Selection for breed-specific growth hormone and IGF-I alleles in a synthetic beef cattle cross, Canchim

This study was developed to evaluate selection effects on gene frequencies in a synthetic beef cattle cross (5/8 Charolais, 3/8 Zebu) named Canchim. A sample of 154 Canchim animals, representing three generation classes, was analyzed for seven molecular markers. Thirty-six Charolais cattle were also studied for comparison. A highly significant variation (P < 0.01), with a linear increase of the allele coding for valine at position 127 of the grow th hormone peptide, was observed through generations. This allele was found in the Charolais sample but is not observed in Brazilian Bos indicus breed s. Four alleles were found for a microsatellite in the 5' flanking region o f insulin-like growth factor I gene in Canchim. Allele sizes ranged from 23 1 to 225 bp. There was a significant (P < 0.05) nonlinear increase in the 2 25-bp allele frequency. This allele was not observed in Charolais. Six alle les were observed for microsatellite CSFM50 in the Canchim population. A si gnificant (P i 0.05) linear reduction of the 168-bp allele was observed ove r the generations. The hypothesis of preferential mating for growth hormone polymorphism was supported by Wright's F-statistics. Estimated F-IS value for growth hormone was 0.59 (P < 0.01). These results suggest that selectio n influenced three out of seven loci analyzed. The simultaneous increases o f a Bos taurus-specific growth hormone allele and a Bas indicus-specific in sulin-like growth factor I allele indicate that phenotypic selection favore d different regions of both genomes.