RESUMO
A synergistic effect in the somatotropic axis (GH1-GHR-IGF1) was observed in 736 young Nelore (Bos indicus) bulls under ad libitum grass feeding conditions on irrigated pasture in central Brazil. Stepwise substitution of shorter alleles of the promoter region of the growth hormone gene (GH1) and the P1 promoter of the GH1 receptor gene (GHR) with longer alleles was associated with significantly increased body weight gain (W550, weight at age 550 days; ADG, average daily gain) and fat accrual (FAT, rib eye fat thickness). A threshold effect on ADG was associated with allele size variation at the GH1. A best fit model indicated a 3- to 6-fold effect of GH1 variation on ADG, when compared to the variation at the GHR and a known microsatellite at the somatomedin gene (IGF1, insulin-like growth factor 1). A threshold effect on FAT was associated with substitution of the short GHR allele by the longer GHR alleles; the effect of the GHR variation on FAT was 10-fold that of the variation at the GH1 and IGF1 loci. Among the 10 GH1-GHR-IGF1 multi-genotypes identified, the predominant genotype was homozygous for the large GH1 promoter (long/long, G2/G2 or domestic type), short GHR promoter (short/short or wild type), and short IGF1 microsatellite (short/short or wild type). This predominant multi-genotype suggests that selection pressure in the Nelore breed has been directed towards high ADG and W550, and low FAT. Our results mirror previous findings in the oMtla-oGH transgenic mouse model, in which the level of somatotropic gene expression acts through a threshold mechanism, and low expression results in adipogenesis, while high expression increases body growth.
