Sexually Dimorphic Circadian Regulation of Eclosion and Clock Gene Expression in a Migratory Moth Spodoptera frugiperda

The circadian clock orchestrates essential behavioral and molecular processes, including the timing of eclosion, one of the most tractable and ecologically relevant outputs of the circadian system. Understanding eclosion timing may offer insights into the circadian clock mechanisms that underlie migratory timing. Here, we characterize the diel and circadian patterns of eclosion and core clock gene expression in the fall armyworm (FAW), Spodoptera frugiperda, a globally distributed migratory moth. Using a custom-designed eclosion monitoring system under both 14 hours light-10 hours dark (L14: D10) and constant darkness (DD) conditions, we observed clear diel eclosion rhythms, peaking shortly after lights-off under L14: D10. Under DD, these rhythms became delayed and damped over three consecutive days, consistent with circadian control. Males exhibited more dispersed emergence patterns and distinct eclosion distributions than females under L14: D10 and DD, suggesting sexually dimorphic timing. Gene expression profiling revealed rhythmic oscillations of five canonical clock genes, cyc, clk, tim, per, cry2, with sex-dimorphic differences in mesor, amplitude, or phase, particularly the mesors of males are higher than those of females in five clock genes under L14: D10. These results provide strong evidence for sexually dimorphic circadian regulation at both behavioral and molecular levels in a migratory insect that recently invaded China, suggesting that sex-dimorphic circadian architecture may contribute to allochronic speciation and ecological strategies such as eclosion and migratory timing in a novel environment. Leveraging the established circadian eclosion rhythm assay, which captures core features of clockwork mechanisms, further investigation may reveal how these sex-based differences contribute to the evolution of circadian function and adaptive traits.