Regulation of Metamorphosis in Holometabolous Insects - PubMed

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Holometabolous insects' metamorphosis is regulated by three genes: chinmo, broad, and E93, corresponding to larval, pupal, and adult stages.
A temporal endocrine landscape involving ecdysteroids, juvenile hormone (JH), and myoglianin controls life stage transitions.
Chinmo suppresses broad and E93, acting as an anti-metamorphic gene.
JH target gene Krüppel-homolog 1 (Kr-h1) also suppresses metamorphosis, with JH's role varying by larval stage.
Early larval molts are JH-independent, while later ones require JH to maintain larval molting and suppress E93 until a size threshold is met.
Myoglianin and/or ecdysteroids induce broad expression, and declining JH levels can stimulate E93.
Entry to metamorphosis is mediated by either E93 (Tribolium) or broad (Drosophila), depending on JH necessity.
Inhibitory interactions between broad and E93, along with JH's return in the prepupa, ensure pupa formation precedes adult differentiation.
In hemimetabolous insects, Chinmo and JH (via Kr-h1) maintain the nymphal stage and suppress E93 and adult differentiation.
Broad and chinmo collaborate in nymphal growth, with chinmo supporting isomorphic growth and broad supporting positive allometric growth.
Mutual inhibition between these transcription factors was a key evolutionary innovation, enabling imaginal primordia expansion in larvae before pupal morphogenesis.

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