Methoprene-Tolerant Orchestrates the Number of Larval Molts at the Second Instar Stage in an Herbivorous Ladybird

Abstract

Insect nymphal/larval instars vary widely across insect taxa and at the intraspecific level, they are orchestrated by two hormones, juvenile hormones (JH) and 20-hydroxyecdysone (20E). For insects whose nympha/larva molting events are more than 3, lack of JH signal causes premature metamorphosis and additional JH exposure induces supernumerary juvenile molts. In Drosophila melanogaster, the number of larval molts is fixed at two (dimolter) and is not affected by either JH or 20E signals. Larvae of a subset of Coleopterans are trimolters. Whether JH signal governs the number of larval molts in these Coleopteran trimolters deserves investigation. In the current study, we found that Methoprene-tolerant (Met) gene was actively transcribed from the first instar to pupal stages in an herbivorous ladybird Henosepilachna vigintioctomaculata. An injection of dsMet at the second, third and fourth (final) larval stages successfully knocked down the target gene, and disrupted the expression of both JH and 20E signaling genes. RNA interference for Met at the second instar larval stage reduced the number of larval molts from 3 to 2, and consequently caused premature metamorphosis and miniature pupae, as well as the impairment of larval-pupal transition. Contrarily, depletion of Met at the third and fourth instar larval periods arrested the larval development at the prepupal stage. Our findings suggest that JH signaling orchestrates the number of larval molts at the second instar stage in the H. vigintioctomaculata larvae.