GENETIC POLYMORPHISMS OF MILK PROTEINS IN HUNGARIAN DAIRY SHEEP BREEDS AND CROSSES
Institute for Animal
Breeding and Nutrition
Two main protein groups are found in sheep milk:
- Caseins (_ S1, _ S2, _, );
- Whey proteins (_ lactalbumin, _ lactoglobulin).
A four allele polymorphism (A, B, C, D) of _ S1 casein has been evidenced. The primary structure of A, C, D (Welsh) variants was determined by Ferranti et al. (1995).
The Welsh variant was detected for the first time by King (1966) in the Cluny Forest breed, Welsh Mountain and Suffolk. Later it was also evidenced in Italy in the Sarda breed (Bolla, 1989; Piredda, 1993).
The _ S1 casein D allele (Welsh) is associated with lower fat and protein content in the Sarda breed (Bolla, 1989).
_-lactoglobulin is the major whey protein which has three genetic variants: A, B (Kolde, Braunitzer, 1983) (Schlee et al. 1993) and C (Erhardt, 1989) evidenced by isoelectric focusing.
This protein is able to bind different hydrophobic molecules but its biological role is unknown.
The _-Lg was assigned to chromosome 3 in sheep.
The polymorphism evidenced already in milk (alleles A and B) has been detected in DNA amplifying a PCR fragment of intron I and exon I of the ovine _-Lg. An Rsa I endonuclease restriction site has been revealed (GT/AC) for the A allele, while allele B has no such restriction site.
The determination of milk protein genotypes is of particular importance. Piredda et al. showed that milk produced by homozygous Welsh ewes is associated with lower casein content and casein index, a greater whey protein content and worse lactodynamographic parameters.
The genotype BB of _-lactoglobulin seems to be associated with higher milk yield; on the other hand genotypes AA and AB seem to be superior in protein and casein content and curd yield (Garzon and Martinez, 1992).
The aim of our experiment is to find the relationship between _-Lg and _ S1 casein on one hand and production traits and milk properties on the other hand. Data published up to now in this field are very contradictory.
Keywords: sheep, _ lactoglobulin, _ S1 casein, PCR-RFLP
MATERIALS AND METHODS
We collected 1 294 blood samples from different breeds of sheep existing in Hungary such as Hungarian Merino, Tsigaia, Lacaune, British Milksheep, Awassi and Crosses.
We determined the genotype of b-lactoglobulin (Figure 1.) using the PCR-RFLP technique, where the primers used were: 5' CTT CCC ACC CCC AGA GTG CAA C3', 5' TGG GGA GTG GGG GTT CCA TGT T3'. The samples were amplified for 31 cycles, 94 C, 1 min.; 65 C, 1 min.; 72 C 1 min. Restriction enzyme applied was Rsa I.
For the identification of Welsh Casa 1 variant we used a method developed by Rammuno et al (1997) based on PCR-ASA technique.
Up to now we have analysed 500 samples from Lacaune, Tsigaia, British milksheep and Awassi breeds. No Welsh or heterozygous variants have been found yet.
Our results concerning the distribution of _-lactoglobulin genotypes of different breeds of sheep and crosses are presented on Figure 1.
The allele C of _-lactoglobulin remains to be determined in the future.
Figure 1. The distribution of _-lactoglobulin genotypes of different breeds of sheep and crosses
A few milk characteristics associated with _-lactoglobulin genotypes are presented in Figure 2.
Figure 2. The fat content and the cheese yield of the milk
Significant differences in sugar and protein content of milk were not detected.
The examination of _S1 casein will be carried on for the rest of our samples.
Bolla P. - Caroli A. - Mezzelani A. - Rizzi R. - Pagnacco G. - Fraghi A. & Casu S. 1989. Milk protein markers and production in sheep. In: Anim. Gen., 20. Suppl. 1., 78.
Erhardt G. 1989. Evidence for a third allele at the _-Lg locus of sheep and its occurrence in different breeds. In: Anim. Gen. 20. 197-204.
Garzon A. I. & Martinez J. 1992. _-Lg in Manchega sheep breed. Relationship with milk technological indexes in handcraft manufacture of Manchego cheese. XXIII. Int. Conf. Anim. Genet., Interlaken.
King J.W.B. 1966. Proceeding of 10th European Conference on Animal Blood Groups and Biochemical Polymorphisms, Paris, 427-431.
Kolde H.J. & Braunitzer G. 1983. The primary structure of ovine _-Lg, In: Milckwissenschaft 38: 70-72.
Piredda G. - Papoff, C.M. - Sanna, S.R. & Campus, R.L. 1993. Scienza e Technica Lattiero-Casearia 44 (3) 135-143.
Rammuno L. - Cosenza, G. - Rando, A. - Macciotta, N.P.P. - Pappalardo, M. & Masina, P. 1997. Identification of carriers of the Welsh Casa I variant using on allele-specific PCR method. In: Anim.Genetics, 28: 154-155.
Schlee P. - Krause I. & Rottmann O. 1993. Genotyping of ovine _-Lg alleles A and B using The PCR. Arch. Tierz. 36: 519-523.