In their early stage, the study notes, the majority of neurons in these worms' brains are shared by males and females. During sexual maturation, however, some connections become differentiated by gender. Hobert calls this "the punchline of the paper," and it may one day help explain how human sexual identity emerges from neural circuits.
“This study gives us new insight into the molecular mechanisms that guide development of male and female wiring patterns in the worm brain," said Coryse St. Hilaire-Clarke of the National Institute of Neurological Disorders and Stroke, which funded the research. "Some of the genes and proteins identified in this process are also found in higher-order animals, suggesting that these findings may be relevant to other species.”
By now, scientists have a precise brain map of the nematode, which is also known as C. elegans: 294 neurons are shared by both sexes; eight neurons are specific to females; and 91 are specific to males. There's a specific survival reason for this huge discrepancy — but more on that later.
"Only in C. elegans can we approach the question of how sex affects the brain and behavior at all levels — molecular, neuronal and circuits," said Oren-Suissa, who teaches with Hobert at Columbia University. "We can’t do this in any animal model."
Not until the worms reach maturity do the "gendered" neurons differentiate. This happens through a kind of pruning process, similar to the general pruning that takes place in human brains during puberty — the "use it or lose it" stage of maturation.
"What arises from shared neurons is sex-specific behavior," Hobert said, and that behavior is set in adulthood by males eliminating the sex-specific circuits of females and vice versa.
The researchers also looked at whether the sex-specific wiring triggers different behaviors. They found that chemosensory neurons — the brain cells that detect chemical cues in the environment, from a predator, a potential mate or food, for example — function differently in nematodes depending on their gender.
In mature male roundworms, those neurons primarily assisted with mating cues, whereas in females they helped the worms avoid noxious elements in the environment.
"The male basically has to make a choice," Hobert explained. "What’s more important: to sense the environment for what’s good or bad, or do I totally focus on finding a mating partner? For males, this is much more important."
And where are all those 91 sex-specific male neurons located? In a guy nematode's tail, Oren-Suissa said.
"It's kind of funny," she said. "It's like they have two brains."
"And they're using all those additional brain cells to do sex acts," Hobert added. "All they do is essentially mate."