Background of Insect Metamorphosis: Numerous Functions of Ecdysteroid

ABSTRACTInsect development is mainly controlled by juvenile hormone (JH) and ecdysone, and their hemolymph titer determines the insect direction; larva, pupa, or adult. The mediators of them are Krüppel homolog 1 (Kr‐h1), Broad‐Complex (BR‐C), and E93. They activate genes that characterize larval, pupal, and adult feature, which gives rise to the metamorphosis. Before individual ecdysis, these master factors activate target genes to produce larva, pupa, or adult. Prothoracicotropic hormone (PTTH) from the brain activates prothoracic gland (PG), resulted in the ecdysis. Other factors that activate ecdysteroid production are reported. The produced ecdysteroid was observed early stages of the last larval instar of Bombyx mori, where the ecdysteroid titer is different from previous stage. Two stages are different in JH and ecdysone titer, and the interaction of the JH and ecdysone production determines their titer. Ecdysone brings about the prominent change, which needs many gene transcriptions and the interaction of ecdysone‐responsive transcription factors (ERTFs). Their target genes are successively expressed, which brings about the metamorphosis. For the activation of genes, ecdysone gives rise to chromatin remodeling and histone modification. Ecdysone and other factors bring about cell division of the wing disc of the last larval instar; for the proliferation and differentiation, which gives rise to the growth and differentiation of the wing disc for the metamorphosis. In addition, hormone‐responsive miRNAs work, several ERTFs function for one gene, and the suppressive TF function along with metamorphosis. Thus, several attractive things underly around the insect metamorphosis. We will be near the understanding of the metamorphosis through these things.