diapause hormone Usage And Synthesis

Description

DH is synthesized by the subesophageal ganglion (SG). It is released into the hemolymph via the corpus cardiacumcorpus allatum (CC-CA) complex, and targets the developing ovaries during pupal-adult development to induce the embryonic diapause of the silkworm Bombyx mori.

Structure

DH is a C-terminally amidated peptide consisting of 24 aa residues with a C-terminal sequence of FGPRLNH2.Therefore, DH is a member of the FXPRL-NH2 family. DH is widely conserved among lepidopteran insects.

Discovery

DH was first discovered as a hormone that is synthesized by SG and released into the hemolymph to induce embryonic diapause in B. mori.It was isolated from B. mori SG, and its structure was determined in 1991.In addition, DH has been shown to induce the termination of pupal diapause in the corn earworm, Heliothis/ Helicoverpa species.

Distribution

DH is detected in seven pairs of neurosecretory cells (DH-PBAN-producing neurosecretory cells; DHPCs),which consists of the SMd, SMx, and SLb neuromeres along the ventral midline and in SL cells located within the B. mori SG.

Biological functions

B. mori DH acts on the developing ovaries during the middle pupal stage and transfers the DH signal into the oocyte via DHR. At 2–3 days after egg-laying, the eggs reach the diapause stage, with the enlargement of both extremities of the embryo and the completion of mesoderm segregation. In developing ovaries, DH is known to enhance trehalase activity on the oolemma, which leads to the accelerated incorporation of hemolymph trehalose by the oocyte and finally, to higher levels of glycogen deposition, compared to the oocyte without the DH signal. This glycogen is utilized for the production of sorbitol and glycerol along with the diapause. DH promotes trehalase activity via upregulation of the trehalase gene, during the period when Ca²⁺ and protein kinase C are involved in signal transduction.In Orgyia thyellina, DH induces seasonal reproductive polyphenism, including the egg diapause.

Regulation of synthesis and release

B. mori DH is biosynthesized in the neurosecretory cells in the SG. It is transported via nervus corporis cardiaci-III, maxillary nervus, nervi corporis cardiaci-ventralis, and nervi corporis cardiaci-nervus recurrens to the CC-CA complex, and is then released into the hemolymph.The expression of the B. mori DH-PBAN gene is switched on by the interaction of a cis-element (-1117 to-1088nt) on the 50 upstream region with a transcription factor (B. mori Pitx1).In the pupae of diapause eggs, which are the producers of the bivoltine strain (Kosetsu), the DH titer was measured at 8.85 ± 2.71pM, which was twofold higher than in the nondiapause egg producer.