title: Knockout silkworms reveal a dispensable role for juvenile hormones in holometabolous life cycle.
abstract: Insect juvenile hormones (JHs) prevent precocious metamorphosis and allow larvae to undergo multiple rounds of status quo molts. However, the roles of JHs during the embryonic and very early larval stages have not been fully understood. We generated and characterized knockout silkworms (Bombyx mori) with null mutations in JH biosynthesis or JH receptor genes using genome-editing tools. We found that embryonic growth and morphogenesis are largely independent of JHs in Bombyx and that, even in the absence of JHs or JH signaling, pupal characters are not formed in first- or second-instar larvae, and precocious metamorphosis is induced after the second instar at the earliest. We also show by mosaic analysis that a pupal specifier gene broad, which is dramatically up-regulated in the late stage of the last larval instar, is essential for pupal commitment in the epidermis. Importantly, the mRNA expression level of broad, which is thought to be repressed by JHs, remained at very low basal levels during the early larval instars of JH-deficient or JH signaling-deficient knockouts. Therefore, our study suggests that the long-accepted paradigm that JHs maintain the juvenile status throughout larval life should be revised because the larval status can be maintained by a JH-independent mechanism in very early larval instars. We propose that the lack of competence for metamorphosis during the early larval stages may result from the absence of an unidentified broad-inducing factor, i.e., a competence factor.
authors.affiliation: Division of Insect Sciences, National Institute of Agrobiological Sciences, Owashi 1-2, Tsukuba City, Ibaraki, 305-8634, Japan; daimontakaaki@affrc.go.jp.
authors.forename: Takaaki
authors.initials: T
authors.lastname: Daimon
authors.affiliation: Division of Insect Sciences, National Institute of Agrobiological Sciences, Owashi 1-2, Tsukuba City, Ibaraki, 305-8634, Japan;
authors.forename: Miwa
authors.initials: M
authors.lastname: Uchibori
authors.affiliation: Division of Insect Sciences, National Institute of Agrobiological Sciences, Owashi 1-2, Tsukuba City, Ibaraki, 305-8634, Japan;
authors.forename: Hajime
authors.initials: H
authors.lastname: Nakao
authors.affiliation: Genetically Modified Organism Research Center, National Institute of Agrobiological Sciences, Owashi 1-2, Tsukuba City, Ibaraki, 305-8634, Japan.
authors.forename: Hideki
authors.initials: H
authors.lastname: Sezutsu
authors.affiliation: Division of Insect Sciences, National Institute of Agrobiological Sciences, Owashi 1-2, Tsukuba City, Ibaraki, 305-8634, Japan;
authors.forename: Tetsuro
authors.initials: T
authors.lastname: Shinoda
mesh_terms: D000483:Alleles; D000818:Animals; D001483:Base Sequence; D012831:Bombyx / Q000196:embryology / Q000235:genetics* / Q000254:growth & development* / Q000648:ultrastructure; D003433:Crosses, Genetic; D004625:Embryo, Nonmammalian / Q000187:drug effects / Q000196:embryology; D047108:Embryonic Development / Q000187:drug effects / Q000235:genetics; D004720:Endonucleases / Q000378:metabolism; D005260:Female; D018507:Gene Expression Regulation, Developmental / Q000187:drug effects; D055786:Gene Knockout Techniques; D018390:Gene Targeting; D017344:Genes, Insect; D007605:Juvenile Hormones / Q000378:metabolism* / Q000494:pharmacology; D007814:Larva / Q000187:drug effects / Q000648:ultrastructure; D008018:Life Cycle Stages / Q000187:drug effects / Q000235:genetics; D008297:Male; D008969:Molecular Sequence Data; D009030:Mosaicism; D009154:Mutation / Q000235:genetics; D010641:Phenotype; D015398:Signal Transduction / Q000187:drug effects / Q000235:genetics; D015534:Trans-Activators / Q000378:metabolism
chemical_list: D007605:Juvenile Hormones; D015534:Trans-Activators; D004720:Endonucleases
keywords: Bombyx; TALEN; ecdysone; juvenile hormone; metamorphosis