Application of Congenics to Introgress a Myostatin Mutation in Nellore Cattle via SNP-Assisted Backcrossing

Congenic strains are usually generated by mating two inbred strains, and backcrossing the descendants for 5-10 generations with one of the original strains. In the present work, this approach was used to transfer a chromosome specific region from one cattle breed to another through backcrossing allowing the insertion of heterologous economically important mutations that are not present in the recipient breed. We used a high density SNP panel to monitor ancestry proportions in a cohort 7/8 Nellore x 1/8 Belgian Blue crossbred cattle population which is being backcrossed to produce Nellore population carrying the Belgian Blue myostatin deletion. Thirty-eight crossbreed individuals, eleven purebred Nellore and three purebred Belgian Blue individuals were genotyped using a high density SNP chip (BovineHD Illumina^®). Sliding window linkage disequilibrium pruning strategy was adopted to create an informative SNP subpanel based on the composite population and corrected for possible ascertainment bias. A total of 77,850 genome-wide markers were pruned out, which were considered for unsupervised fast model-based estimation of ancestry (ADMIXTURE) assuming two ancestral populations (K1=Nellore and K2=Belgian Blue), and Principal Components Analysis. Based on pedigree record, individuals were expected to equally carry 87.5% of the founder Nellore genetic information. ADMIXTURE analysis revealed 14 individuals with lower Nellore ancestry than expected, with a minimum of 79.9%. Average Nellore and Belgian Blue ancestry were 87.91% and 12.09%, respectively. This pilot study indicates that SNP-assisted backcrossing is a powerful tool for the introgression of specific alleles followed by restoring of one of the base population’s genetic makeup.