POLYMORPHISM OF THE Β-LACTOGLOBULIN GENE AND DAIRY PRODUCTIVITY OF DIFFERENT SHEEP’S GENOTYPES
Aim. To carry out the Ukrainian breeding sheep populations structure genetic analysis according to the parameters of the β-lactoglobulin polymorphic gene. In addition, to study the associations between individual genotypes of this locus and the ewes' dairy productivity level. Methods. Molecular genetics, population statistical, biometric. Results. The genetic structure of the Ascanian Fine-Fleeced (AFF) and Ascanian Karakul (AK) breeds’ sheep populations was investigated by variants of the beta-lactoglobulin gene (β-LG). It was found that in the studied generation of both Ascanian Merino and Ascanian Karakul sheep, the heterozygous genotype β-LG A/B received an advantage in distribution, 56.3 and 61.5%, respectively. The second place is occupied by the β-L GB/B homozygote (31.2; 23.1%), and the last – the β-LG A/A homozygote (12.5; 15.4%). Accordingly, in terms of the locus allelic variants manifestation frequency in both sheep populations, the β-LGB allele received a large proportion (0.594; 0.538).
Analysis of the phylogenetic relationships between the studied gene pools of different origins sheep and parameters of the coat (fine wooled and coarse-wooled) did not reveal significant interbreed differences. In particular, there are no significant differences in the level of heterozygosity (He = 0.481; 0.500) and locus polymorphism (ne = 1.93; 1.99). In addition, both populations are in a state of Hardy-Weinberg genetic equilibrium (χ2 = 0.77; 0.78).
The β-LG gene is a candidate gene for dairy productivity in animals. In almost all mammals, except for rodents and primates, β-lactoglobulin is the main whey protein in milk. Its content in sheep milk is more than 50%, which largely determines the quality of the product. We have begun a study to establish possible associations between molecular genetic markers and the main productive traits of Ascanian origin sheep development level, including the trait of ewes' dairy productivity. It was found that the total average daily milk yield in all studied animals was 354.48 ml, while sheep of the Ascanian Fine-Fleeced breed exceeded the Karakul sheep in terms of this indicator (371.09 ml versus 337.87 ml), SNF (12.25 versus 12.10%), protein (4.63% versus 4.58%) and lactose (6.68% versus 6.59%). However, Karakul ewes were positively distinguished by a high fat content in milk (7.16% versus 6.28%).
The correlations of β-lactoglobulin genotypes with signs of sheep dairy productivity were also investigated. It is shown that there is a multi-vector nature of associations in each of the populations. In particular, in the medium of one of them (AFF), homozygotes β-LG A/A are the best in terms of dairy yield – 594 ml versus 330 and 354 ml in other genotypes, and in the AK medium, on the contrary, animals with homozygous genotype β-LG B/B – 407 ml versus 240 and 318 ml.
According to the β-LG gene influence level on individual ewes' dairy productivity signs, determined by unilabiate analysis of variance, it was found that this genotypes’ locus total influence force within individual breeds has rather significant differences. In particular, in AFF, this strength for most indicators, except for the fat content, occupies values close to the average value (η2 = 38.0–46.0%), while in AK this dependence is two times lower (η2 = 18.0–21.0%).
Conclusions. In sheep populations of different genesis, the β-lactoglobulin gene is in a polymorphic state and is determined by two codominant alleles (β-LGA, β-LGB). To date, in the studied gene pools, in terms of the frequency of manifestation, the β-LGB allele predominates – 0.594; 0.538, and in terms of concentration - heterozygotes genotype β-LG A/B – 56.3%; 61.5%. According to the locus polymorphism level and the heterozygosity degree, the value of these indicators, depending on the different orientation of the sheep wool productivity, is almost the same, 1.93–1.99; and 0.481–0.500, respectively. According to the Wright fixation index in both herds, a right-sided deviation of this parameter is observed, which indicates a selection in favor of heterozygous genotypes (Fis = +0.37; +0.71). Comparison of the actual and theoretically calculated distribution of genotypes revealed the presence of a genetic equilibrium of populations for these genes. That is, selection and breeding work carried out in herds does not have a significant effect on their genetic structure state for the studied polymorphic genes. Since the β-lactoglobulin gene is one of the ones controlling the formation of dairy productivity in sheep, therefore, we investigated the genotypes influence level on their productive trait and found that the opposite in the genesis of breeds leads to a multi-vector nature of associations between these factors. In particular, among the Ascanian Fine-Fleeced breed, ewes with the β-LG A/A genotype are the best in terms of average daily milk yields - 594 ml versus 330 and 354 ml among their peers, and in terms of protein content, milk fat, lactose, dry skim milk residue and density In milk, the β-LG B/B genotype has an advantage (p < 0.05). In the Ascanian Karakul breed, in terms of milk yield, on the contrary, individuals with the β-LG B/B genotype turned out to be better, and in terms of other indicators - animals with an alternative genotype. Heterozygous ewes in all respects occupied a position close to the average for the herd. Thus, in sheep of different origins and directions of productivity, β-LG gene markers have different levels of influence on the development of dairy productivity in the studied gene pools.
The obtained data in combination with other existing methods for assessing the sheep genotype can be used as a biochemical test for the breed gene pool state, as well as for predicting, on their basis, the dairy productivity signs development level in animals.
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