the thylacine – back from the dead?

Blogging on Peer-Reviewed ResearchNot quite (although that's implied in some of the rather breathless reporting of an extraordinary paper that was published on-line this month). Nevertheless, the real story describes a striking achievement: the cloning of DNA from the extinct thylacine, or Tasmanian tiger.

Here's the abstract from the paper, so you can see how the authors described what they've done:

There is a burgeoning repository of information available from ancient DNA that can be used to understand how genomes have evolved and to determine the genetic features that defined a particular species. To assess the functional consequences of changes to a genome, a variety of methods are needed to examine extinct DNA function. We isolated a transcriptional enhancer element from the genome of an extinct marsupial, the Tasmanian tiger (Thylacinus cynocephalus or thylacine) obtained from 100-year-old ethanol-fixed tissues from museum collections. We then examined the function of the enhancer in vivo. Using a transgenic approach, it was possible to resurrect DNA function in transgenic mice. The results demonstrate that the thylacine Col2A1 enhancer directed chondrocyte-specific expression in this extinct mammalian species in the same way as its orthologue does in mice. While other studies have examined extinct coding DNA function in vitro, this is the first example of the restoration of extinct non-coding DNA and examination of its function in vivo. Our method using transgenesis can be used to explore the function of regulatory and protein-coding sequences obtained from any extinct species in an in vivo model system, providing important insights into gene evolution and diversity (Pask, Behringer & Renfree, 2008).

thylacine image from PLoS ONE.png

(a) A young male thylacine in Hobart Zoo in 1928; (b) one of the preserved pouch young specimens (head length 34mm) from which DNA was extracted, from the Museum Victoria collection.
(Pask, Beringer & Renfree, 2008. doi:10.1371/journal.pone.0002240.g001)

The image above shows what the adult animal looked like, and one of the 'pouch young' (ie a very young infant, collected from the mother's pouch) from which DNA was extracted. Pask and his colleagues were particularly interested in looking at non-coding regulatory DNA, because mutations in these sequences have the potential to have a considerable effect on variation between species and larger taxa. For this reason they chose to attempt to obtain a transcriptional enhancer element that's been well-studied in other organisms – and were pleased to find that they'd succeeded in doing so. (They confirmed its origin, and that it wasn't a human DNA contaminant, by phylogenetic comparison of the DNA sequence – it definitely belonged to a marsupial.) 

The next step was to use the thylacine sequence to make transgenic mouse embryos, and see where the gene was expressed. They found that it was active in developing cartilage (the 'chondrocyte-specific expression' mentioned in the abstract) and nowhere else.

mouse embryos expressing thylacine DNA.png
Stained transgenic mouse embryos showing varying levels of reporter gene expression
within the developing cartilage (blue).
Pask, Behringer & Renfree (2008). doi:10.1371/journal.pone.0002240.g003


They concluded that the thylacine Col2a1 gene had a conserved developmental role in cartilage formation, and that its promoter directed expression in chondrocytes in this extinct marsupial mammal. It also suggests that the associated transcription factors and the minimal enhancer element required for inducing expression from this promoter have remained sufficiently conserved between mice and marsupials to direct expression despite approximately 148 million years of divergent evolution between these species.

Which is pretty darned amazing, really. (And they don't say, media comments notwithstanding, that this opens the door to resurrecting extinct species, maybe even dinosaurs. A single enhancer does not a thylacine – or a dinosaur – make.)


A.J. Pask, R.R. Behringer & M.B. Renfree (2008) Resurrecton of DNA function in vivo from an extinct genome. PLoS ONE 3(5): e2240. doi:10.1371/journal.pone.0002240

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