As the days draw in, it's a sign for many creatures to stop producing offspring and prepare instead for winter. Insects build up fat and some of their tissues, such as their ovaries, go into a form of suspended animation known as diapause: essentially, they stop developing to save energy. The process is coordinated by a network of small signalling molecules released from neurons (known as neuropeptides) in the brain, with specific regions of the brain controlling when organs begin their preparations for winter. Jili Xi from The University of Osaka, Japan, explains that a section of the brain known as the pars lateralis mediates the signals that cause the ovaries to stop developing as daylength decreases in insects such as the bean bug (Riptortus pedestris), an agricultural pest. But it wasn't clear which signalling molecule released by the pars lateralis triggers the ovaries to begin shutting down. One candidate was a signalling molecule called corazonin, which apparently stops the development of pupating tobacco hornworms when the days grow short. Might it be a key signalling molecule produced in the brains of bean bugs to shut down their ovaries as winter closes in?
First, Xi, Yoshitaka Hamanaka and Sakiko Shiga (University of Osaka) used dyes – directed to neurons producing corazonin by antibodies – to identify the regions of the brain producing the signalling molecule. The resulting detailed microscope images revealed corazonin-producing neurons extending to regions of the brain that produce hormones involved in the insect's fertility: the corpus cardiacum and corpus allatum. In addition, the team identified corazonin-producing neurons extending to the aorta, ‘indicating that neurons in the pars lateralis release the corazonin molecule into the insect's blood, where it can travel around the body to send signals to important organs’, says Xi. The team also discovered that neurons from the nearby region of the brain that controls how insect bodies react to changing day length – the accessory medulla – feed into the corazonin-producing neurons, revealing a route for how seasonal day length changes are transmitted to the fertility-regulating corazonin-producing neurons.
Next, the team needed evidence that corazonin produced in the brain directly causes the bean bug's ovaries to stop developing. They injected a specially designed RNA molecule – which deactivates and removes corazonin – into female bugs that had grown up when the days were short to find out whether the loss of corazonin would allow the insects’ ovaries to continue developing despite the short daylength. Sure enough, it did, ‘suggesting that corazonin peptides in pars lateralis neurons play a crucial role in inducing reproductive diapause under short-day conditions’, says Xi.
But which tissues does the signalling molecule target to send a female bean bug's ovaries into suspended animation over winter? This time the team searched for receptors, which detect corazonin carried in the insects’ haemolymph, in different tissues and revealed that the receptors occur in the ovaries, as they had hoped, and a second tissue, the fat body, which is the insect equivalent of adipose tissue combined with the liver, to help the insect store more energy in preparation for winter. Corazonin receptors turning up in the fat body also makes sense because it produces a protein, vitellogenin – a vital component of egg yolk – which is no longer required when the ovaries go into suspended animation.
Xi and colleagues have successfully mapped how declining day length causes production of the signalling molecule corazonin in the brains of female bean bugs. This is then released into the blood, causing the insects to build up fat stores and the ovaries to cease developing in preparation for winter.
