Extensive coloured identification of dog chromosomes to support karyotype studies: the colour code

The identification of individual dog chromosomes is problematic because the 38 pairs of autosomes are small and acrocentric. Here we describe the design and application of a FISH tool that enables definitive identification of each dog autosome in a normal karyotype, without relying on subjective interpretation of DAPI banding patterns. From a high-resolution physical map of the canine genome, we have chosen a panel of 80 canine chromosome-specific BAC clones. DNA from each clone is labeled with one of five different fluorochrome-conjugated nucleotides. By selecting one to three spatially separated BACs per chromosome, and labelling them with a distinctive combination of colours, each autosome can be identified objectively and orientated accurately, irrespective of the quality of DAPI chromosome banding. This tool, or part of it, can be used for any purpose where accurate identification of canine autosomes in a normal karyotype is essential. In this study, we demonstrate use of the 'colour code' for chromosome identification following CGH analysis of unbalanced genomic aberrations in a canine brain tumour. Our method is an improvement of an earlier procedure, featuring chromosome-specific BACs and sequential FISH hybridisations, as it enables simultaneous identification of all chromosomes in a single hybridisation.