these esti mates with the expected values generated by a limited number of mating pairs contributing progeny to a fruit, suggest an effective size of a bout 30 individuals. Even though 95% bootstrap confidence intervals fo r F-IS estimates comparing the supplemented and control halves do not overlap, computer simulations suggest that we cannot support the hypot hesis that selection is acting nn allozyme variation.