First Decoded Marsupial Genome Reveals "Junk DNA" Surprise

Stefan Lovgren
for National Geographic News
May 10, 2007

The genome of a marsupial—the tiny short-tailed opossum (Monodelphis domestica)—has been sequenced for the first time.

The study reveals a surprising role in human evolution for "jumping genes"—parasitic bits of "junk DNA" that until now were thought to be nothing more than a nuisance—and may also lead to a number of medical breakthroughs.

In particular, the study highlights the genetic differences between marsupials such as opossums and kangaroos and placental mammals like humans, mice, and dogs.

Marsupials typically spend their youths tucked in a mother's pouch, while placental females maintain a temporary organ called the placenta to nourish their embryos.

"The opossum is a wonderful comparison to the human," said Eric Lander, director of the Broad Institute of MIT and Harvard University in Cambridge, Massachusetts, which led the sequencing project.

The study, which appears in today's issue of the journal Nature, helps to explain the evolutionary origins of human DNA, Lander pointed out. (Read a genetics overview.)

And opossums are often used as models in a wide variety of research on human health and disease, including the malignant melanoma form of skin cancer. (Related: Macaque Genome Deciphered; May Herald Medical Breakthroughs" [April 12, 2007].)

The new findings show that marsupials have a much more complex immune system than previously thought.

Jumping Genes

Marsupials are the closest living relatives of placental mammals. The two groups split from a common ancestor about 180 million years ago.

Scientists were able to pinpoint the genetic elements that are present in placental mammals but missing from marsupials to learn more about what makes the two groups different.

The researchers were surprised to find that placental and marsupial mammals have largely the same set of genes for making proteins. Instead, much of the difference lies in the controls that turn genes on and off.

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