Males and females sometimes look and behave so differently that they could be mistaken for different species. Males often display vibrant pigmentation and striking ornaments to court inconspicuous females, who accept or reject male advances. How did such differences evolve? After all, the sexes share a genome and so selection must shape this diversity from shared genes. Carolina Rezával and colleagues at the University of Oxford wanted to understand what differences in male and female brains control these sex differences in behaviour. To answer this question, the team studied the courtship display in Drosophila melanogaster fruit flies, where males (but not females) sing and dance to court potential mates.
The team knew that in D. melanogaster, reproductive behaviour is controlled by doublesex neurons. But it was unclear whether sex-differences in these neurons explain why males court and females do not. To find out, Rezával and colleagues artificially activated doublesex neurons in the brains of female Drosophila and found that this caused the females to act like courting males, following, tapping and serenading other flies. This result shows that female brains have latent neurons that, when activated, trigger male-like behaviours.
If females have circuity that allows them to act like males, why don't we see male-like behaviours in females more often? The team found two clues that may hint at the answer. First, when the team turned on different bundles of neurons in turn using a temperature-dependent molecular switch, they found that activating just one neuron cluster (called pC1) caused females to act like males.
Second, females with activated doublesex neurons did not perform courtship displays when housed alone, while males with activated neurons sang and danced even without an audience. This suggests that females require additional stimuli to begin courting. By systematically blocking different senses in females paired with male flies, the team found that females only failed to court when their antennae were removed and so they had no sense of smell. By exposing flies to different compounds, the team showed that females only begin courting once they get a whiff of pheromones.
Taken together, these data suggest that differences in the pC1 node may explain why females do not court, even though they can. Males and females have different numbers of both PC1 neurons and projections from these neurons. Perhaps there are too few pC1 neurons to trigger male-specific courtship behaviours in females. Or perhaps these neurons are less excitable or less well connected and so receive less excitation (or more inhibition) than in males, so females only begin acting like males when their pC1 neurons are activated outside of their normal range.
Sex differences in behaviour have been attributed to neurons that are unique to each sex. However, these results show that females actually have neurons capable of triggering male-specific behaviour and that, instead, differences in how each sex uses shared circuitry can explain why males woo females and females choose amongst them.