TGF-β-dependent regulation of juvenile hormone biosynthesis in insect development and metamorphosis

Insects undergo precisely coordinated developmental transitions regulated by ecdysteroids and juvenile hormone (JH). Recent studies in the hemimetabolous cricket, Gryllus bimaculatus, have revealed that transforming growth factor-β signaling directly regulates JH biosynthesis. In this system, Decapentaplegic (Dpp) promotes juvenile hormone acid methyltransferase (jhamt) expression and JH production, whereas Myoglianin (Myo) represses jhamt transcription. Functional analyses using RNA interference and CRISPR/Cas9-mediated gene knockout demonstrate that loss of myo leads to sustained JH overproduction, supernumerary molts, and failure of metamorphosis, establishing Myo as a key regulator of developmental timing during nymphal development. This review integrates findings from diverse insect lineages to place Myo-dependent regulation of JH biosynthesis within an evolutionary framework, highlighting its conserved endocrine roles in hemimetabolous insects and its lineage-specific diversification in holometabolous species. Together, these studies provide new insights into how Myo signaling links growth status to endocrine control of developmental progression and metamorphosis.