Senita sterols and specialization in Drosophila pachea

A fly from a decomposing stem

In the nineteen-sixties, William Heed and Henry Kircher studied Sonoran Desert flies by comparing their breeding sites with laboratory cultures. Drosophila pachea emerged from fermenting senita fragments, but development on ordinary banana medium often stopped during early larval stages. Adding fresh or autoclaved cactus tissue enabled another generation to develop. The factor’s persistence after autoclaving directed attention towards tissue chemistry. [Heed and Kircher 1965]

An experiment with isolated sterols

Sterols including schottenol and lophenol were isolated from cactus tissue and then added separately to the medium. Schottenol supported Drosophila pachea development and reproduction under the conditions reported in 1965. Lophenol did not. Synthetic schottenol produced a response similar to the plant isolate, linking the effect to a defined compound rather than simply to a piece of cactus. [Heed and Kircher 1965]

To distinguish the fly’s role from microbial activity, the researchers established an axenic culture without detectable microbial contamination. Flies continued maturing and reproducing on sterile supplemented medium. The result demonstrated utilization of the tested sterol without prior microbial processing. It concerned that specific nutritional function, whereas natural cactus tissue decomposition remains a microbially mediated process. [Heed and Kircher 1965]

Larval development and adult female fertility

Comparing sterols revealed separate requirements at successive life stages. In one treatment, the compound designated Δ5,7-stigmastadien-3β-ol allowed larvae to reach adulthood, yet the resulting females laid no eggs on the same medium. Siblings transferred to senita-supplemented medium regained fertility. Females in the first group were inseminated but contained immature eggs. Reaching adulthood and producing a further generation were therefore distinct experimental endpoints. [Heed and Kircher 1965]

Pilocereine restricts competitors

A follow-up study published in 1967 compared Drosophila pachea with eight other fly species. Media received senita tissue or its alkaloids, pilocereine and lophocereine. The basic tests of the other species used 15 females and 15 males in duplicate vials. Adult survival and the number and reproductive viability of offspring were assessed. Pilocereine strongly impaired these outcomes in many tested species, with responses differing among species and developmental stages. [Kircher et al. 1967]

Drosophila pachea showed much greater tolerance, although high concentrations also reduced offspring production. With 14% added pilocereine relative to dry dietary ingredients, two and seven offspring were obtained after six weeks in the two vials. Senita-only controls produced 441 and 454. These laboratory concentrations were used to compare tolerance. They show how adult survival and completion of the reproductive cycle can give different pictures of the same compound’s effects. [Kircher et al. 1967]

An enzyme linking diet to hormones

In 2012, Lang and colleagues linked this specialization to Neverland. In Drosophila pachea, the enzyme retains activity with lathosterol but has lost the ability to convert cholesterol into 7-dehydrocholesterol, a steroid hormone precursor. Enzyme assays and transgenic Drosophila melanogaster experiments confirmed this difference. Hormone analysis implicated lathosterol as the utilized precursor. The historical schottenol feeding test and the later identification of the hormonal pathway therefore address different levels of explanation. [Lang et al. 2012]