Regulation of Muscle Gene Expression Over the Moult in Crustacea

Muscle growth in crustacea occurs over the moult and involves a corresponding increase in fractional rates of sarcomeric protein synthesis. Heterologous and homologous cDNA probes for the myofibrillar protein, actin and myosin, have been used to investigate the factors responsible for controlling muscle growth in crustaceans at the molecular level. Factors such as passive stretch, caused by ecdysial expansion; moulting hormones such as ecdysteroids, which regulate the cycle of moult related events; and temperature, an environmental factor known to influence the rates of many biochemical reactions, all play a role in modulating muscle growth in crustacea. In isopod crustaceans, the regulation of muscle growth produces new problems as these crustaceans are characterised by a biphasic moult cycle in which the posterior half of the old exoskeleton is shed hours to days before the anterior half. These animals therefore provide an ideal model for the study of differential muscle growth. In both decapod and isopod crustaceans, transcriptional regulation alone may not be sufficient to account for the the upregulation of sarcomeric protein synthesis during intermittent growth. Both passive stretch and elevated ecdysteroid titre increase protein synthesis rates for actin in lobster muscles, in vivo. These factors may be involved in the promotion of ribosomal activity and translational processing during ecdysial muscle growth. Temperature has a direct effect on protein synthesis rates and levels of actin mRNA expression and may be a principal factor in the control of seasonal moult cycles.