Myoglianin triggers the pre-metamorphosis stage in hemimetabolan insects

What is it about?

Juvenile hormone represses metamorphosis in insects. Therefore, a decrease in this hormone in the final juvenile instar triggers adult morphogenesis. But, what induces this juvenile hormone decrease? We have addressed this question using as model the German cockroach, Blattella germanica, a basal insect showing progressive metamorphosis. We found that, during the penultimate nymphal instar, the protein Myoglianin is highly expressed in the corpora allata, which are the glands that produce juvenile hormone, and in the prothoracic gland, the glands that produce ecdysone, the hormone that triggers molting and metamorphosis. In the corpora allata, high Myoglianin levels repress the expression of a gene coding for a critical enzyme of juvenile hormone biosynthesis. In the prothoracic gland, low circulating juvenile hormone levels trigger gland degeneration, which is necessary for metamorphosis. Moreover, the prothoracic gland peak of Myoglianin indirectly stimulates the expression of ecdysone biosynthesis genes, thus enhancing the production of the metamorphic ecdysone pulse in final nymphal instar. The Myoglianin expression peak in penultimate nymphal instar also represses cell proliferation, which can contribute to enhance ecdysone production. The data indicate that Myoglianin triggers the pre-metamorphic nymphal instar in B. germanica, and possibly in other insects with progressive metamorphosis.

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