Milk protein genetic polymorphisms: A comparison between Maiwa yak and Jiulong yak

Zh. Yucai,¹ Zh. Guanghui,¹ M. Yongjiang,¹ P. Xianwen,¹ J. Mingfeng,¹ W. Yong,¹ Z. Sixiang² and Ch. Jie²

1. Department of Animal Science, Southwest Nationalities College, Chengdu 610041, Sichuan, P.R. China
2. College of Animal Medicine, Nanjing Agricultural University, Nanjing 210095, Jiangsu, P.R. China

Summary

Milk protein polymorphisms were studied by polyacrylamide gel electrophoresis (PAGE) in 100 Jiulong yak and 109 Maiwa yak. Genetic polymorphisms at the a_s1-casein, k-casein and β-lactoglobulin proteins were found in the two breeds but no polymorphism was identified at the a-lactalbumin or k-casein proteins. Four variants of the a_s1-casein (B, C, D and E) protein were detected in both breeds and two variants were identified for the k-casein and β-lactoglobulin proteins. Significant differences in allelic frequencies at the a_s1-casein and k-casein proteins were found between the Maiwa and the Jiulong yak breeds.

Keywords: Milk, protein polymorphism, yak

Introduction

Milk protein polymorphisms have received considerable research interests in recent years because of the significance of some genetic variants in cheese making properties of milk and their relationships to milk, fat and protein yields (Lin et al. 1992). Although most researches on milk proteins were related to major components (such as the a_s1-casein and β-casein), MUC1, a minor protein component of milk, has also received a lot of interest due to its potential practical values (Patton 1999). Milk protein polymorphism and the quantitative differences in the expressions of some alleles have been studied by gene analysis as well as by PAGE methods (Medrano and Aguilar-Cordova 1990; Lum et al. 1997). DNA variants within the 5'-flanking region of milk-protein-encoding genes have also been reported (Bleck et al. 1996). However, relatively limited information to date is available on yak milk protein polymorphisms (Zhang et al. 1991; Zheng and Han 1996). The purpose of this study was to determine the genetic variants of milk proteins in Jiulong yak and Maiwa yak.

Materials and methods

This study involved two breeds: 100 Jiulong yak from Jiulong County and 109 Maiwa yak from Hongyuan County. Following hand milking in the morning, small (approximately 30 mL) quantities of milk were collected from individual yak cow. A few milk samples of Jiulong yellow cattle and Chinese Holstein cows were also collected as controls. These samples were frozen (–20°C) and transferred to laboratory for analysis.

Milk protein typing was performed on 15 × 15 cm PAGE using the procedure of Medrano and Sharrow (1989).

Results and discussion

The electrophoretic distributions of the main casein fractions are showed in Figure 1a. Only the A variant of the β-casein (β-CN) was identified in Jiulong and Maiwa yak. Larger loading volumes were needed for the detection of the k-casein alleles (data not shown). The electrophoretic pattern of whey proteins is showed in Figure 1b. All the samples were homozygous for the BB allele of the α-lactalbumin (α-LA). Quantitative differences in the expression of β-lactoglobulin (β-LG) alleles were observed. Heterozygotic yak are producing more β-LG B than of β-LG A as indicated by the intensity of the two stained bands. It confirms previous finding (Zheng and Han 1996) and could be the result of transcriptional or post-transcriptional regulation of the β-LG gene (Lum et al. 1997).

Y = yak, H = Chinese Holstein, JY = Jiulong Yellow cattle, as1-CN = as1-casein, b-CN = b-casein, SA = serum albumin, a-LA = a-lactalbumin, b-LG = b-lactoglobulin. From left to right, the genotypes of as1-casein are DD, DD, BB, CE, BD, CE; the genotypes of b-LG are DD, DD, BB, BD, AB, BD, AB.

Figure 1. Polyacrylamide gel electrophoresis (PAGE) patterns of casein (a) and whey proteins (b).

Milk protein genotypes and allelic frequencies at the Jiulong and Maiwa yak are indicated in Table 1. α_s1-casein DD genotype was the most common genotype in Maiwa yak. DD, DE genotypes were observed at similar frequencies in Jiulong yak. Two variants were identified for the k-casein and β-LG loci. The genotypic distributions of the α_s1-casein and k-casein proteins variants were significantly different between Jiulong and Maiwa yak (P<0.01) and did not follow Hardy-Weinberg equilibrium within the two breeds (c²-test, P<0.01). Similarly, the genotypic distribution of the β-LG variants was not in agreement with Hardy-Weinberg equilibrium in Jiulong yak (P<0.05).

Table 1. Genotypic frequencies of milk proteins in Maiwa and Jiulong yak.

The most common allele for the α_s1-casein locus is allele D, for the k-casein locus it is allele B and for the β-lactoglobulin locus it is allele D (Table 2). In most breeds of cattle as1-casein B and β-LG B are the commonest alleles. Also, the k-casein A allele is the commonest in Holsteins and Ayrshires cattle, whereas in the Jerseys it is the k-casein B allele (Lin et al. 1992; Medrano and Sharrow 1989). The different variants and genotype frequencies observed in this study at the α_s1-casein and β-LG loci between yak and cattle breeds illustrate the difference in milk composition observed between the two species. The allelic frequencies observed at the β-LG locus in Maiwa and Jiulong yak is different from the ones reported by Zhang et al. (1991) in Qinghai yak.

Table 2. Allelic frequencies at milk protein loci in the Maiwa and Jiulong yak.

Although milk protein typing directly by PAGE is simple and cheap, poor resolution of some protein variants could be a problem. Genetic typing of bovine milk protein loci following DNA PCR amplification has been developed in recent years (Medrano et al. 1990; Lum et al. 1997). The accurate and early identification of milk protein genotypes at the DNA level in both females and sires is allowing to establish breeding programs aiming to increase the frequency of the desired milk protein alleles in dairy cattle population. A similar approach will be useful in yak.

Acknowledgments

This work was supported with funds from the National Natural Science Foundation of China, the Science Committee of Sichuan Province and the Laboratory of Animal Physiology and Biochemistry of Nanjing Agricultural University.

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