As the days grow shorter, many animals prepare for the approaching winter by suspending reproduction. Insects, for example, accumulate energy stores while halting ovarian development through a process known as reproductive diapause. In a recent study published in The Journal of Experimental Biology, researchers at The University of Osaka uncovered a key neuroendocrine pathway underlying this seasonal shift in the bean bug Riptortus pedestris, identifying the neuropeptide corazonin as a molecular signal that suppresses reproduction in response to changes in day length.
Photoperiodic diapause allows insects to anticipate unfavorable seasons and adjust their physiology in advance. Although reproductive diapause is clearly triggered by day length, how the brain communicates this information to the reproductive system has remained largely unclear.
In R. pedestris, earlier studies have shown that photoperiodic information is first received through the compound eyes, then transmitted to the optic lobes-specifically to the anterior region of the medulla, where cells expressing the core circadian clock protein PERIOD and pigment-dispersing factor (PDF) are located. This region is considered a hub for photoperiodic signal processing. In addition, glutamate levels in the brain are regulated in a photoperiod-dependent manner, and the pars lateralis of protocerebrum has been implicated in the regulation of reproductive diapause. However, the neural pathway linking these photoperiodic clock-associated brain regions to neurosecretory cells responsible for diapause induction-and the signaling molecules involved-has remained unknown.
To address this question, a research team including Specially Appointed Assistant Professor Jili Xi, Lecturer Yoshitaka Hamanaka, and Professor Sakiko Shiga investigated R. pedestris, a species highly sensitive to photoperiod. Females exposed to short-day conditions (e.g., 12h light/12h dark) enter reproductive diapause and cease egg production, whereas those under long-day conditions continue ovarian development.
By combining of RNA interference (RNAi), neuroanatomical mapping, and tissue-specific gene expression analysis, the researchers discovered that corazonin, produced in neurons of the pars lateralis, plays a role in mediating photoperiodic diapause.
"We found that the neuropeptide corazonin plays a suppressive role in reproduction under short-day conditions," explains Xi. "When corazonin expression was knocked down by RNAi, females exposed to short-day conditions developed mature ovaries and laid eggs, effectively bypassing diapause. The neurons that produce corazonin send fibers toward the corpus cardiacum-corpus allatum complex, which serves as a neurohemal release site. Although corazonin fibers pass through this area, its receptor is not expressed there. Instead, receptors are localized in peripheral tissues such as the fat body and ovaries-indicating that corazonin acts directly on these organs rather than via a traditional hormonal cascade."
Interestingly, the team also detected that corazonin-expressing neurons are anatomically connected to PDF neurons. This finding suggests that seasonal photoperiodic signals may be relayed from the circuit of PERIOD and PDF neurons to the reproductive system via corazonin signaling.
Through this work, Xi and colleagues have successfully mapped a neuroendocrine pathway linking seasonal day length to reproductive suppression in the brain of R. pedestris. They suggest that declining day length stimulates corazonin production in the brain, which is then released into the hemolymph to promote fat accumulation and suppress ovarian development-an adaptation preparing insects for winter. These findings illuminate the neural and hormonal mechanisms underlying photoperiodic reproductive diapause and highlight the suppressive role of corazonin in seasonal reproduction control, and provide a potential molecular target for developing environmentally friendly pest control strategies.
Fig. 1
Proposed neuronal pathway underlying reproductive suppression under short-day photoperiod conditions in the bean bug Riptortus pedestris
Credit: XI JILI
Fig. 2
Effect of corazonin RNAi on diapause: Over 50% of females developed ovaries even under short-day conditions
Credit: XI JILI
Notes
The article, "Corazonin mediates photoperiodically induced reproductive diapause in the bean bug Riptortus pedestris," was published in The Journal of Experimental Biology at DOI:
