Genotype and sexual influences on growth and muscle development of chicken embryos

The influence of sex and genotype on growth of chick embryos was studied using eggs from commercial broiler hens mated with broiler strain males (BrBr) or with bantam males (BaBr) and using eggs from bantam females mated to bantam males (BaBa) or to broiler strain males (BrBa). Male BrBr embryos were first significantly heavier than females at 9 days of incubation. The influence of sire's genotype was expressed between 6 and 16 days of incubation but not at 20 days. Dam's genotype, and the associated difference in egg weight, significantly affected embryo weight by 12 days of incubation and by 20 days it alone influenced embryo weight. Plasma insulin-like growth factor-I (IGF-I) levels of BrBr and BaBr embryos did not differ at 8, 10, 12, or 14 days of incubation, nor were there sex differences. There were no genotype differences in whole body protein concentration, DNA concentration or protein:DNA ratios at 4 or 8 days of incubation. At 12 days the pectoralis major DNA concentration of BaBa embryos was significantly higher than that of all other groups. Pectoralis muscle protein concentration of BrBa embryos was significantly lower than those of BaBr embryos at 12 days of incubation, but by 16 days it was highest. The protein:DNA ratios of embryos developing in bantam eggs were significantly lower than those in broiler eggs at 12 days. In summary, genotypic and sex differences in body weight and muscularity that develop after hatching appear to be predetermined by differences in embryonic growth patterns, but there is no clear relationship between embryonic growth, plasma IGF-I concentration or the concentration of DNA or protein in pectoralis muscle.