Scan for selective sweeps associated with muscular devolpment in Belgian Blue beef cattle

The Belgian Blue beef cattle is well known for its double muscling phenotype resulting from fixation of a deletion in the myostatin gene. Since this fixation, further intensive selection for muscular development has been particularly succesful. This response to selection might be due to fixation of more genetic variants increasing muscular development. In the present study, we search for selective sweeps in the Belgian Blue genome which might result from the fixation of such variants. To that end we used data from 593 sires genotyped with the BovineHD SNP array. In addition, we used the Belgian Blue dual purpose and the Holstein breeds as controls. We first performed scans for regions of complete homozygosity resulting from fixation. Large fixed regions were found around major genes known to be fixed in the Belgian Blue cattle breed (MSTN, PLAG1 and MC1R) but no other regions of the same magnitude was found. Among the smaller identified regions, only few of them were unique to the Belgian Blue breed. Statistical tests based on long range haplotype information were also implemented, revealing large regions in the genome of reduced haplotype homozygosity specific to the Belgian Blue breed. Some of these regions corresponded to known major genes (MSTN, roan locus, PLAG1 or MC1R) while other regions were new. To determine whether these regions might be the result of selection for muscular development, we performed association studies for muscular development. None of the identified QTL matched with the regions of reduced haplotype homozygosity and the largest QTLs did not presented evidence of strong selective sweeps. These results suggest that the response to selection for muscular development in Belgian Blue beef cattle is probably the result of polygenic selection.