Figure 1. The study focuses on the diapause-based variations in voltinism (i.e., generations per year) in the silkworm. Based on forward genetics and other approaches, the research team has characterized the circadian clock gene Cycle as a molecular switch that controls seasonal diapause in silkworms. The findings reveal how clock genes regulate both seasonal rhythms and daily circadian rhythms

Supported by the National Natural Science Foundation of China (Grant Nos. 32225008, 32021001), the research team led by Dr. Shuai Zhan at the Center for Excellence in Molecular Plant Sciences, Chinese Academy of Sciences, has made new progress in the study of insect seasonal adaptation. The team has characterized a molecular switch that controls diapause in the silkworm and elucidated the genetic mechanisms underlying its life-history diversity. Their findings, entitled "Functional polymorphism of CYCLE underlies diapause variation in moths," were published online in Science on May 30, 2025 (Beijing time). Article link: https://www.science.org/doi/10.1126/science.ado2129.

Insects are the most widely distributed animal groups on Earth. To adapt to various environmental and climatic conditions, insects have evolved diversified seasonal adaptation strategies and flexible life-history strategies. As a common strategy for seasonal adaptation in insects, diapause denotes a developmentally arrested state induced by specific environmental cues (such as photoperiod, temperature, etc.), enabling insects to conquer adverse conditions such as low temperature, short daylength, and food scarcity. The occurrence of diapause significantly prolongs the life-cycle of insects and thus shapes the great diversity of life-history (often termed as voltinism) in insects. For example, northern populations of many pest species breed one or two generations per year because of diapause, while southern non-diapause populations may produce multiple generations annually.

In this study, the team focused on the voltinism variations in silkworms, the model species of Lepidoptera (including moths and butterflies). By applying forward genetic approaches and functional studies, they identified Cycle as the key molecular switch controlling the diapause entry in silkworms. Cycle is a core circadian clock gene in insects, usually involved in the regulation of daily circadian rhythm. The team found that, in the silkworm, Cycle encodes three isoforms (A–C) via alternative splicing; compared to diapause strains, the isoform C has been functionally disrupted in non-diapause strains due to a specific mutation, while their isoforms A and B are unaffected. The team further demonstrated that the isoform C controls diapause in silkworms and other lepidopterans, while the other isoforms might play the native role of CYCLE in regulating circadian rhythm.

This study not only identified a molecular "switch" controlling the environmental induction of diapause in Lepidoptera, but also proposed a model that explains how a pleiotropic gene such as the clock gene reconcile the stability of daily rhythm functions with the plasticity in mediating seasonal rhythms. In addition, these findings provide novel insights into evolutionary innovations and adaptive mechanisms of complex traits in animals.