Inactive prothoracic glands in larvae and pupae of Aedes aegypti: Ecdysteroid release by tissues in the thorax and abdomen

doi:10.1016/0965-1748(92)90032-A

Insect Biochemistry and Molecular Biology

Volume 22, Issue 6, September 1992, Pages 553-559

https://doi.org/10.1016/0965-1748(92)90032-A Get rights and content

Abstract

We report that the tissue identified as the prothoracic gland in fourth instar Aedes aegypti does not release ecdysteroids. The thorax, without the prothoracic gland, released the same amount of ecdysteroids in vitro as the intact thorax. Ecdysteroids were also released in vitro from the abdomen. Thoracic and abdominal ecdysteroid release was observed in the pupa and in all larval instars. During the fourth instar, a single increase and decrease in the rate of ecdysteroid release was observed from both the thorax and the abdomen. The thoracic and abdominal tissues of both sexes release ecdysteroids synchronously and in equal amounts. Hemolymph ecdysteroid titers are correlated with thoracic and abdominal release of ecdysteroids in vitro. Ecdysteroids are released from all abdominal segments, and the same amounts of ecdysteroids are released from the dorsal and ventral halves of the abdomen. More free ecdysteroids are released in vitro from thoracic and abdominal tissues than are present in the tissues themselves. Additionally, incubation of tissue homogenates with enzymes that hydrolyze ecdysteroid conjugates fails to increase ecdysteroid recovery, suggesting that as yet unidentified cells in both the thorax and abdomen synthesize ecdysteroids de novo.

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There is precedent for a non-PG primary source of ecdysteroids during embryogenesis in D. melanogaster (the epidermis; see above) and during metamorphosis in other insects, with the oenocytes and epidermis implicated in various species (reviewed in Delbecque et al., 1990). An alternative primary source during D. melanogaster metamorphosis has never—to our knowledge—been formally tested with either biochemical analyses of the presence of Halloween enzymes in non-PG tissues during the PAP, nor with ex vivo ecdysteroid secretion experiments like those performed in other insect species (Delbecque et al., 1990; Jenkins et al., 1992; Telang et al., 2007). Despite this absence of supportive biochemical data, there is a small amount of (non-shd) Halloween gene expression in non-PG larval tissues (Leader et al., 2018) and whole-body expression of most of these genes is roughly sustained past pupation (Graveley et al., 2011).
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Phylogenetic studies, however, indicate that Torso is structurally distinct from the IR and OEHR (Vogel et al., 2013), while functional assays indicate that Torso signals through the mitogen-activated protein kinase (MAPK) pathway (Rewitz et al., 2009). The mosquito literature suggests ECD stimulates molting and metamorphosis, but notably identifies the body wall of larvae rather than PTGs as the source of ECD (Jenkins et al., 1992; Telang et al., 2007). While mosquitoes encode orthologs for ptth and torso (Predel et al., 2010; Akbari et al., 2013), their functional roles in ECD biosynthesis are unknown.
A critical step in mosquito reproduction is the ingestion of a blood meal from a vertebrate host. In mosquitoes like Aedes aegypti, blood feeding stimulates the release of ovary ecdysteroidogenic hormone (OEH) and insulin-like peptide 3 (ILP3). This induces the ovaries to produce ecdysteroid hormone (ECD), which then drives egg maturation. In many immature insects, prothoracicotropic hormone (PTTH) stimulates the prothoracic glands to produce ECD that directs molting and metamorphosis. The receptors for OEH, ILP3 and PTTH are different receptor tyrosine kinases with OEH and ILP3 signaling converging downstream in the insulin pathway and PTTH activating the mitogen-activated protein kinase pathway. Calcium (Ca²⁺) flux and cAMP have also been implicated in PTTH signaling, but the role of Ca²⁺ in OEH, ILP3, and cAMP signaling in ovaries is unknown. Here, we assessed whether Ca²⁺ flux affects OEH, ILP3, and cAMP activity in A. aegypti ovaries and also asked whether PTTH stimulated ovaries to produce ECD. Results indicated that Ca²⁺ flux enhanced but was not essential for OEH or ILP3 activity, whereas cAMP signaling was dependent on Ca²⁺ flux. Recombinant PTTH from Bombyx mori fully activated ECD production by B. mori PTGs, but exhibited no activity toward A. aegypti ovaries. Recombinant PTTH from A. aegypti also failed to stimulate either B. mori PTGs or A. aegypti ovaries to produce ECD. We discuss the implications of these results in the context of mosquito reproduction and ECD biosynthesis by insects generally.
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Given the literature on many other insects, it is somewhat ironic that surprisingly little is known about the regulation of moulting in mosquitoes. An early study showed that larval thoracic and abdominal wall tissues produce ecdysone yet also reported that PGs do not (Jenkins et al., 1992). These findings together with the Drosophila literature indicate our understanding of peptide hormone functions in moulting are far from complete for Diptera.
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Inactive prothoracic glands in larvae and pupae of Aedes aegypti: Ecdysteroid release by tissues in the thorax and abdomen

Abstract

Steroids

Int. J. invert. Reprod. Dev.

Experientia

Experientia

Science

J. Insect Physiol.

J. Insect Physiol.

Appl. ent. Zool.

Biol. Zentralbl.

The ecdysone titer in Drosophila hydei and the metabolism of moulting hormones in the salivary glands

External factors and ecdysone release in Calliphora erythrocephala

Experientia

Endocrinology of the prothoracicotropic hormone

Arthropod molting hormone: radioimmune assay

Science

The stomodaeal nervous system, the neurosecretory system, and the gland complex in Aedes aegypti (L.)(Diptera:Culicidae)

Can. J. Zool.

Molting of roaches without prothoracic glands

Science

Aedes aegypti (L.) the Yellow Fever Mosquito: Its Life History, Bionomics and Structure

Aedes aegypti (L.) the Yellow Fever Mosquito: Its Life History, Bionomics and Structure

Aedes aegypti (L.) the Yellow Fever Mosquito: Its Life History, Bionomics and Structure

Aedes aegypti (L.) the Yellow Fever Mosquito: Its Life History, Bionomics and Structure

Aedes aegypti (L.) the Yellow Fever Mosquito: Its Life History, Bionomics and Structure

Metamorphosis of the corpus allatum and degeneration of the prothoracic glands during the larval-pupal-adult transformation of Drosophila melanogaster: a cytophysiological analysis of the ring gland

Dev. Biol.

Haemolymph-ecdysteroid peaks occur in headless larvae of Calpodes ethlius

J. Insect Physiol.

Conversion of radiolabelled 2,22,25-trideoxyecdysone in Tenebrio pupae

Insect Biochem.

A technique for transplantation for Drosophila

Am. Nature