A short note on linkage between the β- and κ-caseins in the Churra and Manchega ovine breeds

The studies of the linkage of the genes of ovine caseins band k-Cn (CSN2 and CSN3) are limited to the work carried out by Di Gregorio et al. (1991) and Leveziel et al. (1991). Both studies indicate that the genes of ovine caseins form a linkage group in a similar way to other ruminants. The cluster of the casein genes is a highly significant economic region of the ruminant genome since caseins account for approximately 80% of the protein content of milk. Ovine caseins have been assigned to chromosome 6 (Maddox et al., 2001). The aim of this paper was to demonstrate the linkage between the genes of the band k-Cn caseins in the Churra and Manchega ovine breeds by analysing the DNA polymorphisms. These breeds are two of the most important autochthonous Spanish dairy sheep breeds. Six full Churra (2) and Manchega (4) breed families, chosen at random and not related, were analysed. A total of 49 animals six rams, 14 ewes and 29 offspring were studied. The band k-Cn ovine caseins were analysed using the RFLP technique. The DNA samples were obtained from leukocytes using the Goossens and Kan technique (1991). The DNA of each animal was directed with the Hind III, Pst I and Pvu II endonucleases. The C468 and C371 plasmids, containing cDNA of the bovine 5’b and k caseins, respectively, were used as probes (Stewart et al., 1984, 1987). Electrophoresis, DNA transfer to nylon membranes, prehybridation, hybridation and autoradiography were carried out as specified in Di Gregorio et al. (1991) and Ordás et al. (1997). Analysis of the family data for detecting genetic linkage between the b and k caseins was in accordance with the ‘lod-score’ method (Morton, 1955). The value of Z : ] 3 or 5 2 is considered an indicator of the presence or absence of linkage, respectively. The CSN2 locus showed two fragments of 1.2 and 1.6 kb with Hind III. The CSN3 locus showed two fragments of 2.3 and 3.2 with Hind III, three fragments of 3.5, 4.6 and 5.8 kb with Pst I and two fragments of 21.0 6.8 and 27.8 kb with Pvu II. The legacy of the different fragments at each of these loci is Mendelian (Di Gregorio et al., 1991; Ordás et al., 1997). The genetic linkage of the two ovine casein genes was shown by analysis of the segregation of gametes in families with double heterozygote males. This characteristic was present in three males (two Churra and one Manchega) of the six analysed. Table I presents the analysed relationships, indicating as an exponent the endonucleases used to detect RFLPs, the males analysed in each family, the number of informative offspring (those in which the paternal fragments can be determined without any ambiguity), the segregation and ‘lod-score’ values (Z) for u 0 (limiting values of Z when u tends to 0.00). In all cases the analysis of linkage leads to the exclusive observation of parental types (13) and the absence of recombinants (0) in offspring. The ‘lod-score’ values show that there is genetic linkage between the b-Cn and k-Cn genes, given that their accumulated value is slightly above three (Z 3.01). Thus, the results obtained indicate that the loci of the b-Cn and k-Cn caseins are linked in the Churra and Manchega breeds. This corroborates the statements of Di Gregorio et al. (1991) and