Mitochondrial DNA polymorphism and determination of effects on economic traits in dairy cattle

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Abstract
Sixty‐seven maternal lineages stratified into four milk production groups were tested for mitochondrial restriction fragment length polymorphisms (mtRFLP). Purified mtDNA and genomic DNA were digested by 31 different restriction enzymes, surveying about 1500 nt of the mtDNA genome (9%). Genomic blots from agarose and acrylamide gels were hybridized to a radiolabeled bovine mtDNA probe. Thirty‐three mtRFLPs were found for 20 enzymes. Except for TaqI, all mtRFLPs could be explained as simple gains or losses of restriction sites. There were 16, 6, 14 and 14 polymorphic sites in the High, Average, Low and Elite groups, respectively. Nucleotide diversity was 11×10‐4, 3.3×10‐4, 7.7×10‐4, and 3.5×10‐4, respectively. Distribution of rare alleles among these groups was not random (p<.001), and higher for the High and Low groups, indicating a possible relationship between these polymorphisms and milk production. Two maternal lineages had ≥ 9 rare alleles, all other lineages had ≤ 3 rare alleles. Heteroplasmic mtDNA among individuals of a single maternal lineage was found for BstNI, and heteroplasmic mtDNA within individuals was found for TaqI in two maternal lineages. The frequency of the rare allele was < 0.1 for all mtRFLPs and > 0.05 for only three loci. With selective genotyping and ≥ 10 recorded animals per lineage, a mtDNA locus with an effect ≥ 0.25 standard deviations could be detected by genotyping 2‐3 individuals of each of 120 maternal lineages.