Karyotyping of dog chromosomes is a difficult task owing to the high diploid number of chromosomes (2n = 78) and the similar morphology of autosomes, all of which are acrocentrics. In this report 22 of the 39 G-banded chromosome pairs and their corresponding ideograms have been standardized. The ideogram comprises altogether 235 bands. The need for the introduction of molecular techniques such as chromosome painting and physical mapping of genetic markers for the identification of small acrocentrics is discussed.

In contrast to many other animals, knowledge about the canine karyotype is quite sparse. This is due in part to the rather difficult canine karyotypic pattern. Except for the X and the Y chromosome, there are only acrocentric chromosomes, which appear to be quite small and difficult to identify unambiguously. In previous reports, schematic representations of the canine karyotype have been described. However, a nomenclature comparable to that of the human karyotype or the karyotypes of sheep, cattle, or goats does not yet exist for the dog. Based on high-resolution banding of metaphase chromosomes from canine fibroblasts, we propose an ideogram of the canine karyotype with 460 numbered bands and characteristic landmarks. In addition, the centromere positions of the canine chromosomes are determined by a combined GTG-banding/ FISH approach, and the R- and G-banding patterns are compared.

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We have hypothesized that metacentric and submetacentric chromosomes frequently observed in malignant canine tumors are a result of telomeric fusions. Therefore cells from a canine mammary pleomorphic adenoma were transformed with a plasmid containing the SV40 ‘early region’, known to cause telomeric associations. Compared with non-transformed adenoma cells, the cells had a higher proliferative capacity and expressed the large SV40-T-antigen. Karyotype studies showed the conversion from a normal to an aberrant karyotype with an increase of bi-armed chromosomes resulting from fusions of acrocentric chromosomes. In addition, the length of the telomeric repeats (TTAGGG) was determined for an early and a late passage of the transformed cells by Southern hybridization. The length of the telomeric repeats was apparently longer in the 5th than in the 38th passage. In situ hybridization with a telomere-specific DNA revealed interstitial telomeric repeats in the bi-armed chromosomes. We have concluded that these findings reflect the clonal expansion of head-to-head-telomeric fusions of canine acrocentric chromosomes leading to dicentric chromosomes with a very short distance between the two centromeres. Our results support the idea that the apparent centric fusions that have been described in some canine tumors may in fact be the cytogenetic products of head-to-head-telomeric fusions.

There is an increasing interest in genomic research on the domestic dog (Canis familiaris). However, these investigations are complicated by the canine karyotype comprising 76 acrocentric autosomes of similar size and shape and the metacentric sex chromosomes. None of the numerous published ideograms and karyotypes has yet been generally accepted. The present article gives a review of these descriptions of the canine karyotype. The two most recent nomenclatures and the current efforts toward a standardized canine karyotype made by the Committee for the Standardized Karyotype of the Dog are discussed in detail and recommendations for future use of a nomenclature for the canine karyotype are given.