What Can We Learn About Extinction?

We have produced the first accurate data for the thylacine mitochondrial genome. This will be a key ingredient for mapping changes in the thylacine population structure, starting several thousand years ago when it still survived on the Australian mainland, and monitoring changes in genetic diversity leading up to extinction. Unlike studies with species that went extinct thousands of years ago, or species that are not yet extinct, we have a precise date for the extinction -- 7 September 1936. For instance, we know that our samples are from specimens from the last few generations of the species; there are no assumptions, extrapolations, or other forms of guesswork. That is, we have direct empirical data from the very brink of extinction.

Moreover, our data include sequences from the nuclear genome, which contains essentially all of the functional genetic information, plus sequences from micro-organisms found in the sample and perhaps coming from the living animal. Data like ours can potentially answer fascinating questions about the thylacine's extinction. Here is one example:

On pages 202-203, the book by Robert Paddle discusses an epidemic that hit thylacines in the years around 1900. It was described as resembling distemper or mange, resulting in a loss of hair and producing scabs on the body. It took the disease about six years to spread from the east coast of Tasmania to the west coast. Among captive animals the disease could be fatal; during 1900-1903 the Melbourne Zoo lost 16 of their 17 thylacines. The disease may have persisted in the thylacines up to the very end; the book by Guiler (p. 64) describes an animal that died in the Hobart Zoo in 1935, and thus was one of the very last to exist, as follows: "its skin was in such poor shape as to be useless." Currently, the extent to which the epidemic contributed to the thylacines' extinction is not known. It is possible that the disease contributed to the inability of zoos to maintain a breeding population, and that a reduced diversity in the thylacine's immune system spelled doom for the species.

The sequencing approach used in our study may one day shed light on the potential role of this disease in the thylacine's extinction. Analysis of the microbes found in the thylacine hair shafts could potentially identify the microbe that caused the disease. Furthermore, sequencing of the nuclear genome will tell us about the genes related to immunity, such as those in the major histocompatibility complex, which in turn will inform us about the ability of the species to defend against bacteria and viruses.

Reference: Thylacine: The Tragedy of the Tasmanain Tiger by Eric R. Guiler, Oxford Univ. Press, Melbourne, 1985.