Sense of smell controls fruit fly fertility

Some smells are simply delicious. It's worth following your nose to find out what treat is in store, and fruit flies (Drosophila melanogaster) love nothing more than following their sense of smell to some appetising fruit. A delicious smell also tells the insects that they are likely to be well fed, providing them with the energy that is essential to produce eggs and reproduce. But could this sense tie in with the insect's fertility, allowing insects that pick up good smells to produce large numbers of eggs, while less pleasant odours that warn of hardship impair the insect's fertility? Curious to find out how much of a role a fruit fly's sense of smell may play in its fertility, Madhumala Sadanandappa and Giovanni Bosco from the Geisel School of Medicine at Dartmouth, USA, investigated fruit flies with no sense of smell.

First, the team bred fruit flies that could no longer produce the scent receptors on the surface of the olfactory nerves, which initially detect an odour. Sure enough, when they checked the flies’ sense of smell, the insects couldn't smell much, and when they checked how many eggs the flies laid in a day, the flies that lacked a sense of smell produced ∼20% fewer eggs; they were less fertile than the insects with a functioning sense of smell. Then, the team disrupted the flies’ sense of smell by preventing the nerve from releasing the neurotransmitters that carry nerve signals, and the fertility of these flies was also reduced. It wasn't just the lack of a scent receptor that disrupted the flies’ fertility, it was the loss of the entire signalling process, which conveys the sense of smell, that had impacted their ability to produce eggs. The sense of smell is essential for fruit fly fertility.

But which aspect of the process that generates eggs in the flies’ ovaries does the sense of smell target to regulate the insect's fertility? Sadanandappa and Bosco examined the follicles that produce eggs in the ovaries of fruit flies with no sense of smell and they found that the eggs stopped developing early. Next, the duo wondered whether disruption to the fly's sense of smell simply slowed the eggs’ development, but when they monitored the condition of the eggs in the ovaries of flies over the insects’ lives, it was clear that fewer eggs developed; they weren't developing more slowly, the eggs were unable to develop. And when the duo checked why the flies’ eggs failed to develop, they discovered that the cells that usually develop into eggs were dying, preventing the flies with no sense of smell from producing as many eggs as the flies that could sniff out an odour. In addition, when the researchers triggered the odour-sensitive neurons to fire electrical signals as if they had picked up a smell, the flies developed larger ovaries and produced more eggs; the insects’ sense of smell was clearly driving their fertility.

‘Our results suggest that activation of brain-derived olfactory circuits regulates egg production’, say Sadanandappa and Bosco. And they point out that this makes sense, because odours inform fruit flies when food is plentiful, giving them the power to control their fertility when times are good and it's wise to reproduce. But when conditions are lean and food is scarce or a threat is present, their sense of smell could help them to reduce the number of eggs they produce until the adversity has passed and it's safe to produce the next generation.