Nature: “How a fly came to love the vomit fruit”

You know, I went looking for some non-coronavirus science news, and Nature provided this gem of a headline about a plucky Drosophilia fruit-fly species and its unusual infatuation with toxic noni fruit (Morinda citrifolia):

In these flies, and in other animals, researchers have observed intriguing neural differences between close relatives that might explain their differing behaviours. But until recently, it has been impossible to test these neural–behavioural correlations directly. Writing in Nature, Auer et al. adapt genome-editing approaches for use in D. sechellia.

The noni fruit — nicknamed the vomit fruit for its pungent smell — might have been one of the only food sources consistently available to the ancient castaways. At first, noni would have been unappealing and even deadly to the flies, but over time they evolved to tolerate the toxins and love the smell. Present-day D. sechellia feed on the fruit exclusively (Fig. 1). By contrast, D. sechellia’s sibling species Drosophila simulans and more distant cousin, Drosophila melanogaster, retain their dislike for noni.

In 2003, scientists studying the sense of smell in this group of flies uncovered an intriguing clue. One class of sensory neuron that expresses the protein odorant receptor 22a (Or22a) was more abundant in D. sechellia than in any other fly species they analysed.

Auer et al. focused on how changes in Or22a contribute to D. sechellia’s selective diet. Inactivating the Or22a gene left the fly almost completely unable to locate its favourite fruit from just under one metre away. This result confirmed that neurons expressing Or22a process cues that help D. sechellia to target noni.

They substituted Or22a in D. melanogaster with the version of the receptor from D. sechellia, and vice versa. The two species’ receptors are nearly identical, harbouring just a few changes in amino-acid residues that tweak sensitivity to different compounds. To continue the musical analogy, we might compare this experiment to swapping strings between a violin and a viola and asking how the mismatched, differently tuned strings on each instrument affect the recital. Remarkably, the receptor swap gave D. melanogaster a slight taste for noni and diminished D. sechellia’s attraction to the fruit. This definitive test, made possible by the group’s cutting-edge toolkit, clearly showed that changes in Or22a tuning contribute to D. sechellia’s partiality for noni.