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Female flies only lay eggs after mating. Here’s why.

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03/06/20 | Female flies only lay eggs after mating. Here’s why.

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A female fruit fly can lay hundreds of eggs in her short life –but only after mating. Now, researchers at the Howard Hughes Medical Institute’s Janelia Research Campus have untangled the neural pathway that gives a fly’s egg-laying machinery the green light at the opportune time, they report February 26, 2020, in Nature. The neural circuit directly links egg-laying to a fly’s mating status.

“This is now perhaps one of the most completely understood behaviors in the fly,” says Group Leader Barry Dickson. “It's a great model for looking at how the brain integrates internal and external information.” 

Most of the time, the neurons that prompt a female fly to lay eggs are inhibited, unable to send messages. A protein found in fly sperm removes these inhibitory signals, unblocking the pathway after mating. Dickson’s team traced the circuit from the sensory neurons that detect mating to the command neurons that actually prompt egg laying. 

Mating alone isn’t enough to trigger the fly to actually lay, notes Fei Wang, a postdoc at Janelia who co-led the study alongside fellow postdoc Kaiyu Wang. The egg-laying neurons also seem to be activated by sensory cues that suggest a fly is in an ideal spot – perhaps that she’s sitting atop a soft piece of rotting fruit that will provide a rich food source for her offspring, rather than a hard, sterile countertop. Neurons in the female fly’s brain integrate those different cues, and then, when the time is right, signal to neurons that direct egg release. 

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Citation:

Fei Wang, Kaiyu Wang, Nora Forknall, Christopher Patrick, Tansy Yang, Ruchi Parekh, Davi Bock, and Barry J. Dickson. Neural circuitry linking mating and egg-laying in Drosophila femalesNature, Published online February 26, 2020.