It belongs to a South-American oposum

May 10, 2007 09:10 GMT  ·  By

Marsupials and placental mammals ( humans included) had a common ancestor about 180 million years ago, during the Jurassic era (middle dinosaur epoch) and chose very different reproductive paths: marsupials rear their offspring externally, sometimes in a pouch, while placental mammals deliver well developed youngsters.

Now, researchers at the Broad Institute of MIT and Harvard in Cambridge, Massachusetts, have sequenced for the first time a marsupial genome: that of the gray, short-tailed opossum, a small arboreal South-American species (not an Australian one (!), where most current marsupials, like kangaroos and koala, live).

This species, Monodelphis domestica, is a common lab animal,used in the investigation of human diseases, like cancer and neurological conditions.

The research completes a greater investigation comparing the genetics of different vertebrate groups, to see how they differ between them and from humans.

"The idea is to obtain genome sequence information from organisms that are appropriately spaced relative to us on the tree of life," explained Adam Felsenfeld, from the US National Human Genome Research Institute.

"For example, by lining up the sequences it is possible to detect regions of the genome that have not changed, so are conserved and perhaps important; or, alternatively, regions that are changing very rapidly."

Monodelphis is an easy breeder, delivering large litters and having no pouch.

"This marsupial is a bit different in that the young are not kept in a pouch; they just dangle off the teats," observed Jenny Graves from the Australian National University.

Like in all marsupials, newborns are something more than a mouth and gut and crawl immediately to their mother's teats. They do not possess yet an immune system but can survive in an open, "dirty" environment, an aspect that researchers are interested in decoding in the animal's genes.

Newborns can also regrow totally severed spinal cords.

"The genome hasn't told us exactly how they do that but the genome provides us with a blueprint for further study and for being able to apply some of that knowledge to humans," commented Chris Gunter, a senior biology editor at Nature.

The opossum was found to possess 18,000 to 20,000 protein-coding genes, less than in placental mammals, but quite a similar number. The main difference is in the DNA sequences controlling the genes: when and where they turn on and off.

"20 % of all the regulatory instructions in the human genome were invented after we parted ways with the marsupial.

Evolution is tinkering much more with the controls than with the genes themselves." explained Eric Lander, the director of the Broad Institute.