Candidate gene markers for production traits in beef cattle

In this study, two candidate gene markers, bovine growth hormone gene and mitochondrial DNA, were characterized using PCR-RFLP (Polymerase Chain Reaction - Restriction Fragment Length Polymorphism) analysis. Previous studies on variation in the growth hormone gene have contributed to our understanding of the relationship between this gene and milk production in dairy cattle, but not for production traits in beef cattle. In mitochondrial DNA, variation has also been demonstrated mostly in dairy cattle, and associated with variation in milk production. The major aims of my study, therefore, were to: 1. Identify polymorphisms at two regions in the growth hormone gene (GH-L1 and GH-L2), and in mitochondrial DNA (D-loop and ND- 5); 2. Compare polymorphisms within and between 8 breeds of beef cattle; 3. Test for associations between growth hormone and mtDNA genotypes and production (milk, fertility and growth) traits in Hereford and composite breeds of beef cattle. PCR-RFLP proved a very useful tool to examine genetic diversity. This technique is considerably less time consuming than the more conventional RFLP analysis, especially when dealing with large samples. The requirement for only a small amount of DNA template for PCR-RFLP also made it the technique of choice. Polymorphisms were found in two of the growth hormone gene regions; in GH-L1 using Alu\, and in GH-L2 using Mspl. Sequence data indicated that the Alul polymorphism resulted from a leucine/valine substitution at position 127, and the Mspl polymorphism from a C-T transition at position +837. Polymorphisms were found in the mitochondrial D-loop using Taql, Pstl, Sspl, Apal and Avail, and in mitochondrial ND-5 using Spel and Hindlll. New polymorphisms, most likely due to nucleotide substitution, were found both in the mitochondrial D-loop and ND-5. Variation in the growth hormone gene and mitochondrial DNA may have a profound effect on morphological and physiological characters, since those characters are controlled by the genetic information carried by DNA. Bali cattle (Bos javanicus) were genetically divergent in mtDNA from the 8 Bos taurus breeds examined. This reinforces the high conservation value for Bali cattle. Although there was a trend for composite beef cattle (1/4 Brahman, 1/4 Charolais, 1/4 Friesian, 1/8 Hereford, 1/8 Angus) to be more genetically diverse than purebred Hereford cattle at the growth hormone gene loci, this was not significant, and the breeds did not differ in mtDNA diversity. This is against theoretical expectations for neutral genes, but may indicate that growth hormone and mtDNA have been under artificial selection in the parent breeds of the composite. The effect of variation in the growth hormone gene and mitochondrial DNA on production traits were statistically tested using two ANOVA models. No production trait in this study was affected by the growth hormone Alul genotype although previous studies indicated that Alul genotypes have significant effects on milk production in Holstein cows (Lucy et al, 1993) and carcass gain and meat value (Schlee et al., 1994a). Both ANOVA models indicated that growth hormone Mspl genotypes significantly affected daily gain and birth weight, but not milk production or fertility. The results suggest that animals carrying growth hormone MspI(—) are superior for growth traits. This may be a direct or linked effect of the growth hormone gene. Hoj et al. (1993a) found the Mspl polymorphism to be correlated with milk fat production in dairy cattle, but this is the first report of an association with production traits in beef cattle. The only trait affected by variation of mitochondrial DNA was female fertility. This trait was significantly affected by the Taql / Pstl / Sspl polymorphism in the D-loop and by the HindUl / Spel polymorphism in ND-5. This is the first report of a correlation between mitochondrial polymorphism and fertility in beef cattle, although in dairy cattle Schutz et al. (1994) reported associations with milk yield. Because the mitochondrial genome is inherited maternally, without recombination, these effects may be due to linkage. This finding suggests that cytoplasmic genetic effects are also important in determining performance traits, and therefore it is important to consider maternal inheritance in breeding programs