Developmental plasticity in male courtship in Bicyclus anynana butterflies is driven by hormone regulation of the yellow gene

The organizational role for hormones in the regulation of sexual behavior is currently poorly explored. Previous work showed that seasonal variation in levels of the steroid hormone 20-hydroxyecdysone (20E) during pupal development regulates plasticity in male courtship behavior in Bicyclus anynana butterflies. Wet season (WS) males, reared at high temperature, have high levels of 20-hydroxyecdysone (20E) during pupation and become active courters. Dry season (DS) males, reared at low temperatures, have lower levels of 20E and lower courtship rates. Rescue of WS courtship rates can be achieved via injection of 20E into DS male pupae, but it is still unknown whether 20E alters gene expression in the pupal brain, and if so, the identity of those targets. Using transcriptomics, qPCR, and behavioral assays with a transgenic knockout, we show that higher expression levels of the yellow gene in DS male pupal brains, relative to WS brains, represses courtship in DS males. Furthermore, injecting DS males with 20E downregulates yellow to WS levels 4 hours post-injection, revealing a hormone sensitive window that determines courtship behavior. These findings are in striking contrast to Drosophila, where yellow is required for active male courtship behavior. We conclude that 20E plays an organizational role during pupal brain development by regulating the expression of yellow, which is a repressor of the neural circuity for male courtship behavior in B. anynana. This work shows that similar to vertebrates, hormones can also play an organizational role in insect brains, leading to permanent changes in adult sexual behavior. Significance StatementBehavioral plasticity in adult insects is known to be regulated by hormones, which activate neural circuits in response to environmental cues. Here, we show that hormones can also regulate adult behavioral plasticity by altering gene expression during brain development, adjusting the insects behavior to predictable seasonal environmental variation. We show that seasonal changes in the hormone 20E alters expression of the yellow gene in the developing pupal brain of Bicyclus anynana butterflies, which leads to differences in male courtship behavior between the dry and wet seasonal forms. This work provides one of the first examples of the organizational role of hormones in altering gene expression and adult sexual behavior in the developing insect brain.