Long-term decline in the genetic diversity of the Tasmanian tiger or thylacine, an extinct Australian marsupial predator, began before the arrival of humans in Australia, reports a paper published this week in Nature Ecology & Evolution. The thylacine genome also reveals that the striking similarities between the thylacine and canids (dog-like mammals, including domestic dogs) were likely driven by changes in genetic regions that regulate the location and time of expression of certain protein-coding genes.
The thylacine (Thylacinus cynocephalus) was a large marsupial carnivore broadly distributed across Australia until around 3,000 years ago. An isolated Tasmanian population was deemed a threat to the sheep industry by European settlers and was targeted for eradication by the government. The last known thylacine died at the Hobart Zoo in 1936. The resemblance between thylacine and evolutionary distant canids is one of the most remarkable examples of convergent evolution (the independent evolution of similar features) - apart from the typical marsupial pouch, the thylacine was almost indistinguishable from placental canids.
Andrew Pask and colleagues sequenced the thylacine genome from DNA extracted from a 108-year-old, alcohol-preserved thylacine specimen. The authors identify a steep decline in thylacine genetic diversity around 70,000-120,000 years ago, which precedes the human colonisation of Australia and overlaps with climate changes associated with the penultimate glacial cycle (a time of very cold temperatures). Comparing head morphology of several extinct and modern mammals, the authors demonstrate exceptional convergent evolution between thylacines and canids. To elucidate the genetic basis of thylacine-canid similarities, they compared the thylacine genome with a reconstructed genome of the ancestor of canids. They find similarities between the two genomes, likely due to chance rather than natural selection, and the genes and pathways that were targeted by natural selection were not the same in thylacines and canid.
The authors conclude that thylacine-canid convergent evolution is not a result of selection acting on the same protein-coding genes, but rather on regions that regulate how and when genes are expressed.
In accompanying News & Views, Ross Barnett and Eline Lorenzen write: “The thylacine study highlights the continued importance of historic museum collections in advancing our understanding of the natural world.”