Abstract:
Breeding genetically polled animals provide a welfare friendly alternative and a long-term solution to dehorning. The availability of high through-put genomic technology enables the study of the genetic mechanisms of polled and scurs phenotypes with higher precision, compared to microsatellite markers. The majority of research to date on the POLLED locus has been performed in European and Australian cattle breeds. The primary aim of this study was to investigate the genetic basis of the polled and scurs phenotypes in the South African Bonsmara breed by using genomic SNP data from phenotyped animals. To validate the Celtic mutation as the causative mutation for polledness in the Bonsmara breed, 386 animals (164 Bonsmara, 133 Drakensberger and 89 Hereford) were screened for the Celtic mutation of the POLLED locus. Phenotypically polled and scurred animals were found to carry at least one copy of the Celtic allele (PC) whereas horned animals were homozygous wildtype. The majority of Bonsmara animals tested were heterozygous polled (PC = 0.591) with a low frequency of homozygous polled animals observed (PCPC = 0.177). A total of 224 heterozygous polled animals were genotyped with the GGP 150K bovine array (139 376 SNPs) to map the SCURS locus implementing a case-control study design. The scurs phenotype in the Bonsmara was associated with six different SNPs across three different chromosomes (BTA3, BTA10 and BTA17). The current study identified four candidate genes (FBN1, LRIG2, PHGDH, GUCY1B1) across three chromosomes which are involved in various molecular functions and biological processes, including amino acid metabolic processes, regulation of cell signals and processes involved with the central nervous system development. Further characterization of the identified candidate genes from this study may be useful for identifying possible causal mutations for scurs. This research indicated discrepancies and difficulties in recording of the polled phenotype which is exacerbated by the prevalence of the scurs phenotype in the Bonsmara breed. An outcome of this study was the compilation of a phenotyping protocol for Bonsmara farmers for accurate identification of the polled and scurs phenotypes. Accurate phenotypic identification of the polled and scurs phenotypes in the Bonsmara will contribute towards more effective selection and accelerated fixation of the polled allele in Bonsmara herds. Genotypic identification of polledness to distinguish between heterozygous and homozygous polled animals are recommended. Since homozygous polled animals are limited in the Bonsmara breed, breeders are advised to place more selection pressure on polledness to increase the number of polled animals within the breed.
