Ovine footrot gene marker screening in a Katahdin sheep flock
An extensive polymorphism at the DQA2 and DQA2-like loci located within the Major Histocompatibility Complex (MHC) in sheep have been identified and subsequently have been utilized to develop a gene marker testing procedures for footrot resistance. Although hair sheep breeds, such as Katahdin, are reputed to have better disease resistance, no genotypic markers have been reported for footrot resistance. The footrot gene-marker test reports five basic footrot scores (1, 2, 3, 4 and 5) corresponding to alleles of the MHC DQA2 and DQA2-like loci. This gives 15 possible score combinations (1,1; 1,2; ... 4,5; 5,5), where 1,1 is claimed to have the highest resistance and 5,5 the lowest resistance to footrot infection. Blood samples were collected on FTA blood DNA collection paper cards from 600 Katahdin sheep and Katahdin crossbred ewes and rams from Lincoln University farms, Jefferson City, Missouri. DNA extraction and the gene marker test were performed at the Lincoln University Gene Marker Laboratory, Lincoln, NZ. Blood samples from 583 sheep with gene marker test results were analyzed. A chi-square test was used to test difference in variant alleles, animal genotypes, and score group distribution frequency. As footrot tolerance genes are assumed to exert a dominant effect, an expressed value for a pair of alleles was derived and animals were grouped into five categorical groups (1 to 5). Variant allelic distributions in 5 score groups were 10.0%, 21.2%, 45.3%, 17.6%, and 5.9% respectively. Whereas, animals classed into five gene marker score groups were 18.5%, 33.3%, 42.0%, 5.8%, and 0.3% respectively. Both allelic distribution and genotypic distributions were significantly (P< 0.01) different among the five gene marker score groups. The percentage of animals in the footrot tolerance score (1 and 2) was 52%, with a moderate score of 3 was 42%, and with scores of 4 and 5 were 6%. Therefore, by using footrot marker screening, the potential to select a high resistant flock is possible within three to five breeding seasons.
Keywords: Katahdin, footrot, DNA marker