Myoglianin is a crucial factor for the transition to the juvenile hormone-dependent phase during hemimetabolous nymphal development

Abstract

In hemimetabolous insects, the developmental process of nymphs is divided into three growth phases, i.e., juvenile hormone (JH)-independent, JH-dependent, and JH-free phases. The wing primordium in hemimetabolous insects is formed latently in the JH-independent phase and manifests and grows in the JH-dependent phase. Myoglianin (Myo) is known to be a key factor of metamorphosis in the JH-free phase of nymphs, regulating negatively JH synthesis. Here we find the role of Myo in earlier phases in the cricket Gryllus bimaculatus via gene knockout analysis using CRISPR/Cas9. In the myo knockout (KO) mutants, developmental delay during embryogenesis was observed, and nymphal body size and the timing of molting were affected. The KO nymphs underwent multiple molts, typically around seven, but remained significantly smaller in body size compared to wild-type individuals. The KO nymphs also did not exhibit the expected growth of wing primordia, implying that transition to JH-dependent phase was failed. This failure in phase transition could have been caused by excessive JH because titers of JH I and JH II were remarkably increased in the KO mutants. Our results suggest that Myo plays a crucial role not only in regulating timing of molting but also in the transition to the nymphal growth phases associated with growth of wing primordia and nymphal body size.