ADAPTATION OF FISH TO DIFFERENT ENVIRONMENTAL-TEMPERATURE BY QUALITATIVE AND QUANTITATIVE CHANGES IN GENE-EXPRESSION

In fish that occupy a specific thermal habitat the contractile apparatus has been adapted during evolution for that specific temperature range. In Antarctic fish the myofibrillar GTPase activity is relatively high at temperatures around 0 degrees C, but this is at the expense of enzyme thermostability as this enzyme from antarctic fishes is heat inactivated at comparatively low temperatures (Johnston and Goldspink, 1975c; Johnston and Walesby, 1977). Such thermal characteristics may be a general feature of proteins from Antarctic fish (Somero, 1991) and so may contribute to their extreme cold stenothermy. Some eurythermal species of fish have the ability to rebuild their contractile systems by expressing a different set of genes at low temperature to that expressed at warm environmental temperatures. We have cloned and are in the process of characterising warm and cold as well as embryonic myosin heavy chain isoform genes of carp. These genes encode different types of myosin crossbridges (the force generators for muscular contraction) and therefore determine the contractile characteristics of the muscle. The physiological result of the switches in gene expression are that the muscles of fish acclimated to low environmental temperatures develop more force and more power at these temperatures than muscles from fish acclimated to warm environmental temperatures. Carp acclimated to warm temperatures have myosin with a greater thermostability but at a lower myofibrillar ATPase.