Abstract

Injections of the hormones ecdysone and ecdysterone produce molting in crayfish. The first observed effect of the hormones is apolysis, and the eventual result is early molting. When ecdysterone is injected at any time during premolt, it speeds the progression to the next substage of the molting cycle. In order to study the action of ecdysterone at the molecular level, we have focused on the process of chitin synthesis, which is just one aspect of the phenomenon of molting. We found ecdysterone does stimulate chi tin synthesis, provided it is injected into crayfish in stages D0 and D1', when chitin is normally synthesized very slowly. The ecdysones have been found to act via gene activation in other arthropods. In the crayfish, ecdysterone stimulates DNA synthesis in the epidermis. This is significant because major changes in gene activation seem to require concurrent DNA synthesis. The premolt increase in chitin synthesis in the crayfish can be inhibited with an RNA synthesis inhibitor. Therefore, ecdysterone probably activates synthesis of the RNA needed for synthesis of certain chitin-synthesizing enzymes. There are three of the enzymes in the pathway of chitin synthesis that seem most likely to be the ones affected by ecdysterone. The first of these is the enzyme that catalyzes the synthesis of glucosamine-6-phosphate. It's activity during the molting cycle parallels the rate of chitin synthesis, and N-acetylglucosamine is converted to chitin more rapidly than is glucose, as if the latter conversion is slowed by a limiting reaction at glucosamine-6-phosphate formation. The second enzyme is UDP-acetylglucosamine pyrophosphorylase. The activity of this enzyme during the molting cycle also parallels the rate of chitin synthesis, and injections of ecdysterone were found to increase the activity. The third enzyme is chitin synthetase. During stage D3, when chitin synthesis is well underway, compounds late in the synthetic pathway accumulate in the epidermis and in other tissues . Then, during early postmolt when chitin synthesis is at its peak, the concentrations of these compounds become much lower. This suggests that chitin synthetase limits the conversion of these compounds to chitin in stage D3, allowing them to accumulate. During postmolt, chitin synthetase no longer limits this conversion.

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