Twenty-hydroxyecdysone produced by dephosphorylation and ecdysteroidogenesis regulates early embryonic development in the silkmoth, Bombyx mori

Ecdysteroids are key regulators of embryonic development as well as molting and metamorphosis in insects. Although an active form of ecdysteroids, 20-hydroxyecdysone (20E) is known to be produced through ecdysteroidogenesis from cholesterol and dephosphorylation of 20E-phosphate during embryogenesis in Lepidoptera, the importance of these production mechanisms in embryonic development has been unclear. Here, we investigated the activation timing of ecdysteroidogenesis from cholesterol and 20E-phosphate dephosphorylation during early embryogenesis in non-diapause eggs of the silkmoth Bombyx mori by observing morphological development, quantifying 20E and 20E-phosphate, measuring transcripts of enzymes involved in 20E production, and detecting activity of these enzymes using egg extracts. Stage-dependent 20E fluctuation and changes in mRNA amounts of enzymes suggest that the two 20E-producing mechanisms are activated at different stages during embryogenesis. Furthermore, knockdown of a dephosphorylation enzyme delayed development at early embryogenesis, whereas knockdown of an ecdysteroidogenic enzyme delayed development at early-middle embryogenesis. These results suggest that 20E is primarily produced initially by dephosphorylation of 20E -phosphate, and then by ecdysteroidogenesis from cholesterol to induce progression of embryonic development in B. mori.