To recap, the Tasmanian devil (Sarcophilus harrisii) is a species in serious trouble. In a phenomenon almost unique to science, the already small population is suffering from a transmissible form of cancer called Devil Facial Tumour Disease (DFTD), with over 70% of the population infected. It is almost completely lethal, causing swelling in the mouth and face, leading to suffocation and starvation. 80% of the population has been wiped out since its discovery in 1996, and it is predicted that the species could be extinct within 25-35 years.
I care deeply about this for a number of reasons. First, I think that they are beautiful animals. Biologically they are important, propping up a fragile ecosystem in the self-contained, isolated biome that is the island of Tasmania, which I desperately wish to revisit. Scientifically, too, they are fascinating, being the largest remaining carnivorous marsupial on Earth. For this reason, they are also of great significance for conservation. Finally, they are important for historic reasons. In living memory, Tasmania was once home to a larger carnivorous marsupial, the thylacine, which was hunted to the brink of extinction. The last of its kind died of neglect in Beaumaris Zoo, Hobart, in 1936. Tasmania does not need the legacy of allowing another of its unique creatures to die out at our hands.
The plight of the Tasmanian devil is so unique and critical that the slightest development in the search to unravel the cause and cure of the cancer automatically generates headlines. A recent paper by Miller et al. led to a flurry of newspaper and magazine articles, and a well put editorial from New Scientist, and I’d like to comment, briefly, on the paper and its implications.
The new study follows the completion of the devil genome last year, which gave hope that mutations might be discovered in both individuals (to search for immune resistance) and in the cancer itself. The new study sought to determine the extent of genetic diversity within the species. They looked at two individuals from opposite ends of the islands, as well as samples from 87 wild and full mitochondrial genomes from 14 living and museum specimens. The two individuals they fully sequenced should have been very different: Cedric, from the north-west, was a bumbling fellow who sparked hopes among scientists in 2007 when he failed to contract DFTD when injected with facial cancer cells. He was thought to be immune, surviving multiple experimental infections. He eventually succumbed. The north-west of Tasmania remains the last stronghold for resistant individuals. Spirit, on the other hand, lived in the south-east and was dying from DFTD when captured.
Between the two individuals they found 914,827 nucleotide substitutions - where one single letter in the extensive genetic code differs. That may sound like a lot, but the equivalent number between human groups is 4,800,466, and in orang-utans it is nearly twice this again. They also looked at the prevalence of different versions of particular genes and found historical mixing of devil populations, which could explain the low diversity between them. But when did this low diversity arise?
To answer this they examined the mitochondrial DNA of seven modern and six museum samples, as well as a tumour taken from Spirit that they had shown contained cells not belonging to Spirit. Mitochondria are tiny energy-producing compartments inside every cell. They contain their own DNA, shorter and less complicated than the full DNA sequence of the nucleus, and are passed intact through the maternal lineage. This makes them key in determining historical genetic variation, because their genes are not reshuffled with each conception of new offspring. Instead they are preserved, variation only arising from the ravages of time. Miller et al. conclude that, with one exception, all of the genetic diversity seen in samples from the 1800s exists today, or, to look at it another way, the extremely limited genetic diversity seen today has been the case for at least 100 years.
As this New Scientist editorial points out, 15 years ago, Tasmanian devils were listed by the International Union for the Conservation of Nature (IUCN) as "of least concern": there were plenty of them on the island. Then DFTD came along.
But had we known 15 years ago how genetically alike the entire population are, things might have been very different. We can't have foreseen DFTD, but the knowledge of their low genetic diversity would have set off alarm bells, for it made them highly susceptible to any new illness, unique to science or not. DFTD was a disaster waiting to happen. Despite high numbers, the species was most definitely not "of least concern".
Miller's paper ends with a note of optimism:
We believe that this project illustrates the promise of high-throughput sequencing and genotyping methods for helping to assess intraspecies genetic diversity, including comparison with historical levels, and for planning how to maintain the remaining diversity.Such techniques are now not only possible but relatively affordable. Yet still the IUCN does not require genetic testing of species when evaluating extinction risk. Countless species might appear safe and yet, like the Tasmanian devil - those happy, fluffy growling marsupial bears - be sitting ducks for an unknown future plague. Molecular biology has the tools to help conservation: it is time we used them.