Molecular dissection of the color-sided phenotype in cattle reveals a novel mechanism of chromosome evolution involving circular shuttling intermediates.

The color-sided (Cs) phenotype is a dominant coat color pattern segregating in several breeds including Belgian Blue Cattle (BBC) and Brown Swiss (BS). A genome-wide association study performed in BBC unambiguously positioned the Cs locus on chromo- some 29 (BTA29); however, SNP arrays and CGH detected an equally perfectly associated <480 kb duplication encompassing the KIT gene on chromo- some 6 (BTA6). FISH analysis reconciled these results by revealing an intrachromosomal duplication, which transposed a fragment of BTA6 to BTA29. The organization of the duplicated segment, including breakpoint definition, was determined by high-throughput resequencing and revealed that the transpo- sition occurred via a circular intermediate. The trans- posed KIT copy was shown to be transcriptionally competent, suggesting that dominant color-sidedness results from dysregulated expression of KIT.
Similar analyses of the color-sided phenotype conducted in BS revealed linkage on BTA6, a <120- kb-BTA6 duplication (which overlaps with the BBC duplication), and a <414-kb-BTA29 duplication adja- cent to the BTA29 breakpoint defined in BBC. FISH analysis showed the duplicated portion of BTA29 was located on BTA6 and adjacent to the KIT gene. SNP genotyping indicated that the BTA6 and BTA29 haplotypes associated with color-sidedness in BS and BBC were near identical, demonstrating the non-independence of the two chromosomal events. High-throughput resequencing of a color-sided BS animal defined the corresponding breakpoints and suggests that the BS Cs allele is derived from the BBC duplication