Dispersal in cactophilic Drosophila

Three species of Drosophila each breed in necrotic tissue of specific colum nar cacti endemic to the Sonoran Desert. Drosophila pachea breeds in senita (Lophocereus schottii). D. nigrospiracula breeds in saguaro (Carnegiea gig antea) or cardon (Pachycereus pringlei), and D. mojavensis uses organ pipe (Stenocereus thurberi) in Sonora, Mexico and southern Arizona. Patches of t hese three host cacti have very different spatial distributions, with those of senita bring quite frequent and close together, while those of the othe r hosts are much father apart. Testing all three species simultaneously. we used capture-mark-release-recapture methods to ask if dispersal differed i n these species and if differences were those predicted by the spatial avai lability of the host patches. D. pachea dispersed the shortest distance in all experiments. Furthermore. D. pachea was the only species showing sex-bi ased dispersal, with male Bits exhibiting the greater propensity to dispers e. The observations suggest that across similar spatial scales. D. pachea s hould allow greater population genetic structure than the other two species , and that mitochondrial DNA, because of its maternal inheritance, might sh ow greater evidence of structure than nuclear markers.