Abstract
The aim of this study was to analyze the genetic structure of the casein cluster in eight selection lines of the Holstein Friesian (HF), German Simmental (GS) and German Black Pied cattle (“Deutsches Schwarzbuntes Niederungsrind”, DSN) breeds. A total of 2962 milk samples were typed at -casein (-CN), -casein (-CN), -casein (-CN) and -casein (-CN) loci using isoelectric focusing. The number of alleles per locus ranged from one (-CN) to five (-CN), and the average expected heterozygosity and polymorphic information content of all loci were 0.33 and 0.27, respectively. The unrooted dendrogram revealed that the selection lines of the endangered DSN breed were clearly separated from the HF and GS breeds due to their predominance of the -CN A1 allele and the comprehensive haplotype BA1A (in the abbreviation of ---CN). Temporal changes in allele distributions indicated decreasing genetic diversity at the casein loci, explaining the moderate level of genetic differentiation among selection lines (7.1 %). The variability of the casein should be exploited in future using breeding programs to select genetic lines for specific protein production in bovine milk but also to preserve biodiversity.
Highlights
Since domestication 8000–10 000 years ago, natural as well as man-made factors including geography, environment, culture and directional artificial selection contributed to cattle trait modifications phenotypically and genetically (Loftus et al, 1994)
A total of 11 alleles were detected in eight selection lines at four casein loci
The results of the present study indicate that different selection strategies indirectly contributed to the variability of the casein polymorphisms linked to milk production traits
Summary
Since domestication 8000–10 000 years ago, natural as well as man-made factors including geography, environment, culture and directional artificial selection contributed to cattle trait modifications phenotypically and genetically (Loftus et al, 1994). Positive (Darwinian) selection promotes the spread of beneficial alleles, so that frequencies for these alleles increase and the selected alleles might be fixed over generations (Maynard Smith and Haigh, 1974; Kreitman, 2000). Negative or purifying selection hinders the spread of unfavorable alleles, causing decreasing allele frequencies up to the complete loss from the population (Kreitman, 2000). Selection causes a “hitchhiking” effect on the frequency of neutral alleles at linked loci (Maynard Smith and Haigh, 1974). The cattle genome represents an opportunity for the identification of genetic variation that contributes to phenotypic diversity and for inferring genome responses to strong artificial selection. The different methods to detect selection signatures are based either on the distribution of allele frequencies, on the properties of haplotypes segregating within populations or on genetic differentiation between populations (reviewed by Hohenlohe et al, 2010)
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