Effect of selection for growth rate on embryonic breast muscle development in turkeys.

Muscle development at 20 and 25 d of incubation was studied in a randombred control line (RBC2), a subline (F) of RBC2 selected only for increased 16-wk BW, a commercial sire line (B), and reciprocal crosses of the F and B lines. Muscle samples from three males and three females of each genetic group were collected in such a manner to avoid contraction. After fixing, the muscles were stained with hematoxylin and eosin, measurements of muscle fiber width, muscle fiber bundle length and width, number of fibers within a 15.6 microm2 area, and extracellular matrix perimysial (PW) and endomysial (EW) width were taken with an Olympus XI 70 microscope equipped with an Olympus Magna Fire digital camera linked to Image Pro software. From each slide, 20 measurements were taken for each characteristic analyzed. In most of the muscle traits measured, additive genetic variation, as indicated by line differences, occurred when the RBC2 line was included in the comparison of pure lines. However, when only the B and F lines were compared, line differences were less frequent. In comparisons of the B and F lines and their reciprocal crosses, heterosis, as measured by contrasts of the average of the pure lines and the average of the reciprocal crosses, was an important source of variation for individual fiber measurements (negative) and extracellular space (positive) at 20 d of incubation but was less important at 25 d of incubation. No significant interactions between genetic group and sex were noted at 20 d of incubation, but such interactions were frequent at 25 d of incubation. These results suggest that muscle organizational differences between the two sexes begin to occur between these two ages and are not the same for different genetic groups.