From gene to cheese : the caprine alphas1 casein polymorphism, its effects and its evolution

Abstract

The caprine alpha_s1 casein locus is remarkable for its high level of polymorphism and for the fact that there exists clear differences in the level of protein synthesised between alleles or groups of alleles. The early studies established that the polymorphism was due to a minimum of 7 alleles which corresponded to 4 different levels of synthesis : the A, B and C alleles are associated with a "high" level of alpha_s1 casein (around 3.6 g/l), the E allele is associated with a "medium" level (1.6 g/l), the D and F alleles are associated with a "low" level (0.6 g/l) and the O allele is a nul allele (with no alpha_s1 casein in the homozygote). In 1985 the E and F alleles predominated in the French milking breeds, Alpine and Saanen, in part accounting for the low level of milk protein in these breeds and limiting their cheese yield. This posed the question of the value of selection in favour of "high" alleles and the means by which such selection might best be achieved. Each of the "low" variants is characterised by the deletion of an amino acid sequence (residues 59 to 95 in F, 59 to 69 in D, and 14 to 26 in G) which results from anomalous splicing of the RNA messenger. In the case of the F allele, this anomaly appears to be due to the deletion of a nucleotide, in the case of the G allele to the substitution of a nucleotide. On the other hand, the reduction in the level of synthesis found with the E allele appears to be caused by an insertion of 458 nucleotides in the last exon of the gene. The B1 subdivision of the "high" B allele is the original allele of the species. The alleles with reduced levels are therefore defective mutants. The study of the on-farm performance of the offspring of 5 bucks heterozygous at the alpha_s1-Cn locus provided evidence for their different protein levels which agrees with the estimates given above (around 2.5 g/kg difference between the A and F alleles, 2 g/kg for the A and E alleles). This polymorphism did not affect the amount of milk produced while unexpectedly, significant differences were found for the fat percentage between the A allele and the E and F. Overall, the A allele has, when compared to the F allele, a significant effect on the total amount of protein per lactation but not on the amount of fat. Analyses of the physico-chemical properties of milk from goats homozygous for the 3 principal alleles (A, E, F) confirm the effects of the genotypes on the casein and fat percentages, and shows, in addition significant effects on the diameter of the micelles and on their calcium content, which is lower in A/A milk. These characteristics explain the observed differences in the ability of the milk to be coagulated by rennin, this ability being greatest for A/A milk. The most important differences concern the maximum curd firmness (A/A>E/E>F/F) and the rate of firming (A/A>E/E and F/F). A/A milk also has, on average, an intermediate time for coagulation between that of E/E milk (longer) and that of F/F milk (shorter). Trials of the traditional manufacture of "Pélardon des Cévennes" type cheese provide evidence of clear differences in the adjusted cheese yield : +7.4 % between A/A and E/E milk and + 14.8 % between A/A and F/F milk. Seasonal variation in yield follows that in the fat + protein yield. Some differences in the firmness of cheese (A/A>E/E>F/F), for which evidence was previously found by laboratory measurements, were confirmed by a taste panel. According to the same panel, the goat flavour tended to be less pronounced in the cheeses made from A/A milk. In the European milking breeds, we observe, according to the breed, a preponderance of "high" alleles A and B, of the "medium" allele E or the "low" allele F. The latter predominates in the French and Italian Alpine and Saanen breeds and also in the Corse population. In the Alpine and Saanen breeds the "high" alleles are almost absent in goats which have a low index or protein percentage, but are in the majority in the Alpine breed (0.72), and are frequent in the Saanen breed (0.42) in animals which have a high index or protein percentage. In the Alpine breed, the frequency of the A allele has clearly increased in recent years in males at test (around 0.5 in 1992) and in the stud bucks (0.6). In the Saanen breed the E allele remains predominant, but the frequency of the A allele is increasing. The scientific contributions and implications of this work and the prospects for its application in breeding programmes are discussed.