The fruit-fly and the cactus

Aesop (slave and story-teller believed to have lived in ancient Greece 620–560 BC(E)) is credited with many fables that delivered morality tales, etc. But one you won’t find amongst that collection is ‘the fruit-fly and the cactus’. However, this modern-day cautionary tale has a ‘take home message’ (THM) that would not be out of place alongside his ‘The woman and the fat hen’ (whose THM apparently is that ‘relying on statistics does not always produce results’, or ‘figures are not always facts’). Anyway, back to the modern-day. We’ve probably got so used to fruit-flies (‘the arabidopsis of the animal biology world’) being lab-based model organisms that we may be surprised to hear that they do actually live outside of the research facility in the wild. And there are many different species too – not just Drosophila melanogaster. And some of them are really ‘unusual’ (botanist’s speak for ‘rather interesting, if in a non-plant way’…). Take for instance D. pachea, which is wholly dependent upon the senita cactus (Lophocereus schottii) in the Sonoran Desert (South-western USA/Northern Mexico), and whose distribution is therefore ‘patchier’ than many other fruit-fly species. Why is it so fussy? Work by Michael Lang et al. shows that the fly is unable to transform cholesterol into 7-dehydrocholesterol – an important reaction in the usual biosynthetic pathway to the insect hormone ecdysone, which permits the fly’s maturation – and that is generally a bad thing. However, the fly is able to use cactus-produced lathosterol in place of cholesterol. Thus, the mutation of the fly’s appropriately named Neverland enzyme – which deprives it of the ability to transform cholesterol and thus its ability ‘to grow up’ – has firmly tied the fly’s fortunes to those of the cactus. Fortuitously, the fly also has resistance to the otherwise toxic compounds of the cactus (which, the study’s authors speculate, may actually have been the first step along the road to its ultimate obligate specialist status as this enabled it to escape competition from other fly species). If you’re wondering what the cactus may get out of this, Virginie Orgogozo (last-named author of the Science study) says, ‘D. pachea flies live on rotten parts of the senita cactus. We don’t know if there is any benefit for the cactus to host these flies’. But maybe removal of rotten parts of the plant by the fly lessens the chances of microbial infection of the cactus as a whole, which helps to ensure continuity of the fly’s ‘fix’ of  lathosterol? Anyway, in its own microcosmic way – and in a satisfying nod in Aesop’s direction – this study demonstrates a universal truth: Animal fortunes on this rock we call home are intimately bound up with those of plants (and survival of both is probably contingent upon survival of each). Here endeth the lesson (for now…)