Сельскохозяйственная биология, 2010, № 4, с. 51-54.

УДК 636.2.034:547.96:575.822

POLYMORPHISM IN κ-CASEIN AND PROLACTIN GENES AND THEIR INFLUENCE ON DAIRY PRODUCTIVITY OF COWS OF THE BLACK-AND-WHITE BREED

T.S. Goryacheva, G.M. Goncharenko

In cows of the Black-and-White breed (the «Pashinckii» breeder, Novosibirskaya oblast’) the polymorphism of κ-casein and prolactin genes was evaluated by the PCR-PDRF method. It was shown, that AA genotype has the largest distribution (frequency of 73-77 %). A comparison of the indices of dairy productivity in cows with different genotypes on these genes shown that animals with BB genotype on κ-casein have an advantage on yield of milk for first and third lactation and also on content of the fat and the protein in milk. The cows with BB genotype on prolactin have an advantage only in content of fat in milk.

Key words: polymorphism, κ-casein, prolactin, gene, milk productivity, Black-and-White breed.

The rapid identification of a genotype regardless of species, breed, sex and age of an animal is a real advantage of molecular genetics studying the nature of inheritance of qualitative and quantitative determinants and this is also the way to speed up breeding process. In dairy cattle husbandry, the BB-genotype on κ-caseine is considered to be the most promising marker associated with higher protein content in milk and its better technological qualities. The prolactin gene is of some interest as well, owing to its main function connected with stimulation of mammary glands in mammals, as well as with milk formation and secretion (1). The active participation of prolactin gene products in the formation of milk productivity is the basis for finding associations of polymorphic gene variants with dairy productivity indices (2-4). Meanwhile, prolactin provides some other physiological effects - osmoregulation, reproduction and behavioral reactions of animals (5, 6). Many biological functions, such as the expression of casein gene, are carried out via prolactin receptors (7). However, the information on polymorphism of these genes and their connection with milk productivity is not enough for evaluation and prediction of their genetic potential.

The purpose of this study was to identify the frequency of κ-casein and prolactin genes alleles in dairy cows the Black-and-White breed and to determine their connection with dairy productivity indices.

Technique. The study was performed in 2008 upon 400 dairy cows the Black-and-White breed (the breeder farm - JSC Plemzavod “Pashinskii", Novosibirsk province) using PCR-PDRF method (polymerase chain reaction with subsequent restriction of fragments).

DNA was isolated accoridng a standard technique of the laboratory "Medigen" (Novosibirsk). For PCR amplification of κ-casein gene fragment, the primers VAR 3 and VAR 5 were used, for prolactin - PRL 1 and PRL 2 primers by the method of the Scientific Center for Biotechnology of All-Russia Development and Research Institute for Livestock Husbandry (Dubrovitsy settlement, Moscow province (2).

The amplification product obtained for the κ-casein gene was restricted by endonuclease HindIII, for prolactin gene – by RsaI (“Sibenzime”,  Novosibirsk) at 37 °C for 16 h. The restricts were separated by agarose gel electrophoresis (for  κ-casein – 2%, for prolactin - 4%) with the addition of ethidium bromide and identified under UV light in transilluminator.

The genes for κ-casein and prolactin were tested on two most common allelic variants - A and B, which are inherited as codominants.

The calculations of alleles’ frequencies and genotypes distribution, as well as statistical data processing were performed using Microsoft Excel along the standard method (8).

Results. The method of PCR-PDRF has allowed the identification in the experimental animals of 2 alleles and 3 genotypes for κ-casein gene showing the significantly different frequencies of their occurrence (Table 1).

For κ-casein gene, the allele А was detected in most of the animals (P < 0,001). This fact has resulted in a dominance within the herd of AA genotype -  its frequency was 55,5 and 75,5% higher than the frequency of АВ genotype (P < 0,001).

The similar distribution was observed for prolactin alleles and genotypes: the frequency of A allele exceeded the frequency of B at 0,710 (P <0,001), and the homozygous genotype AA was found to be the most prevalent in the herd (48,3 and 70,7% higher frequency compared with AB and BB, respectively; P <0,001). According to the Hardy-Weinberg principle, the studied population was in the gene equilibrium for these genes, i.e. no selection on them was performed.

In the group of cows carrying BB genotype on κ-casein, milk yields during the 1st lactation were 490 and 747 kg higher than in the groups with AA (P <0,05) and AB genotypes, respectively (P <0,01) ( Table 2). At the same time, the cows with BB genotype on κ-casein demonstrated the higher yields of milk fat - by 19,9 and 31,5 kg, respectively (P <0,01). The higher contents of fat and protein during the 1st lactation were observed in the group with BB genotype. These cows exceeded the carriers of AA and AB genotypes in milk fat content – by 0,03 and 0,05%, in milk protein content - by 0,06 and 0,07%, in yields of milk protein – by 19,2 (P <0,05) and 27,6 kg (P <0,01), respectively.

During the third lactation, the best parameters of milk productivity were also observed in the cows with BB-genotype on κ-casein: their milk yields were 744 and 930 kg higher than that of animals with AA (P <0,05) and AB genotypes (P <0,01), respectively. The genotype BB provided the highest content of fat and protein in milk: the first indicator was higher than that of genotypes AA and AB, respectively, by 40,5 and 45,1 kg (P <0,001), the second one – by 0,09 and 0,08% (30,1 and 36,2 kg) (P <0,01).

The comparison of milk productivity of cows depending on their genotype on prolactin revealed no significant differences except the fat content in milk (see Table 2.).  The higher values were found in individuals with the genotype BB: during the 1st lactation, they exceeded the animals with genotype AB at 0,18% (P <0.05), and during the 3rd lactation – at 0,29 and 0,32% (P <0.001) compared with the carriers of genotypes AA and AB, respectively.

Thus, in the studied population of cows the Black-and-White breed, the most frequent (70% individuals) was the allele A and genotype AA on both prolactin and κ-casein. The desired genotype BB was detected in, respectively, 1,5 and 2,3% of the examined cows. The genotype BB on κ-casein provides a positive effect on milk yields, fat and protein content in milk, while the genotype BB on prolactin - on the fat content in milk.

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