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Effect of genetic selection on MyoD and myogenin expression in turkeys with different growth rates.

Skeletal muscle development requires the ordered expression of specific myogenic regulatory factors, which include MyoD, Myf5, myogenin, and MRF4. The MyoD and Mrf5 factors are required for the determination of myoblasts, whereas myogenin and MRF4 play a pivotal role in terminal differentiation. In the current study, males and females of a turkey genetic line selected only for increased 16-wk BW (F line) and an unselected randombred control (RBC2 line) from which the F line was developed were used to investigate the developmental expression of MyoD and myogenin mRNA in embryonic pectoralis major muscle and myogenic satellite cells. Pectoralis major muscle was isolated at embryonic d (ED) 14, 16, 18, 20, 22, and 24. The mRNA levels of MyoD and myogenin were measured using a real-time quantitative polymerase chain reaction method. Both MyoD and myogenin expression declined during embryonic development. The decrease in MyoD expression started at ED 16 for the F line and at ED 18 for the RBC2 line for both sexes. Myogenin expression in both lines began to decline at ED 14. The F line males had lower myogenin expression at ED 14, 16, and 18 than the RBC2 line males, which was similar for the F line females compared with the RBC2 line females except there was no significant difference at ED 18. The RBC2 line males had greater expression than the females for myogenin at ED 16 and 18 for the RBC2 line. Proliferating myogenic satellite cells in both lines and sexes expressed low levels of MyoD and myogenin. After the initiation of differentiation in both lines and sexes, there was a sharp surge in MyoD expression at 24 h followed by a decrease at 48 h and then an increase in expression through 72 or 96 h of differentiation. There were line and sex differences in myogenin expression during the differentiation process. These data are suggestive of growth- and sex-related differences in the expression of myogenic regulatory factors key to muscle cell proliferation and differentiation, which will affect muscle growth rate.