Respective contributions of juvenile hormone and 20-hydroxyecdysone to the regulation of major haemolymph protein synthesis in Bombyx mori larvae

Major haemolymph proteins are produced by the larval fat body of Bombyx mori mainly at the end of the feeding period. The endocrine regulation of their synthesis and accumulation of their mRNAs have been investigated in vitro and in isolated abdomens. These haemolymph proteins are subjected to a dual regulation by both juvenile hormone and 20-hydroxyecdysone. 20-Hydroxyecdysone decreases the actual amount of their mRNAs irrespective of the age at which it is applied, and is probably responsible for the cessation of the activity of the corresponding genes during the spinning period; juvenile hormone analogue slows down the accumulation of their mRNAs when applied during the first part of the last instar or triggers a decrease in their amount throughout a prothoracicotropic effect when applied during the last 3 days of the feeding period. Thus according to the developmental stage, juvenile hormone influences major haemolymph protein production through two processes, one dependent and one independent of 20-hydroxyecdysone. On the basis of in vitro experiments, we suggest that both hormones can act on the fat body and modify the synthesis of major haemolymph proteins independently of any other neuroendocrine centre; the characteristics of their respective actions are very different.