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Tricky tree. DNA bits called retroposons reveal that marsupials share a South American ancestor but form distinct South American and Australian groups.

Adapted from M. A. Nilsson et al., PLoS Biology, 8 (July 2010)

Jumping Genes Shed Light on Marsupial Migration

Kangaroos, opossums, and Tasmanian devils might differ dramatically in looks and location, but they share the same family tree. Researchers have argued for years about how these marsupials evolved and spread across the world. Now a new study of "jumping" genes may have finally solved the mystery.

More than 300 marsupial species live in the Americas and Australia. The critters are famous for their pouches, built-in baby carriers where they keep and feed their young. They are the closest kin of placental mammals, such as humans, but they branched off to form their own group 130 million years ago. They settled predominantly in South America and Australia, which at the time were part of a supercontintent known as Gondwana.

DNA sequencing and the fossil record tell two different stories of how that settlement went down. The DNA suggests that a single South American ancestor swept into Australia before the continents drifted apart, and the marsupials on each continent then evolved on their own. Fossils, however, support a more complicated picture in which some ancestors made the return journey to South America, meaning that some South American species might have arisen in Australia.

Central to this conflict is the Monito del Monte, or "little mountain monkey." This rare and tiny marsupial lives only in the rainforests of the southern Andes in Chile and Argentina, but it gets classed with its Aussie brethren based on similarities between its ankle and ear bones and those of an extinct Australian marsupial. Some researchers take the Monito as evidence of more complex migration, but others remain unconvinced because the DNA data are inconclusive.

To sort out the mess, evolutionary biologist Maria Nilsson of the University of Münster in Germany and colleagues began looking for weird bits of DNA called retroposons. Retroposons break off from chromosomal DNA and copy and paste themselves back in elsewhere in the genome. Unlike the rest of the genetic code, which is subject to mutations and other random events that can make analysis difficult, retroposons and their locations are predictable and stable after the copy and paste. That makes them a more reliable marker for determining relationships between animals; if two species have the same retroposons in the same spots, they probably share a common ancestor.

Nilsson compared the retroposon lineups of the South American opossum, Australian tammar wallaby (a kangaroo), and 20 other species, including the common wombat, wallaroo, and marsupial mole. All marsupials shared the same 10 retroposons, confirming that they all share a common ancestor. The South American marsupials had undergone fewer retroposon copy-and-paste incidents, which suggests that they form the older branches of the tree. That opossums lacked two nearly universal retroposons may indicate that they are the closest, genetically speaking, to the marsupial ancestor, Nilsson says. Even the mysterious Monito found its place: It shares certain genetic features with Australian marsupials, but it is definitely more closely related to its fellow South Americans.

The retroposon data also clearly divided Australian and South American marsupials into distinct groups—a hint that the animals enjoyed little contact as they evolved.

Together, the results suggest that a single migration from South America to Australia gave rise to the variety seen today, including the Monito. "I was really thrilled," Nilsson recalls. "I think this is going to solve the controversy."

The findings, published online today in PLoS Biology, represent "a really big step forward in terms of [understanding] marsupial relationships," says evolutionary biologist Matthew Phillips of the Australian National University in Canberra. The only shortcoming of the retroposon technique is that it doesn't account for dead ends—extinct marsupials that might further fill in the family tree, he says. Combining the retroposon technique with fossil data could help researchers get a more detailed picture of where and how marsupial ancestors migrated. "This study provides a useful technique and foundation for future studies" that may finally lay the controversy to rest, Phillips says.