TEMPERATURE AND DEVELOPMENTAL PLASTICITY OF MUSCLE PHENOTYPE IN HERRING LARVAE

Myogenesis, the expression of myofibrillar protein isoforms and the de velopment of muscle innervation were investigated in Clyde herring (Cl upea harengus L.) in two successive spawning seasons at temperatures r anging from 5 degrees C to 15 degrees C. Myotube formation occurred in a rostral to caudal progression at similar somite stages at all tempe ratures. Superficial mononuclear muscle pioneer fibres were present at the horizontal septum. Myofibrillogenesis was retarded with respect t o somite stage at low temperatures; for example, by the 50-somite stag e, myofibrils were observed in the muscle pioneers of the first 31 som ites at 12 degrees C, but only the first 20 somites at 5 degrees C. In the electron microscope, the earliest stages of myofibril assembly we re observed in the muscle pioneer cells and in a proportion of the mul tinucleated myotubes within the same somite. By the end of somitogenes is, the density of myofibrils in the rostral myotomes was much higher at 15 degrees C than at 5 degrees C. Embryonic isoforms of myosin ligh t chain 2 (LC2), troponin I and troponin T were identified in the pres umptive white muscle using two-dimensional gel electrophoresis. Expres sion of the embryonic isoforms was gradually switched off during the l arval stages. The size range over which embryonic isoforms were presen t was inversely related to rearing temperature. For example, the adult pattern of myosin LC2 expression was established at 11 mm total lengt h (TL) at 15 degrees C, but not until 15 mm TL at 5 degrees C. Acetylc holinesterase staining was apparent at the myosepta in 31-somite stage embryos at 15 degrees C, but not until approximately the 40-somite st age at 5 degrees C. The red muscle fibres of larvae were initially inn ervated only at their myoseptal ends. The temperature at which the red muscle fibres became multiply innervated was inversely related to bod y size, occurring at 12-14 mm at 12 degrees C, but not until 16-19 mm at 5 degrees C. We conclude that the temperature during early developm ent determines the relative timing and degree of expression of the myo genic programme, resulting in significant phenotypic variation in the swimming muscles of the larval stages. Our results highlight a potenti al mechanism whereby early thermal experience could influence survival and hence the strength of particular year classes of fish.