Why are those ugly devils not dead yet?

Devil facial tumour disease (DFTD) is a transmissable cancer that affects Tasmanian devils and has substantially depleted their population, rasing concern that the species faces extinction. However, a new study offers some hope. Epstein et al. report that three populations of Tasmanian devil are exhibiting immune-modulated resistance to DFTD owing to modifications in certain genomic regions that may overcome immune suppression (which is how DFTD spreads between individuals). The selective pressure imposed by DFTD may therefore be encouraging its own undoing.

writes Gemma Alderton in Nature Reviews Cancer to highlight a study in Nat Comm by Epstein et al. The evolution of cancer in Tasmanian devils is really interesting, because it is not intra-tumour evolution, like the rest of the stuff I write about, but the evolution of a transmissible cancer from one devil to the next. It seems they like to bite each others faces. And that spreads the cancer.

Now … if transmissible face cancer is what floats your boat, make sure you also read Dan Graur’s take on it: “All #Hype, No Evidence: Have #TasmanianDevils Evolved Resistance to Facial Tumor Disease? Who knows?

The tone of the posts is surprisingly moderate. For example, no one is insulted as the Saddam Hussein of science. And he has some valid points:

There are essentially five explanations for the fact that Tasmanian devils are still with us.

  1. The mortality estimates (100%) were overestimated.
  2. The effects of DFTD on fitness were overestimated. In other words, despite a 100% mortality, Tasmanian devils manage to reproduce successfully before they die.
  3. The mathematical model that predicted their demise was wrong.
  4. The cell line became less virulent in time.
  5. Some Tasmanian devils carried a resistant genotype and were unaffected by the cancer. These genotypes were subject to very strong positive selection and increased their frequency in the population. In other words, Tasmanian devil populations have become immune to the contagious cancer through selection.

Epstein et al. in their Nature Communications paper decided to ignore the simple explanations 1, 2, 3, and 4, and focused solely on explanations 5.

Let’s see how this discussion plays out in the future …


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