The author gives a methodological rationale for molecular and genetic aspects of BoLA-DRB3 gene application according to susceptibility and resistance to necrobacteriosis in cow population of Ukrainian black-pied milk breed.
The study revealed that the distribution of necrobacteriosis in examined populations of dairy cows varied from 5,8% to 15,1%.
The results of polymorphism of the BoLA-DRB3 gene study suggest that from 25 to 31 alleles have been found in cow genotype. In groups of animals sick in necrobacteriosis, the quantity of alleles varied from 22 to 31, and in groups of healthy animals, their quantity varied from 21 to 28 alleles. Taking the samples of sick, healthy animals and total sample with the frequency of more than 5% in terms of three herds the following alleles have been found: *08, *16, *22, *24 and *28. Allele of BoLA-DRB3*24 (P(A) =18,03%) was the most common.
The high number of alleles caused a large variety of genotypes (in the range from 65 to 75) and their regular distribution. The following genotypes were presented the most frequently: in total sample *01/*24 і *22/*24 (P(G) = 4,49%); among sick cows *16/*23 (P(G) =8,57%); among healthy cows *01/*03, *01/*28 і *03/*37 (P(G) = 5,56%).
The DNA markers associated with necrobacteriosis and resistance to necrobacteriosis in cow population of Ukrainian black-pied milk breed were determined based on relative risk analysis according to tests on χ2 and Fisher test for small samples. The analysis of the data clearly indicates that the associativity according to liability to necrobacteriosis in the first population was shown by alleles of BoLA-DRB3*16 (P(A) = 0,0574; RR = 10,6; χ2 = 12,5; P<0,001) і *23 (P(A) = 0,0492; RR = 4,86; χ2 = 6,03; P<0,05), the close association with resistance has been established for alleles of BoLA-DRB3*03 (P(A) = 0,0492; RR = -9,17; χ2 = 6,13; P<0,05) і *22 (P(A) = 0,0738; RR = -4,29; χ2 = 5,48; P<0,05). In the second population the associative disease linkage have been determined for the following alleles: BoLA-DRB3*16 (P(A) = 0,0915; RR = 5,26; χ2 = 7,68; P<0,01), *23 (P(A) = 0,0549; RR = 5,96; χ2 = 5,49; P<0,05) і *24 (P(A) = 0,0671; RR = 4,62; χ2 = 5,12; P<0,05). The linkage based on resistance to necrobacteriosis is determined for allele of BoLA-DRB3*22 (P(A) = 0,122; RR = -5,67; χ2 = 7,63; P<0,01). Necrobacteriosis disease is associated with two alleles and they are BoLA-DRB3*16 (P(A) = 0,0787; RR = 13,6; χ2 = 14,9; P<0,001) and *23 (P(A) = 0,073; RR = 11,92; χ2 = 13,1; P<0,001) for the third population, the resistance to disease is associated with three alleles BoLA-DRB3*01 (P(A) = 0,0955; RR = -3,73; χ2 = 4,14; P<0,05), *03 (P(A) = 0,073; RR = -9,71; χ2 = 6,39; P<0,05) and *22 (P(A) = 0,112; RR = -3,26; χ2= 4,03; P<0,05). The study demonstrated that the associations with necrobacteriosis in three examined populations were determined for alleles of BoLA-DRB3*16 and *23, and associations with resistance to necrobacteriosis were determined for alleles of BoLA-DRB3*22.
It can be concluded that rates of subpopulation fixation index identified low reduction of heterozygosis and low level of divergence among populations (FST < 0,05). It testifies to the fact that the significant genetic variability between herds is absent.
The high level of diversity according to Shannon-Weaver index that ranged from 2.87 to 3.05 was outlined. Genetic variation of the breed has been confirmed by high value of effective number alleles that ranged from 11.11 to 16.93 in unit connections.
The author argues that populations examined according to BoLA-DRB3 gene were characterized by nonuniform distribution of alleles, and the alleles that were determined with the frequency of more 5% contained from ½ to ⅔ of their total number, by absence of dominant genotype and low homozygote-predominance in two populations without statistically significant deviation from HWE. Comparison of polymorphism in cow populations at an interval of 7 years demonstrated the significant changes in the genetic structure according to BoLA-DRB3 gene associated with selection procedures aimed at increasing Holstein pedigree. The increasing of Holstein alleles at 7,29% of total number, proportion of DNA markers associated with the liability (+8,19%) and decreasing the proportion of DNA-markers associated with resistance (-5,61%) to necrobacteriosis were identified in the allele fund of daughter herd. The number of alleles that form milk productivity increased at 5,1%.
