GENOTYPE AND SEXUAL INFLUENCES ON GROWTH AND MUSCLE DEVELOPMENT OF CHICKEN EMBRYOS

The influence of sex and genotype on growth of chick embryos was studi ed using eggs from commercial broiler hens mated with broiler strain m ales (BrBr) or with bantam males (BaBr) and using eggs from bantam fem ales mated to bantam males (BaBa) or to broiler strain males (BrBa). M ale BrBr embryos were first significantly heavier than females at 9 da ys of incubation. The influence of sire's genotype was expressed betwe en 6 and 16 days of incubation but not at 20 days. Dam's genotype, and the associated difference in egg weight, significantly affected embry o weight by 12 days of incubation and by 20 days it alone influenced e mbryo weight. Plasma insulin-like growth factor-I (IGF-I) levels of Br Br and BaBr embryos did not differ at 8, 10, 12, or 14 days of incubat ion, nor were there sex differences. There were no genotype difference s in whole body protein concentration, DNA concentration or protein:DN A 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 e mbryos was significantly lower than those of BaBr embryos at 12 days o f 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 ther e is no clear relationship between embryonic growth, plasma IGF-I conc entration or the concentration of DNA or protein in pectoralis muscle.