Master Factors of Insect Metamorphosis; Chinmo, Kr-h1, BR-C, E93

Kr-h1, BR-C and E93 are called Master factors of insect metamorphosis. Moreover, recently identified factor, Chinmo is included here. These factors are discussed from several stand points here. Insect evolved from ametabola through hemimetabola to holometabola. Holometabolous insect shows metamorphosis through the insect hormones, and JH or ecdysone regulates four factors; Chinmo, Kr-h1, BR-C and E93, punctuating larval, pupal and adult stages. Ecdysone was reported to upregulate CA activity, and JH was also reported to maintain the secretory activity of PG in holometabolous insect. Thus, JH and ecdysone interact each other until penultimate larval instar of holometabolous insect. Both hormones almost disappear at the early stage of the final larval instar, then low concentrations of ecdysteroid begins to be secreted and BR-C is activated, which has been explained to bring the commitment to the pupal stage. Now, by the identification of Chinmo, explanation of pupal commitment should be changed. Chinmo and Kr-h1 repress BR-C and E93 expression. E93 also represses BR-C expression, thus larval, pupal and adult stages are controlled through the interaction of factors. E93 is expressed in embryo of ametabolous and hemimetabolous insect, which is suggested to bring nymphal form, but is not expressed in embryo of holometabolous insect. Chromatin accessibility by Chinmo and E93, and the relation of these factors with insect evolution is to be elucidated.