Scientists have extracted RNA from a Tasmanian tiger, marking the first time this molecule has ever been sequenced in an extinct animal.
Like DNA, RNA (ribonucleic acid) carries genetic information. But instead of having a double strand of nucleotides as DNA does, RNA is made of a single strand. That makes it more likely to degrade over time and harder to extract from long-dead tissue.
But understanding RNA is necessary for learning about the biology of an animal, said Emilio Mármol Sánchez, a postdoctoral researcher at the Centre for Palaeogenetics at the University of Stockholm and the Swedish Museum of Natural History. RNA is the intermediary that translates DNA blueprints into the proteins that build cells; it also regulates cellular metabolism.
RNA "gives you a glimpse of the real biology, of how the cell was metabolically working when it was alive, right before the cell died," Mármol Sánchez told Live Science.
This is particularly interesting for Tasmanian tigers, or thylacines (Thylacinus cynocephalus), carnivorous marsupials that lived in Australia until about 3,000 years ago, when the mainland population died out and the only survivors were left on the island of Tasmania. These survivors were driven to extinction by human hunting and trapping; the last known individual died in a zoo in Hobart, Australia, in 1936. Despite being marsupials, thylacines were remarkably dog-like; this represents a case of convergent evolution, in which two distinct lineages yield an animal with a lot of similarities, likely because it fills an ecological niche.
Mármol Sánchez and his colleagues extracted RNA from a desiccated Tasmanian tiger that died about 130 years ago, and analyzed both muscle and skin tissue. The first hurdle was to show that they could extract RNA from the actual animal, not just DNA or RNA from environmental contamination (like humans handling the hide). By comparing the sequences they uncovered, they differentiated between contamination and actual thylacine RNA, Mármol Sánchez said.
Using the RNA sequences, the team filled in several gaps in the Tasmanian tiger DNA. (Because RNA is transcribed from DNA, it's possible to extrapolate DNA sequences from RNA.) In one exciting finding, the researchers identified a never-before-described sequence of microRNA — which plays a regulatory role in which genes are expressed in a cell — apparently present only in Tasmanian tigers. The researchers also found another microRNA sequence that had not been previously described but that turned out to be common across multiple marsupial species.
In total, the researchers raised the number of known microRNAs in Tasmanian tigers from 62 to 325. They also discerned differences between skin and muscle tissue based only on the RNA in those tissue types. Unsurprisingly, the skin samples had high levels of RNA associated with keratin — the protein in skin, hair and nails — while the muscle samples had high levels of RNA associated with muscle fiber proteins such as actin and myosin.
These results can now be used to compare across species and across evolutionary time, the researchers reported today (Sept. 19) in the journal Genome Research.
Moving forward, Mármol Sánchez said, the team plans to sequence more RNA from other Tasmanian tiger tissue, including preserved organs. The same techniques could be used to investigate not just extinct animals, but ancient viruses, many of which are built only of RNA, not DNA, he said.
Finally, the team hopes to find even older samples of RNA from extinct animals with investigations of mammoths. Mammoths went extinct 4,000 years ago, but the research team is working to extract RNA from samples up to 50,000 years old, Mármol Sánchez said.
"You can expect to find something about RNA in mammoths not so long in the future," he said.
