THE COLONIZATION OF ORANGES BY THE COSMOPOLITAN DROSOPHILA

The guild of ''cosmopolitan'' Drosophila co-exist almost worldwide and yet the mechanisms that underlie this coexistence are unknown. The la rval resource of the guild is decaying fruit and vegetables, but the s pecies show little specialization and can coexist on a single resource , such as oranges. In southern California the guild includes D. simula ns (SIM), D. melanogaster (MEL), D. pseudoobscura (OBS), D. immigrans (IMM), D. hydei (HYD) and D. busckii (BUS). These species show consist ent differences in their colonization of decaying oranges, differences that may promote their coexistence. This study tested whether the col onization pattern of a species is determined primarily by attraction t o specific resource types (decayed or fresh oranges), by ability to co lonize new resource patches, or by dependence on a successional sequen ce of Drosophila species. The experiments compared oranges that were p re-aged prior to a colonization period and showed that the colonizatio n pattern of each species (except OBS) was driven primarily by its dec ay-dependent attraction to oranges. While OBS exhibited a pattern of c olonization independent of pre-aging, the remaining species all showed some preference for older (7-day pre-aged) over fresh oranges. Their overall pattern of attraction, ordered by high relative abundance on f resher oranges, was SIM>MEL=IMM>HYD=BUS. BUS, a specialist on decaying plant material, was the only species that showed a preference for 11- day over 7-day oranges. Pre-aging the oranges under covers, to prevent prior colonization by Drosophila, did not change the interspecific pa ttern of colonization, indicating that microbial decay was driving the changes in attraction. The patterns of attraction separated two ecolo gically similar pairs (SIM from MEL; IMM and HYD) and published data o n ethanol tolerance show that, in each pair, the earliest colonizer ha s the lower tolerance. This suggests an important interplay between co lonization patterns and physiological optima.