Methoprene-Tolerant (Met) Is Indispensable for Larval Metamorphosis and Female Reproduction in the Cotton Bollworm Helicoverpa armigera.

Long Ma,Haijun Xiao,Lin Chen,Wanna Zhang,Chen Liu,Yang Xu,Gemei Liang

doi:10.3389/fphys.2018.01601

Abstract

Juvenile hormone (JH) represses larval metamorphosis and induces adult reproduction in insects. Methoprene-tolerant (Met) is identified as an intranuclear receptor that mediates JH actions. In the present study, we characterized a Met from the severe agricultural pest, Helicoverpa armigera, namely HaMet. In the larval stage, HaMet is predominantly expressed in the epidermis and midgut, and is upregulated before each molting, whereas in adults HaMet is maximally expressed in the ovary, testis, and fat body. The immunofluorescence assay revealed that HaMet was distributed in the longitudinal and circular muscle layers of midgut in larvae, whereas in the ovary of female adults, HaMet was localized in the nucleus of the oolemma. Knockdown of HaMet in final-instar larvae shortened the time of pupation, induced abnormal pupation, and dampened pupation rate. In female adults, HaMet depletion severely suppressed the transcription of Vitellogenin (Vg) and Vitellogenin Receptor (VgR), disrupted the Vg accumulation in fat body and the yolk protein uptake in oocytes, and finally led to an impaired fecundity. Our findings therefore confirmed that HaMet acted as a nuclear receptor of JH and played an essential role in larval metamorphosis, vitellogenesis, and oocyte maturation.

Highlights

Juvenile hormones (JHs) are sesquiterpenoid compounds that are synthesized and secreted by corpora allata
Studies in A. aegypti, T. castaneum, and B. mori have revealed that the heterodimerization of Met with a steroid receptor coactivator was required for the JH-induced transcription of JH target genes (Li et al, 2011; Zhang et al, 2011)
Multiple studies have confirmed that Met is an essential receptor in the JH signaling pathway (Konopova and Jindra, 2007)

Summary

Introduction

Juvenile hormones (JHs) are sesquiterpenoid compounds that are synthesized and secreted by corpora allata. Methoprene-tolerant protein has a typical basic helix-loophelix Per-Arnt-Sim (bHLH-PAS) domain This protein is found to be capable of binding to both natural JH-III and a mimicker of JH (methoprene) with high affinity (Ashok et al, 1998; Miura et al, 2005; Charles et al, 2011). Since the first characterization of Met in D. melanogaster (Wilson and Fabian, 1986), homologs of Met have been identified from a broad range of insect species, including Aedes aegypti (Zhu et al, 2010), Tribolium castaneum (Konopova and Jindra, 2007), Bombyx mori (Kayukawa et al, 2012), Pyrrhocoris apterus (Konopova et al, 2011), and Nilaparvata lugens (Lin et al, 2015). Krüppel-homolog 1 (Kr-h1), a transcription factor with a DNA-binding domain consisting of eight zinc fingers, was reported to work on the downstream of Met in the JH signal pathway (Belles and Santos, 2014)

Methods

Results

Conclusion