Host-plant specificity in the cactophilic Drosophila mulleri species complex

Jaenike's model of optimal oviposition behaviour in phytophagous insects was tested in a guild of 4 closely-related cactophilic Drosophila species (D. mojavensis, D. arizonensis, D. aldrichi and an undescribed form from Navojoa). Rearing records from Sonora and N Sinaloa (NW Mexico) showed a sharp niche separation between species breeding on Opuntia (prickly pear) and those breeding on Stenocereus (columnar cacti). The suitabilities of 4 host-plants were assessed by measuring fitness traits (viability, development time and thorax size) on fermented cactus. Results are in good agreement with the expected predictions of the model in that the resident species had higher overall fitness on its own host-plant. The shift in host-plant use from opuntias to columnars during the evolution of these flies must have entailed a loss of fitness which could have occurred in isolation under stressful environmental conditions. -from Authors