Researchers have identified a small group of neurons in Drosophila that are key to determining how female flies choose where to lay their eggs, a study in Science reports. The neurons are part of a neural circuit that could serve as a model to probe the molecular basis of decision-making, the study's authors say.

Chung-hui Yang, a postdoc in Yuh-Nung Jan's lab at the University of California, San Francisco, was intrigued by the fact that female flies were so picky about where to lay their eggs. "From carefully watching, [Yang] realized that they go through this stereotyped behavior, which has never been reported," Jan said. Each time, the fly bends her abdomen until it is almost perpendicular to the surface she is standing on, and then moves her body back and forth to expel the eggs; immediately afterwards, she grooms herself and momentarily stays still, as if resting.

The researchers then examined this egg-laying behavior in a chamber containing areas of different sucrose concentrations. Although flies feed on sweet substances, they seek out lower sugar concentrations for egg-laying, seemingly comparing their options before settling on a spot. As the researchers observed the flies searching for the perfect site, Jan said, "We began to realize, hmm, they are making a decision."

To identify the neural circuitry at play, Yang, Jan and their colleagues employed a widely used two-component gene expression system to hyperpolarize (and thus inactivate) specific types of neurons. Specifically hyperpolarizing the flies' sweet taste receptor neurons increased egg-laying in higher-sucrose liquids, suggesting that these neurons play a role in determining whether a particular sucrose level is acceptable. They then hyperpolarized another group of neurons that express a peptide called ILP7, which is partly homologous to a mammalian reproductive hormone and is present very selectively in the fly nervous system. This blocked egg-laying completely, suggesting that the ILP7 neurons may be communicating a site's suitability to the animal's reproductive system.

"Basically, what we have described is a starting point," Jan said, adding that the group now plans to use the same technique to inactivate other sets of neurons and conduct genetic screens to further define how this decision-making circuit works.

But how relevant the model will be to decision-making as we humans know is unclear. "I look at the neurobiology of decision making as a window on higher cognition," Michael Shadlen, who studies neuronal activity during decision-making in monkeys at the University of Washington in Seattle, wrote in an Email to The Scientist. "It will be very interesting to see how this particular behavior is achieved by the nervous system of the fly. But for my money (and scientific tastes) I'll bet that it will tell us little about how the brains of humans make decisions or deploy the cognitive faculties that decision-making allows us to glimpse."