Abstract
Independent of the design of the life cycle of any insect, their growth and reproduction are highly choreographed through the action of two versatile hormones: ecdysteroids and juvenile hormones (JH). However, the means by which JH can target tissues and exert its pleiotropic physiological effects is currently still not completely elucidated. Although the identity of the one JH receptor is currently still elusive, recent evidence seems to point to the product of the Methoprene-tolerant gene (Met) as the most likely contender in transducing the action of JH. Studies on the role of this transcription factor have mostly been focused on immature insect stages. In this study we used the viviparous cockroach Diploptera punctata, a favorite model in studying JH endocrinology, to examine the role of Met during reproduction. A tissue distribution and developmental profile of transcript levels was determined for Met and its downstream partners during the first gonadotropic cycle of this cockroach. Using RNA interference, our study shows that silencing Met results in an arrest of basal oocyte development; vitellogenin is no longer transcribed in the fat body and no longer taken up by the ovary. Patency is not induced in these animals which fail to produce the characteristic profile of JH biosynthesis typical of the first gonadotropic cycle. Moreover, the ultrastructure of the follicle cells showed conspicuous whorls of rough endoplasmic reticulum and a failure to form chorion. Our study describes the role of Met on a cellular and physiological level during insect reproduction, and confirms the role of Met as a key factor in the JH signaling pathway.
Highlights
Juvenile hormone plays a prominent and diverse role in the regulation of insect development, being implicated in moulting, metamorphosis, reproduction, ageing, polymorphism and caste differentiation in social insects
Partial cDNAs encoding the juvenile hormones (JH) receptor Met and its downstream targets Kruppel homolog 1 (Kr-h1) and Br-C were isolated from D. punctata
Blastx National Center for Biotechnology Information (NCBI) database searches revealed that the Diploptera orthologs showed substantial similarity to the previously described Met, Krh1 and Br-C sequences and were named DippuMet, DippuKr-h1 and DippuBr-C
Summary
Juvenile hormone plays a prominent and diverse role in the regulation of insect development, being implicated in moulting, metamorphosis, reproduction, ageing, polymorphism and caste differentiation in social insects. JH orchestrates insect moulting and metamorphosis together with the ecdysteroids. The hormone is considered to have an anti-metamorphic action i.e. the presence of JH during certain critical periods which results in the continuation of the larval stage. Following the decline in JH titer in the latter half of the final larval stage, an ecdysteroid surge ensues in the absence of JH, and a metamorphic moult takes place. There is, accumulating evidence that Methoprenetolerant (Met), a basic-helix-loop-helix (bHLH)/Per-Arnt-Sim (PAS) protein, transduces the anti-metamorphic action of JH in both hemimetabolous as well as holometabolous insects [1] and as reviewed by [2,3]. Drosophila melanogaster Met mutants are resistant to toxic doses of JH or its analog methoprene (JHA) [4,5]
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