The chemo-sensing fly: the new canary in the coal mine?

Amanda Alvarez
Aug 2, 2018 · 4 min read
The irresistible allure of fly droppings. Illustration by Milena Menezes Carvalho/RIKEN

“We humans are blind to pheromones,” says Damien Mercier, “but animals are not.” A new tech Mercier has been working on can essentially turn animals, like tiny fruit flies, into living sensors that give human observers an idea of the chemical exchange around us. The broad sensory capacity of the fly can be leveraged for a different kind of output, “taking what the animal is sensing and turning that into visible light through the interface of the animal brain.” And the first experiment naturally looks at how flies deal with that most organic of smells: pheromones served with a side of 💩.

Mercier is a research scientist in the lab of Hokto Kazama in Japan, and they have recently developed a way to apply one of the hottest methods in neuroscience — bioluminescence imaging — to the tiniest lab animals. The method is one part genetic engineering of neurons, one part jellyfish chemistry, and one part really sharp cameras. The result is glowing brains that allow scientists to record the activity of neurons through the skull while flies buzz around freely without tethers or wires. “Getting this to work in smaller creatures is important, because flies can’t carry electrodes [needles] or endoscopes [tiny microscopes] around,” says Mercier, “but everyone thought the [bioluminescent] light is too dim to pass through the fly cuticle [skin]”.

Indeed, the ‘glow’ of the fly brains is not all that bright, only a few hundred photons per second, on average. That’s why the Kazama lab’s experimental fly arena is outfitted with extremely sensitive light detectors on the top and bottom, to capture 95% of the light emitted from the flies, regardless of their orientation. The flies’ positions are also tracked with thermal cameras. But beyond watching the flies’ movements and brains, Mercier diligently records one more thing: ‘landmarks’ or droppings left behind by flies when they land. Unexpectedly, the poo mentioned earlier is packed with pheromone. Believe it or not, these are counted by hand — there are no more than 10 droppings in the arena over a few hours, so it’s not a laborious task — but more important than the quantity is the message hidden in the smell. The Kazama lab specializes in research on fly brains and smells, with tools like a ‘smell-o-drome’ to test which smells flies (and their brains) are attracted to.

And in this experiment, the main attraction is poo. “We’re actually not 100% sure if these landmarks are fecal,” says Mercier, “but they are blue.” The fly food is colored with blue dye so it’s safe to assume the landmarks are indeed droppings, though since they are released from the tip of the fly abdomen they could contain other kinds of secretions. Poo or not, these landmarks are ripe with the male pheromone cVA, or 11-cis-vaccenyl acetate. This chemical has a lot of different meanings for flies — creating attraction, making males aggressive and females more sexually receptive — that can change depending on the context and exposure. But every study so far has used different concentrations and distributions of cVA, making it impossible to compare or know for sure how flies release and respond to it in a natural state. Besides wanting to resolve the meaning of cVA for flies, Mercier cites another attraction to studying this pheromone: it mainly activates a single type of neuron in the fly brain. There are only a few dozen of these olfactory receptor neurons, and their activity can’t be confused for anything else — except attraction to pheromone-rich poo.

In the experiment, both male and female flies leave droppings, but only the males’ droppings are attractive — to both males and females. Even when the male fly is chasing and courting the female, she’ll keep returning to the male’s poos. They are definitely important landmarks for the mating process, but are flies also using them to mark their territory? Figuring that out, says Mercier, is the next step. “There is a dominance in flies, with males fighting over females, so perhaps wild flies have territories, though in the lab they just appear to live in packs.” It’s clear that flies can recognize whether a poo came from a male or a female, but figuring out if flies can also tell the identity of a particular fly from a dropping “would be really cool,” muses Mercier. “Can flies acquire and maintain a representation of self and others, and how can the brain do that?”

We may thus be on our way to exciting studies on self-awareness in flies, all thanks to the power of poo. Until then, fecal landmarks still play an important part in social communication for flies. “It’s a tiny beacon to say ‘I was here not that long ago, so come and find me’.”

Mercier D, Tsuchimoto Y, Ohta K, Kazama H (2018). Olfactory landmark-based communication in interacting Drosophila. Current Biology, DOI: 10.1016/j.cub.2018.06.005