Senita cactus

Unlike their relatives, senita flies are unaffected by these toxins. They flourish on an artificial diet containing even ten times as much toxin as the cactus produces. The plant s toxins fail to deter... 

Although most insects are flexible about their steroid sources, senita flies definitely are not They lack a set of enzymes indispensable for utilizing most steroids, cholesterol included. The enzymes they do have can handle only a few rare natural steroids, and only with these unusual compounds as starting points can they produce their own essential steroids. The single Sonoran source of these rare steroids seems to be senita cactus, and thus senita flies must incorporate the cactus into their diets or perish. As the only Sonoran fruit flies resistant to senita toxins, senita flies experience no competition from their relatives for the one plant that keeps them alive. 

Historically, this lack of competition is probably responsible for the flies total dependency on the cactus The flies somehow lost the biochemical machinery needed for conversion of common plant steroids into those steroids they require. The loss went unnoticed, because the cactus provided an assured source of steroids the flies were still able to utilize. However, no other available plants provide these unusual steroids, so they must now consume senita cactus to survive. 

Schottenol (sterol) Lophocereus schottii (Senita cactus) [Precursor for fruit fly... 

Drosophila pachea is limited to the senita cactus (Lopho-cereus schottii) by a biochemical dependency on sterols found in that cactus. Unlike most insects that can utilize such compounds as P-sitosterol (4) to synthesize ecdysones, flies of this Drosophila species must have the compound schot-tenol (37) in which a particular double-bond position is found (Fig. 23.18) (Harbome, 1982 Herout, 1970 Kircher, 1982). This cactus contains 3-15% dry weight of complex isoquinoline alkaloids that are toxic to both adults and larvae of Drosophila nigrospiracula, D. mojavensis, and D. mela-nogaster. 

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