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Chapter 4 – The science selected for study

4.5 Is the lack of genetic diversity in devils a reason for cancer transmission? transmission?

4.5.1 The Cheetah precedent

Regardless of the uncertainty surrounding the role of genetic diversity in DFTD, the dominant hypothesis, first proposed by Menna Jones in an interview, became devils, like cheetahs, are inbred. Serengeti cheetahs at some stage in the past, through a population bottleneck mated with closest relatives, resulting in genetic uniformity.49 In devils it is proposed inbreeding enables DFTD to be transplanted between devils.50 In the Serengeti cheetah lack of genetic diversity results in their susceptibility to a virus that threatens the survival of the population from a wasting disease. In his book Tears of the Cheetah Stephen O’Brien documents the experiments used to test the hypothesis that a lack of genetic diversity in Serengeti Cheetahs might be the cause of vulnerability to this wasting disease.51 A brief outline of these studies follows.

O’Brien’s scientific observations and the twelve different experiments undertaken to determine the cheetah population’s genetic diversity involved: autografts, the transplanting of tissue from the cheetah’s own body; allografts, transplanting of tissue

Jones ME & Schuster SC, 2011, Genetic diversity and population structure of the endangered marsupial Sarcophilus harrisii (Tasmanian devil), PNAS, Early Edition, pp 1-6. Available at:

www.pnas.org/cgi/doi/10.1073/pnas.1102838108 last accessed 18 December 2012

49 O’Brien, SJ, 2003, Tears of the Cheetah, St Martin’s Press, New York

50 Price M, 2011, For Conservationists, the (Tasmanian) Devil Is in the details, Science. Available at:

http://news.sciencemag.org/sciencenow/2011/06/for-conservationists-the-tasmani.html last accessed 29 December 2012

51 O’Brien SJ, 2003, Tears of the Cheetah, St Martin’s Press, New York

between unrelated cheetahs but of the same species; and xenografts, transplanting of tissue between unrelated species. It was found that the autografts and the allografts took in all cases.52These experiments confirmed that cheetahs were extremely inbred and lacked genetic diversity. Support for the finding was provided by observations of asymmetry in cheetahs, something particular to inbred species. Hence, when compared to the skulls of leopards, they ‘certainly looked very inbred’.53

Zookeepers were the first to notice a problem in cheetahs when they encountered difficulty in breeding them in captivity. It was hypothesised that a lack of genetic diversity might be the problem. Subsequently a number of experiments on the captive cheetahs proved their immune system did not recognise tissue transplants as non-self, and so they appeared to be inbred.54 The experiment was repeated in Pretoria, South Africa on an eastern population of wild cheetahs with the result that seven out of ten allografts, between seemingly non–related cheetahs, were accepted.55 The same experiment was then undertaken on the western population and they found similar results. The cheetah’s MHC genes were analysed to determine if a lack of diversity in this most diverse set of genes could be the problem. It was found that cheetahs did lack genetic diversity probably due to a previous bottleneck in the population.

The bottleneck is proposed to have occurred around 12,000 years ago during an ice age that resulted in a large number of animal extinctions. 56 The cheetah apparently escaped extinction very narrowly and it is possible only one female and her cubs survived to re-breed and populate. The species, based on these few survivors, successfully bred to the

52 ibid, p 25

53 ibid, p 2

54 ibid.

55 ibid.

56 ibid, p 34

present large numbers. Although it was shown beyond doubt that cheetahs are very closely related, the decline in the species may not be due to a lack of genetic diversity, but to human activities such as habitat destruction.57

The highly respected scientist Edward O Wilson in his book The Future of Life states ‘if the species manages to pass through a bottleneck of very low population size and still survive, the depression may in the course of the passage “clean out” the defective genes.58 Such a genetic purge evidently occurred in the cheetah.’59 He goes on to say they ‘did not perish from genetic defects, as might be immediately suspected’ but ‘the principal causes instead were predation by lions and spotted hyenas, along with abandonment by the mothers during periods of food scarcity’.60 According to Wilson a very small or very local population is most vulnerable to demise from a natural disaster, such as storm, fire or drought.

The Tasmanian devil researchers have not undertaken the exhaustive experiments described above to prove the devils’ genetic diversity is similar to that found in the cheetah population. Despite this the DFTD research team did not deviate from the belief that the devils’ lack of diversity in its MHC genes is the reason the cancer is transmissible. Stephen O’Brien visited Hobart on an invitation from DPIPWE and although the meeting was publicized in local newspapers, no reports of his views on the devil disease were made public. However, Pearse in a personnel communication informed me that those who attended were asked to think of ‘pot-stirring’ questions.

She herself asked two questions. The first: are Dasyurids (the family to which the devils

57 ibid.

58 Wilson EO, 2002, The Future of Life, Abacus, London, p 57

59 ibid, pp 56-57

60 ibid, p 57

belong) at the end of their natural existence? To which O’Brien replied – ‘rubbish’.

The second question was, if vaccines were created against the devil cell-lines isn’t there the danger of the devils developing an autoimmune disease? O’Brien replied – ‘sure’.

Regardless of the lack of studies the devil researchers remain committed to the allograft hypothesis and to the conviction that a lack of genetic diversity in the devils’ MHC genes was why the cancer could establish in a new host.