DNA, RNA and total protein content of leg and breast muscles of White Rock chickens selected for 56-day body weight.

Pectoralis and gastrocnemius muscles from White Rock chickens divergently selected for 29 generations for high or low 56-day body weight were analyzed for DNA, RNA and protein (mg/g) content at 12 ages between hatch and 273 days of age. Dimorphism between lines was maximum at day 18 for both muscle types and then declined with age. High-line chickens generally deposited relatively more muscle tissue than those from the low line. Although nonadditive genetic variation was evident for absolute muscle weights, it was more frequent for muscle weight relative to body weight. For both muscle types, DNA unit number, as measured by DNA content (mg/g), was larger for the high line than the low immediately after hatch and smaller in the high than in the low line from day 10 to 56 after which lines were similar. RNA and protein/g muscle were similar for both lines at most ages. Between days 4 and 56, a period of rapid muscle growth, DNA unit size (protein/DNA) of both muscle types was larger in the high than in the low line. Heterosis was positive for protein, DNA unit number and size, and negative for RNA content. While weight of pectoralis muscle was similar to that of one gastrocnemius muscle on day 1, by day 273 its weight was over 3-fold greater. DNA unit number was higher in pectoralis than gastrocnemius muscle from hatch to day 4, similar on days 7 and 10 and lower for pectoralis muscle beyond day 10. RNA content was similar at all ages except 4, 7 and 10 days. DNA unit size followed the same pattern as DNA unit number; however, greater nuclei number at hatch for the high line corresponded with low DNA unit size. This pattern suggests a higher rate of cellular filling for pectoralis than gastrocnemius muscle.