Trait association of a genetic marker near the IGF-I gene in egg-laying chickens

The insulin-like growth factor I (IGF-I) gene was screened for genetic variants associated with trait means and trait correlations. Analysis of an unselected randomlymated White Leghorn population revealed a Pstl restriction fragment length polymorphism (RFLP) in the 5' region of the gene which segregated at a frequency of 0.88 for the Pstl(+) allele (presence of a Pstl restriction site). A comparison of the three genotypic classes revealed that the Pstl(-/-) genotype was associated with significantly lower egg weight measured in three different time periods, while the Pstl(+/-) genotype was significantly associated with a higher eggshell weight estimated from the egg weight and egg specific gravity. For eggshell weight, the effect was age dependent and significant only for the last two periods of egg laying. No genotype associations were found for body weight, feed consumption, and egg laying rates. Significant dominance effects of the IGF-I genotype were observed for two of the egg weight measurements an I three of the eggshell weight estimates. Partial correlation analyses in the two most frequent genotypic classes, Pstl(+/+) and Pstl(+/-), revealed the presence of a regulatory loop between feed consumption, body weight, egg weight, and the rate of egg laying. Several aspects of this regulatory loop were different between the two genotypic classes. In particular, for the Pstl(+/+) genotype, feed consumption was positively associated with egg weight, while there was no significant association for the Pstl(+/-) genotype. Further, the degree of association of body weight with egg weight decreased with age in the genotypic class Pstl(+/-), while it was constant for the Pstl(+/+) genotype. The results indicated that the marker in the IGF-I gene was not only associated with changes in some trait means, but also with changes in the stability of the coordination between feed intake, body weight, and egg production traits.