Vomiting up a droplet of sugar might not seem like the most romantic gesture from a potential suitor. But for one fly species, males that spill their guts are quite a catch.
Drosophila subobscura flies’ peculiar “romantic” barfing might have evolved by repurposing brain cells that usually control digestion for more romantic pursuits, researchers report August 14 in Science.
Most male fruit flies court by following the females around and vibrating their wings to serenade them with a species-specific love song, says Adriane Otopalik. But some fly species, like D. subobscura, spice things up a little. The males will vomit a bit of their last meal and offer it to females they are interested in, says Otopalik, a neuroscientist at Howard Hughes Medical Institute’s Janelia Research Campus in Ashburn, Va.
Nuptial gifts like these are common in some animals, like male spiders attempting to win over their mates without getting their heads bitten off. Scientists think female flies, which can be “very choosy,” might use this romantic barf to pick suitable suitors, says Otopalik, who was not involved in the study.
The thousands of neurons that control most of male fruit flies’ courtship produce a male-specific version of a protein called fruitless. Artificially activating these neurons can make D. subobscura males go through the motions of their seduction dance — even when there aren’t any females around, says Daisuke Yamamoto, an evolutionary biologist at National Institute of Information and Communications Technology in Kobe, Japan.
Yamamoto and his collaborators wondered if somewhere in these courtship brain cells was the key to understanding how nuptial gift giving evolved.
The team found that a group of 16 to 18 digestion-controlling neurons in the D. subobscura brain had the male version of the fruitless protein. Activating just that small number of brain cells triggered males to spill their guts, making them more successful at mating with the ladies of their species.
These same digestion-related neurons in Drosophila melanogaster don’t produce the male-specific fruitless protein. This fly species diverged from D. subobscura about 30 million to 35 million years ago and does not have this vomit-wooing strategy.
D. melanogaster’s digestion neurons are not connected to the rest of the courtship neurons. But when the researchers made those brain cells in D. melanogaster produce the male-specific fruitless protein, male flies vomited as they pursued the females. This also made these cells larger, growing toward and possibly connecting to the courtship neurons in this species.
This is an “amazing example of … cross-species comparison as a way of trying to understand sort of general principles of how a sex-specific behavior comes into being,” Otopalik says.
In D. melanogaster, the digestion neurons are involved in bubbling, a behavior where overstuffed flies vomit some of their stomach contents, Yamamoto says.
“These [digestion] neurons actually directly control the motor system,” projecting into the flies’ gut and mouthparts, Yamamoto says. So nuptial gift giving probably evolved by borrowing an entire preexisting vomit circuit and repurposing it into the courtship system.
The brain cells probably got connected by accident, he says. But the connection was fruitful, and barfing became quite the romantic gesture.
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