Molecular And Enzymatic Mechanisms Of The Pharate-adult CA Reactivation In Samia Cynthia Ricini

Juvenile hormones (JH) play important roles in insect development and reproduction and thus JH biosynthesis in corpora allata (CA) need to be rigorously controlled. The synthesis of JH is regulated by neuropeptides, such as allatotropin (AT) and allatostatin (AST), which are secreted from brain neurosecretory cells. JH synthesis should also be immediately controlled by key enzyme(s) involved in the JH synthesis pathway. Our previous studies have shown that, during nearly the whole pupal stage of the Eri silkworm Samia cynthia ricini (Samcri), CA does not produce any JH, but restarts to synthesize JH at ~6 hours before adult emergence, and increases to a maximal level shortly after emergence before gradually decreasing during the following two days. To uncover the molecular and enzymatic mechanisms of the pharate-adult CA reactivation, we first cloned cDNAs encoding Samcri-AT and–AST. Their expression patterns were then analyzed. The expression levels of Samcri-AT and -AST genes correlated well with the allatoregulatory activities in the brain extracts during the period of adult emergence implying the two neuropeptides tightly control JH synthesis, in a contradictive and cooperative manner. Additionally, tissue expression patterns of the two neuropetpides show their pleiotropic physiological functions. We also cloned cDNAs encoding two potential key enzymes in the JH synthesis pathway: 3-Hydroxy-3-methylglutaryl CoA reductase (HMGR) and juvenile hormone acid methyl transferase (JHAMT). Samcri-HMGR in CA was expressed at constantly high levels during the period of adult emergence, while JHAMT mRNA level and enzyme activity had close correlations with that of JH synthesis. Moreover, JHAMT (mRNA and enzyme activity) and JH synthesis were both much higher in females than in males. These results suggest that the pharate-adult CA reactivation is mainly attributed to the re-expression of JHAMT and the difference in JH synthesis between sexes is also determined by that of JHAMT. Furthermore, changes in JH synthesis in the fifth larvae of S. cynthia ricini and the pharate adults of Bombyx mori (Bommo) were also due to changes in JHAMT expression. Therefore, we conclude that JHAMT but not HMGR is a key enzyme for JH synthesis in this insect species. In addition, we have preliminary data to show that JHAMT is regulated by Samcri-AT at the transcriptional level. This result suggests that JHAMT is likely a mediator for the regulation of JH biosynthesis by allatoregulatory neuropeptides.