MUSCLE-FIBER GROWTH DYNAMICS IN DIPLOID AND TRIPLOID RAINBOW-TROUT

The effect of triploidy on muscle fibre growth was determined by compa ring hyperplasia and hypertrophy of white muscle fibres in all-female, diploid and triploid rainbow trout Oncorhynchus mykiss (100-400 mm to tal length). Conventional morphometry and protein and DNA concentratio ns were used to assess muscle fibre hyperplasia and hypertrophy in whi te muscle samples derived from an anterio-dorsal location. Muscle fibr e distributions were significantly different between triploids and dip loids in trout <300 mm. The proportion of fibres <20 mu m was higher i n diploids than in triploids and the proportion of fibres in the 20-40 mu m category was higher in triploids than in diploids. This indicate s that the hyperplastic fibres of triploids are larger than those of d iploids. Larger hyperplastic fibres in triploids are probably due to t he combined effect of increased nuclear size in triploids and the rela tively high nucleus : cell ratio observed in small muscle fibres. Thes e larger fibres may be less favourable to cellular metabolic exchange because of their smaller surface area to volume ratios, and perhaps ac count for reduced viability and growth observed in triploids during ea rly life stages. On the other hand. the lack of difference in the dist ribution of fibres <20 mu m between diploids and triploids at larger b ody size ranges (301-400 mm) imply that triploid trout may have higher rates of new fibre: recruitment and growth capacity at these sizes. T here was no difference between diploid and triploid trout in the mean size of muscle fibres: however, the number of fibres per unit area was reduced by 10% in triploids. No differences were observed in protein or DNA concentrations in muscle tissues between the two genetic groups . Since triploid nuclei have 1.5 times more DNA than diploid nuclei, t his deviation from the expected muscle DNA concentration (1.3-1.4 time s more DNA in triploids when the 10% reduction in fibre density is con sidered) suggests that the number of nuclei per muscle fibre is reduce d. In both diploids and triploids, mean fibre size increased with body length while fibre density decreased. Similarly, protein concentratio n in the muscle tissue increased and DNA concentration declined with i ncreasing body length. Protein/DNA ratio was strongly and positively c orrelated with fibre size. These results demonstrate that changes in D NA and protein concentrations can be used to assess hyperplasia and hy pertrophy in muscle tissues. However, the morphometric procedure provi des better insight into muscle fibre growth as it enables the direct v isualization and analysis of muscle fibre distribution patterns. (C) 1 998 The Fisheries Society of tile British Isles.