Why does a thylacine look like a dog not a tiger?

  Last updated December 12, 2017 at 11:50 am

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Tasmanian tigers, despite their stripes, look a lot more dog-like than a tiger. But dogs and Tasmanian tigers, also known as thylacines, split away from each other over 160 million years ago, and are some of the most distantly related mammals. Their striking resemblance is one of the best examples of convergent evolution, the idea that animals might take different evolutionary pathways to reach the same destination. Species end up evolving similar structures or features to solve a similar purpose or make use of a certain resource, without being evolutionarily closely related at all.


A thylacine (a) looks very similar to a dingo (canid, b). The ethanol-preserved thylacine pouch young sampled to sequence the genome. (Source: Nature)


Take the spiny mammals. There’s the marsupial echidna, the hedgehog and the porcupine, who have all independently evolved spines, really just modified hairs, as a form of defense. So while the echidna may look related to a porcupine, it’s much more closely related evolutionarily speaking to a platypus.


Similarly, the Tasmanian tiger is much more closely related to wallaby than a dingo. Apart from its pouch, common to all marsupials, it was virtually indistinguishable from from modern canids (dogs). So how did this Aussie marsupial end up looking so much like a dog?


Researchers turned to the Museum Victoria collection, which houses a 108-year-old thylacine pup, preserved in a jar of alcohol. They were able to extract DNA from this specimen and sequence the Tasmanian Tiger genome. Getting intact DNA out of such an old specimen is not easy.


“The DNA tends to leach out of the specimen into the alcohol over time” says Prof Alan Cooper, from the Australian Centre for Ancient DNA at the University of Adelaide and one of the study’s authors, “Museum curators often change the alcohol whenever it gets cloudy, meaning more DNA diffusion into the newly added alcohol.” The team was lucky in this case though, says Alan, “the specimen was not so old and somehow the DNA preservation was great.”


Analysis of the genome shows that Tasmanian tigers experienced a steep decline in genetic diversity around 70,000 – 120,000 years ago, before human colonisation of Australia during a time that coincided with glacial climate changes.


Researchers then compared the head shape of the thylacine to other mammals, including modern canids like wolves, coyotes, red fox, arctic fox and jackals. They showed that there was exceptional convergent evolution between thylacines and canids, with the thylacine showing more closely related skull shape to the red fox or grey wolf than to its nearest genetic relatives.


Stiking similarity between a thylacine skull and red fox skull. Source: Nature


The researchers conclude that this convergent evolution is not due to natural selection acting on similar protein-coding genes, but instead on regions that regulate how and when genes are expressed. These regions have much more “evolutionary flexibility”, and may be particularly important in how body patterns evolve.


Next up the Australian Centre for Ancient DNA will be sequencing the mainland thylacine genome, a population distinct from the isolated Tasmanian population sequenced in this study. But don’t get your hopes up for any Jurassic Park style resurrections. “We’re never going to bring these species back,” says Prof Alan Cooper, “genome sequencing or otherwise.”



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Lisa Bailey


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