The 80-kb DNA duplication on BTA1 is the only remaining candidate mutation for the polled phenotype of Friesian origin

Abstract

Background: The absence of horns, called polled phenotype, is the favored trait in modern cattle husbandry. To date, polled cattle are obtained primarily by dehorning calves. Dehorning is a practice that raises animal welfare issues, which can be addressed by selecting for genetically hornless cattle. In the past 20 years, there have been many studies worldwide to identify unique genetic markers in complete association with the polled trait in cattle and recently, two different alleles at the POLLED locus, both resulting in the absence of horns, were reported: (1) the Celtic allele, which is responsible for the polled phenotype in most breeds and for which a single candidate mutation was detected and (2) the Friesian allele, which is responsible for the polled phenotype predominantly in the Holstein-Friesian breed and in a few other breeds, but for which five candidate mutations were identified in a 260-kb haplotype. Further studies based on genome-wide sequencing and high-density SNP (single nucleotide polymorphism) genotyping confirmed the existence of the Celtic and Friesian variants and narrowed down the causal Friesian haplotype to an interval of 145 kb.

Results: Almost 6000 animals were genetically tested for the polled trait and we detected a recombinant animal which enabled us to reduce the Friesian POLLED haplotype to a single causal mutation, namely a 80-kb duplication. Moreover, our results clearly disagree with the recently reported perfect co-segregation of the POLLED mutation and a SNP at position 1 390 292 bp on bovine chromosome 1 in the Holstein-Friesian population.

Conclusion: We conclude that the 80-kb duplication, as the only remaining variant within the shortened Friesian haplotype, represents the most likely causal mutation for the polled phenotype of Friesian origin.

Figure 1

Polled phenotype of Friesian origin. A) Relative position of the candidate mutations for the polled phenotype of Celtic (P_C) and Friesian (P_F) origin. All reference sequence (RefSeq) alleles are shown in blue and POLLED candidate mutations in orange. The insertion-deletion events (P_5ID, P_202ID and P_80kbID) are presented as bars and SNPs as lines. Note that this figure does not include the fairly distant SNP rs134968598 at position 1 390 292 bp. B) Pedigree of 13 Holstein-Friesian animals used to narrow down the interval containing the POLLED mutation of Friesian origin. Color code is as above. The recombinant haplotypes of the ROSSI and ELBOY RED bulls are shown with the corresponding colors. Polled homozygous (PP) individuals are represented by solid circles (females) and squares (males); declared polled heterozygous animals (Pp) by half-filled symbols; horned animals (pp) by empty symbols; missing and ungenotyped individuals are marked with a diagonal line.