The Tasmanian Devil (TD) is threatened by a dreadful outbreak of facial tumors, which grow rapidly and kill the animals as they become unable to defend themselves and feed. There are fears that the entire population of this unique fierce, wolf-like marsupial may be lost.Drs. Pearse and Swift captured 11 animals and biopsied the tumors, as well as normal tissue. They first described the normal TD karyotype: 14 chromosomes (6 pairs of autosomes and XX or XY).
To their surprise, all the tumors had exactly the same, and a very abnormal karyotype: 13 chromosomes total, but neither copy of chromosome 2, no X, no Y, and 4 additional chromosomes which could not be definitively identified. One animal had an inversion marker in all his normal cells; it was not found in his tumor.
In humans, certain malignancies have a characteristic chromosome break or transposition, but in addition, each patient’s tumors acquires its own particular chromosomal abnormalities. Nothing remotely resembling the condition with the TD has ever been described.
Their conclusion is that all TD have the same tumor, which they are passing from animal to animal. Since they are highly aggressive, encounters often result in biting, and the most likely place for one to bite another would be on the face. It is at that time, probably, that tumor cells are transplanted from one to the other.
Why are the tumors not rejected by the recipient’s immune system? They point out that the TD population shows little genetic heterogeneity, as might be expected when all animals live in a small continuous area like the island of Tasmania. They are all, in other words, closely related.
A colleague of theirs has told them that when one-way MLRs are performed between random members of the TD population, almost no proliferation of T cells is seen is response to MHC on the other animal’s macrophages; just as you’d expect in an essentially-inbred population. So the tumor is not recognized as foreign…
This is about the most dramatic evidence I've seen in favor of being as diverse as possible at MHC loci.
A.-M. Pearse and K. Swift. Allograft theory: Transmission of devil facial-tumour disease. Nature 439, 549 (2 February 2006)
