Consequences of selection on muscle composition. A comparative study on gracilis muscle in wild and domestic pigs

European domestic pigs are derived from the European wild pig by genetic selection and differ in their muscular-growth ability. Thus, it was the aim of this study to investigate the consequences of genetic selection on muscle composition and fibre size in the gracilis muscle. Fibre typing based on the ATPase reaction revealed that this muscle in wild pigs is composed mainly of type-I and -IIa fibres, whereas, in domestic pigs, type-IIb fibres were predominant. For all fibre types, domestic pigs had about threefold larger fibres than wild pigs. Type-I fibres tended to be the smallest fibres in domestic pigs, but the largest fibres in wild pigs, which may be due to long-term training effects in the free-ranging animals. Giant fibres as an indicator of degeneration were obvious only in the domestic-pig samples. Their occurrence, as well as the larger fibre size and the predominance of type-IIb fibres in domestic pigs, may be ascribed to high concentrations of growth hormone. It is concluded that selection for muscular growth may favour metabolic dysbalances within the muscle and, therefore, degenerative processes.