Inter- and intra-specific variation in myosin light chain and troponin I composition in fast muscle fibres from two species of fish (genusOreochromis) which have different temperature-dependent contractile properties

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Abstract
The contractile properties and myofibrillar protein composition of fast muscle have been characterized in pure strains of two tropical fishOreochromis niloticus andO. andersoni. Single fast muscle fibres were isolated from the abdominal myotomes and chemically skinned. The maximum tension-temperature relationships of fibres were similar at 25–30° C, but diverged below 17° C. At 10° C, maximum tension was around 60% higher inO. andersoni (160 ± 15 kN m−2) thanO. niloticus (105 ±13 kN m−2) (mean ±sd). The myofibrillar protein composition of fast fibres was investigated using one-dimensional and two-dimensional gel electrophoresis and peptide mapping. The twoOreochromis species differed with respect to the composition of myosin light chains, troponin I and mysoin heavy chains (V8 protease and chymotrypsin peptide maps). An unexpected finding was the presence of two isoforms of myosin light chain 1 inO. andersoni, with apparent molecular masses of 27.5 kDa (LC1f1) and 26.9 kDa (LC1f2). Individuals with LC1f1 (n=20) and LC1f1+LC1f2 (n=12) were represented in the population studied. The myosin light chain 3 (LC3f) content of fibres was similar in both cases. Breeding experiments established that these intra-specific variations in isoform composition were heritable. Fast muscle fromO. niloticus andO. andersoni contain two isoforms of troponin I (TNIf1+TNIf2) which were both expressed in single fibres. The identity of TNI was confirmed using a stationary phase troponin-C affinity column. Of the 20O. niloticus studied seven contained only TNIf1. The twoOreochromis species studied produce fertile F1 hybrids, are susceptible to ploidy manipulation, have a short generation time and rapid growth rates. They therefore represent a good model for investigating the genetic mechanisms underlying the inheritance of different force-generating capacities in fish.