TGF-β signaling in insects regulates metamorphosis via juvenile hormone biosynthesis

Abstract

Although butterflies undergo a dramatic morphological transformation from larva to adult via a pupal stage (holometamorphosis), crickets undergo a metamorphosis from nymph to adult without formation of a pupa (hemimetamorphosis). Despite these differences, both processes are regulated by common mechanisms that involve 20-hydroxyecdysone (20E) and juvenile hormone (JH). JH regulates many aspects of insect physiology, such as development, reproduction, diapause, and metamorphosis. Consequently, strict regulation of JH levels is crucial throughout an insect’s life cycle. However, it remains unclear how JH synthesis is regulated. Here, we report that in the corpora allata of the cricket, Gryllus bimaculatus, Myoglianin (Gb’Myo), a homolog of Drosophila Myoglianin/vertebrate GDF8/11, is involved in the down-regulation of JH production by suppressing the expression of a gene encoding JH acid O-methyltransferase, Gb’jhamt. In contrast, JH production is up-regulated by Decapentaplegic (Gb’Dpp) and Glass-bottom boat/60A (Gb’Gbb) signaling that occurs as part of the transcriptional activation of Gb’jhamt. Gb’Myo defines the nature of each developmental transition by regulating JH titer and the interactions between JH and 20E. When Gb’myo expression is suppressed, the activation of Gb’jhamt expression and secretion of 20E induce molting, thereby leading to the next instar before the last nymphal instar. Conversely, high Gb’myo expression induces metamorphosis during the last nymphal instar through the cessation of JH synthesis. Gb’myo also regulates final insect size. Because Myo/GDF8/11 and Dpp/bone morphogenetic protein (BMP)2/4-Gbb/BMP5–8 are conserved in both invertebrates and vertebrates, the present findings provide common regulatory mechanisms for endocrine control of animal development.

Expression of juvenile hormone acid O‐methyltransferase and juvenile hormone synthesis in Blattella germanica

“…JHAMT depletion with RNAi in juvenile insects produces precocious metamorphosis in T. castaneum (Minakuchi et al ., ) and in the cricket Gryllus bimaculatus (Ishimaru et al ., ). In addition, similar treatments in adult females undergo a reduction of vitellogenin mRNA in T. castaneum (Sheng et al ., ) and in the planthopper Nilaparvata lugens (Lu et al ., ) and low JH synthesis measured in CA radiochemical assay in the locust Schistocerca gregaria (Marchal et al ., ).…”

Bone morphogenetic protein signaling in distal patterning and intercalation during leg regeneration of the cricket, Gryllus bimaculatus

“…Meanwhile, Mothers-against-dpp (Gb'mad) is the prototype of a family of genes required for signaling by transforming growth factor (TGF)β-related ligands, and it has been shown to be essential for the BMP signaling pathway (Hamaratoglu, Affolter, & Pyrowolakis, 2014;Raftery & Sutherland, 1999;Schmierer & Hill, 2007). It has previously been demonstrated that RNAi targeting of Gb'dpp has no effect on leg development and regeneration, while RNAi targeting of Gb'mad, Gb'tkv, or both Gb'dpp and Gb'glass bottom boat/60A (Gb'gbb), which encodes a BMP family member, leads to precocious adult metamorphosis (Ishimaru et al, 2016). It has previously been demonstrated that RNAi targeting of Gb'dpp has no effect on leg development and regeneration, while RNAi targeting of Gb'mad, Gb'tkv, or both Gb'dpp and Gb'glass bottom boat/60A (Gb'gbb), which encodes a BMP family member, leads to precocious adult metamorphosis (Ishimaru et al, 2016).…”

“…Double-stranded RNAs (dsRNAs) designed to target Gb'mad, Gb'tkv, Gb'dac, Gb'ds, and Gb'ft were prepared as previously described (Bando et al, 2009;Ishimaru et al, 2015Ishimaru et al, , 2016Nakamura et al, 2007). Briefly, following the injection of dsRNAs into the abdomen of third-instar nymphs, tibias were amputated at the distal position between the second and third spines.…”

“…The ABI 7900 Real-Time PCR System (Applied Biosystems, Foster, CA, US) with Power SYBR Green PCR Master Mix (Applied Biosystems) was used to perform qPCR as previously described (Ishimaru et al, 2016;Nakamura, Mito, Miyawaki, Ohuchi, & Noji, 2008). RNA extraction was performed in triplicate biological samples at 2 dpa or 5 dpa.…”

“…RNA extraction was performed in triplicate biological samples at 2 dpa or 5 dpa. The Gb'β-actin gene was selected as an internal control gene (Bando et al, 2009;Ishimaru et al, 2016). Total RNA was reverse transcribed into cDNA using the SuperScript III First-Strand Synthesis System (Invitrogen) with an oligo(dT) 20 primer according to the manufacturer's instructions.…”