Cancer as a contagious disease.

Contagious cancers are malignant cells that are physically transferred between individuals as a natural allograft, forming new clonal tumours. These cancers are highly unusual, but have emerged in 2 mammalian species, the dog and the Tasmanian devil, as well as 4 species of bivalve. The transfer of malignant cells in mammals should initiate a robust immune response and although invertebrates have a less complex immune system, these species still have mechanisms that should prevent engraftment and protect against cellular parasitism. Here the naturally occurring contagious cancers are reviewed to determine what features are important and necessary for the emergence and spread of these types of cancer, with a focus on the mammalian contagious cancers and how they successfully cross histocompatibility barriers.

Class I genes have split from the MHC in the tammar wallaby.

Genes within the Major Histocompatibility Complex (MHC) are critical to the immune response and immunoregulation. Comparative studies have revealed that the MHC has undergone many changes throughout evolution yet in tetrapods the three different classes of MHC genes have maintained linkage, suggesting that there may be some functional advantage obtained by maintaining this clustering of MHC genes. Here we present data showing that class II and III genes, the antigen processing gene TAP2, and MHC framework genes are found together in the tammar wallaby on chromosome 2. Surprisingly class I loci were not found on chromosome 2 but were mapped to ten different locations spread across six chromosomes. This distribution of class I loci in the wallaby on nearly all autosomes is not a characteristic of all marsupials and may be a relatively recent phenomenon. It highlights the need for the inclusion of more than one marsupial species in comparative studies and raises questions regarding the functional significance of the clustering of MHC genes.