INFLUENCE OF DEVELOPMENT AND REARING TEMPERATURE ON THE DISTRIBUTION, ULTRASTRUCTURE AND MYOSIN SUBUNIT COMPOSITION OF MYOTOMAL MUSCLE-FIBER TYPES IN THE PLAICE PLEURONECTES-PLATESSA

Eggs of the plaice Pleuronectes platessa L. were incubated at temperatures of 5, 8, 10, 12 and 15 degrees C in March 1990, 1991 and 1992. The myotomes of yolk-sac larvae contain a single superficial layer of small-diameter muscle fibres which stain intensely for succinic dehydrogenase activity, surrounding 390 to 500 weakly staining inner-muscle fibres of larger diameter. Larvae reared at 15 degrees C only survived for a few days and had significantly more inner-muscle fibres of larger average cross-sectional area than those hatching at 5 to 10 degrees C. Myofibrils occupied 61% of the volume of inner-muscle fibres in 15 degrees C larvae compared with 35 and 36% in larvae hatching at 5 and 10 degrees C, respectively (P <0.01). Following metamorphosis, which occurs between 7 and 10 wk, the myotomes retain the single layer of superficial-muscle fibres characteristic of larvae. A thickening of the superficial-muscle layer is first evident in 4 to 5 mo-old laboratory-reared fish of 20 mm total length (TL) and in 0-group fish caught in June and July. On the basis of the histochemical staining reactions for myofibrillar ATPase and succinic dehydrogenase activities, the myotomes of 1-group (104 mm TL) and adult (280 mm TL) plaice were found to contain a minimum of six distinct muscle-fibre types. Two-dimensional gel electrophoresis and peptide mapping were used to investigate changes in myosin subunit composition during development. Myosin from the inner muscle of larvae contains two isoforms of the phosphorylatable light-chain 2 (LC2(L1) and LC2(L2)). Following metamorphosis and during the first year, inner-muscle fibres co-express LC2 isoforms characteristic of the superficial fast-muscle fibres of adult plaice (LC2(F1) and LC2(F2)) in addition to the larval isoforms. Fast-muscle fibres isolated from deep layers of the myotomes in adult plaice only contain LC2(F2). In contrast, myosin from larval muscle and adult fast muscle contain apparently identical alkali light chains (LC1 and LC3). Peptide maps of myosin heavy chains (MHCs) from 6 wk-old larvae and 10 wk-old fish that had completed metamorphosis are similar, but distinct from those of 1-group plaice. Further changes in white-muscle MHC composition are evident between 1-group fish of 104 mm TL and adults of 280 mm TL.