Photoperiodic control of metamorphic timing via a myoglianin-allatotropin signaling pathway in the cricket Svercacheta siamensis.

Photoperiodic regulation of insect development depends on interactions between environmental cues and endocrine signaling systems. In the cricket Svercacheta siamensis, the photoperiodic transition from the juvenile form to the adult form is a major developmental event controlled by juvenile hormone (JH), with long-day conditions promoting early metamorphosis and short-day conditions delaying metamorphosis through additional nymphal molts. Previous studies have identified myoglianin (Myo), a transforming growth factor-β ligand, as a photoperiod-responsive regulator of JH signaling. However, the organ in which photoperiodically regulated Myo expression occurs, and the mechanism by which Myo influences JH signaling, remain unclear. To address these questions, we first examined the developmental and photoperiodic regulation of myo expression in the brain and corpora cardiaca-corpora allata (CC-CA) complex. We found that myo expression increased during the 7th (final) instar under long-day conditions in the brain, but not in the CC-CA complex, suggesting that photoperiod-responsive Myo signaling originates in the brain. To identify downstream factors that potentially link Myo signaling to JH regulation, we examined four allatoregulatory neuropeptide genes. Among these, only allatotropin (AT) showed developmental and photoperiodic expression patterns, similar to those of myo. RNA interference (RNAi) targeting myo significantly reduced AT expression. Furthermore, AT RNAi increased the expression of juvenile hormone acid O-methyltransferase and Krüppel homolog 1, which are markers associated with JH biosynthesis and signaling, and significantly delayed metamorphosis with additional nymphal molts. These results suggest that AT acts downstream of Myo and contributes to the photoperiodic regulation of metamorphic timing through the modulation of JH signaling.