Allelic variations in the chicken insulin-like growth factor-I gene: Effects on traits of economic importance in poultry

Due to the importance of insulin-like growth factor-I (IGF-I) in regulating many physiological and metabolic processes, the IGF-I gene was chosen as a candidate gene to study trait associated polymorphisms in chickens. A PstI restriction fragment length polymorphism (RFLP) was detected at the 5′ region of the gene and mapped to about 7 Kb upstream of the published promoter sequence. Analysis for association of the marker with traits of economic importance in an unselected, random-bred population of 359 White Leghorns revealed a significant association with egg weight (P ≤ 0.05) and specific gravity (P ≤ 0.05). There was also a trend for association with juvenile body weight (P = 0.08) but not adult body weight. For egg weight the PstI (−/−) genotype was associated with lower egg weight as compared to the heterozygote or the PstI (+/+) genotype. The PstI marker also was found to be significantly associated with differences in trait correlations. A regulatory loop that co-ordinated feed consumption, body weight, egg weight and rate of egg laying was detected, and this regulatory loop differed among the IGF-I genotypic classes. In the PstI (+/−) genotype, the degree of correlation between some of the traits was time dependent, while in the PstI (+/+) genotype it remained constant through the different periods of measurement. Since IGF-I is known to play an important role in immune functions, the association of the IGF-I genotypes with immune traits was also investigated. A significant association was found for delayed type hypersensitivity, interferon production and T-cell count (P ≤ 0.05). Individuals belonging to the PstI (+/−) genotypic class exhibited higher immune response, reflected by the delayed type hypersensitivity reaction and antibody the interactive effects of marker genotypes in the GH, GH-receptor and IGF-I genes on traits and trait correlations indicated that the three are part of an epistatic pathway, wherein the phenotypic consequences of allelic variations in one gene depend on variations in the other two genes. Some traits such as specific gravity were solely dependent on the IGF-I genotype (P < 0.001), others such as the rate of laying indicated a two-way interaction between the GH and IGF-I alleles (P = 0.025), and still others such as feed consumption and feed efficiency had significant three-way interactions between the GH, GH-receptor and IGF-I genotypes (P = 0.009), indicating that each trait was influenced by a different set of interacting genes. The association of the PstI marker in the IGF-I gene with important production and immune traits suggests that it may be used for DNA-based selection in breeding programs.