CHARACTERISTICS OF THE PECTORALIS-SUPERFICIALIS AND SEMIMEMBRANOSUS OF BROILER STRAIN CHICKENS, BANTAM CHICKENS, AND THE RECIPROCAL CROSSES

At hatch, chicks from bantam sire x broiler dam (BaBr) matings were as heavy as those from broiler sire x broiler dam (BrBr) matings. Both w ere heavier than pure bantam chicks (baba) or chicks from broiler stra in sires and bantam dams (BrBa), but weights of the latter two genotyp es were identical. The Pectoralis superficialis and Semimembranosus of pure bantam chicks weighed less than those of pure broiler chicks thr oughout the studies. Muscle weights in the BaBr chicks were markedly h eavier than those of genetically similar BrBa chicks at hatch. Genetic influences were quickly expressed and weights of muscles in these two groups had converged by 14 d of age. Myofiber cross-sectional areas a t 1 d of age were not clearly associated with genotype or maternal env ironment, but clear patterns had begun to develop by 4 d of age. Myofi ber area of both muscles was greater in BrBr than BaBa chicks by this age and after. Myofiber areas in the reciprocal crosses converged towa rd each other and away from the pure line chicks. By 11 d, the area of Pectoralis myofibers of BrBr chicks was greatest, those of the recipr ocal crosses was intermediate and equal to each other, and that of BaB a chicks was least. A similar trend was apparent in Semimembranosus fi bers by 11 d but the differences did not become significant until 21 d of age. Significant differences in myofiber number of the Semimembran osus were present (BrBr >BaBr >BrBa >BaBa). Thus, differences in muscl e mass were the result of differences in both fiber number and fiber s ize.