The kangaroo’s twisted marsupial family tree is now in order thanks to — you guessed it — jumping genes. Genetic evidence shows that a South American ancestor gave rise to all Australian marsupials, and that the South American opossums were the earliest group to branch off from the other six marsupial clans.
Distinctive for raising their live-born young in protective pouches, marsupials all trace back to a common ancestor that split off from the rest of the mammals about 130 million years ago. But fossil and genetic evidence conflict about which marsupial species evolved first, and where.
Jumping genes provide new clues for solving the puzzle. These “junk” bits of DNA make copies of themselves to reinsert randomly in the genome. Half of the marsupial genome consists of jumping genes, so researchers have plenty to work with. Gene-jumping is rare, and each jump is a unique event unlikely to happen again. So if two species share a jumping gene, scientists can deduce that they inherited it from a common ancestor.
Maria Nilsson and her colleagues at Westfälische Wilhelms Universität Münster in Germany looked at similarities and differences in jumping genes in the seven main branches of marsupials. In the July PLoS Biology, the team presents a new marsupial family tree with slightly different familial relationships than other research had predicted.
“It’s a different type of data and it’s much cleaner [than fossil and genetic data],” says evolutionary biologist David Pollock of the University of Colorado School of Medicine in Denver, who was not involved with the research.
According to the new tree, all Australian marsupials arose from a single South American ancestor. In addition, their data puts the gray, short-tailed South American opossum on the earliest branch of the marsupial tree.
There’s always the potential for error in molecular studies, says mammalogist Ines Horovitz of the University of California, Los Angeles. But she says the study “contributes new data, and that’s always important.”
Next, Nilsson says she wants to use jumping genes to probe the relationships among the Australian marsupials to see exactly how they’re related.
