Theyellowgene regulates behavioural plasticity by repressing male courtship inBicyclus anynanabutterflies

Seasonal plasticity in male courtship inBicyclus anynana butterflies is due to variation in levels of the steroid hormone 20E (20-hydroxyecdysone) during pupation. Wet season (WS) males have high levels of 20E and become active courters. Dry season (DS) males have lower levels of 20E and reduced courtship rates. However, WS courtship rates can be achieved if DS male pupae are injected with 20E at 30% of pupation. Here, we investigated the genes involved in male courtship plasticity and examined whether 20E plays an organizational role in the pupal brain that later influences the sexual behaviour of adults. We show that DS pupal brains have a sevenfold upregulation of theyellow gene relative to the WS brains, and that knocking outyellow leads to increased male courtship. We find that injecting 20E into DS pupa reducedyellow expression although not significantly. Our results show thatyellow is a repressor of the neural circuity for male courtship behaviour inB. anynana . 20E levels experienced during pupation could play an organizational role during pupal brain development by regulatingyellow expression, however, other factors might also be involved. Our findings are in striking contrast toDrosophila whereyellow is required for male courtship.