Growth Endocrine Axis and Bovine Chromosome 5: Association of SNP Genotypes and Reproductive Phenotypes in an Angus, Brahman and Romosinuano Diallele

The growth endocrine axis influences reproduction. A QTL associated with enhanced ovulation exists on chromosome 5 in cattle and there are 6 genes underlying this region involved in the mechanisms of GH action. Resequencing exons, 5’ and 3’ untranslated regions and conserved non-coding regions of these genes in a multibreed resource population revealed 75 SNP usable for genotype to phenotype association studies. In the current study, phenotypes included age at first calving, calving interval, days to calving, and pregnancy rate. Data were collected from developing heifers (n = 650) of a diallele composed of Angus, Brahman, and Romosinuano breeds. A SNP in the promoter of the signal transducer and activator of transcription (STAT)2 gene, which is a second messenger of GH, had minor allele frequency > 10% across the three breeds. This SNP did not deviate from HardyWeinberg equilibrium (X = 1.00, P > 0.31), so deemed useful for genotype to phenotype association analyses. Since the remaining SNP appeared to predict breed, they were used to correct for population stratification using STRUCTURE, which revealed three distinctive ancestral clusters. No significant association was detected between the STAT2 genotype and reproductive traits in mixed effects analyses using genotype as a fixed term, sire as a random term, and coefficient of ancestry as a covariate; however, the interaction of SNP genotype and ancestral cluster was associated with the traits days to calving (P < 0.05) and calving interval (P < 0.10). Interaction plots revealed a higher estimated effect of heterozygous genotype in cluster 1 (inferred primarily from Brahman) and lower estimates in clusters 2 and 3 (inferred primarily from Bos taurus). The heterozygous genotype extended these trait levels ~100 d. A SNP in the promoter of the STAT2 gene was associated with fertility trait levels in admixed cows of the breeds Angus, Brahman, and Romosinuano. The effect appeared to be a non-additive genetic relationship as heterozygous genotype extended levels of traits indicative of postpartum rebreeding.