July 30, 2010
Many of the poster animals of Australia—kangaroos, koalas, wombats and wallabies, to name a few—are marsupials, animals best known for carrying around their young in a pouch. Marsupials can also be found in the Americas; in the United States, the Virginia opossum is the only one, but there are dozens of species in Central and South America.
Scientists trying to draw the marsupial family tree have been perplexed by contradictory evidence: DNA studies suggested that the Australian branch was an offshoot of South American animals that migrated to Australia when the two continents were connected and part of Gondwana. Fossil studies, though, seemed to show that some of the Australian marsupials had made their way back to South America.
In a new study in PLoS Biology, researchers from Germany set out to make a marsupial family tree using retroposons, a kind of jumping gene—pieces of DNA that are copied and pasted at random within the genome. The more closely related two species are, the more retroposons they will share.
Comparing the retroposons of the 21 marsupials showed that they all shared 10 jumping genes, thus confirming that they shared one ancestor. But the South American and Australian marsupials formed distinct groups; the Australians shared retroposons that their South American relatives lacked. The researchers were also able to determine that the South American branch was older (meaning that the Australian marsupials had come from South America) because the South Americans lacked two retroposons shared by everyone in the Australian branch.
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