The role of phytosterols in host plant utilization by cactophilic Drosophila

The Cactus‐ Drosophila Model System of the Sonoran Desert consists of four endemic species of Drosophila (D. mojavensis, D. nigrospiracula, D. mettleri and D. pachea ) and five species of columnar cacti (agria, organpipe, saguaro, cardón and senita). Extensive collection records indicate that each cactus species has only one species of Drosophila as the primary resident. The elimination of six of the twenty possible random combinations of Drosophila species and cactus species can be attributed directly to phytosterols. Drosophila pachea has a strict requirement for Δ 7 ‐sterols such as 7‐cholestenol and 7‐campestenol. Since Δ 7 ‐sterols are found only in senita cactus, D. pachea cannot use agria, organpipe, saguaro or cardón as host plants. The lipid fractions of agria and organpipe are chemically similar and contain high concentrations of several 3β,6α‐dihydroxysterols. Larval viability tests using chemical constitutents of organpipe cactus demonstrate that the sterol diols are toxic to D. nigrospiracula but not to the resident, species, D. mojavensis . Agria and organpipe are therefore unsuitable as host plants for D. nigrospiracula . These results suggest that phytosterols play a major role in determining host plant utilization by cactophilic Drosophila in the Sonoran Desert.