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Last Updated: Wednesday, 9 May 2007, 18:00 GMT 19:00 UK
Tiny marsupial records DNA first
Opossum (Paul Samollow, SFBR, San Antonio)
The small creature is normally found in the trees of South America
The grey, short-tailed opossum has become the first marsupial to have its DNA read through by scientists.

The creature is commonly used in labs as an experimental "model" in which to study the causes of human disease, such as cancer and neurological problems.

Knowing its genome will boost those efforts, and give new insight into the different evolutionary paths taken by marsupial and placental mammals.

Details of the work appear in the journals Nature and Genome Research.

The study is part of a grand scheme to compare the biochemistry of a range of animals with that of Homo sapiens, to understand better how the human body is built and maintained, and how all species differ from one another.

"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."

'Dirty' trick

In the wild, the opossum Monodelphis domestica is found in the trees of South America.

It was chosen as a subject to have its full complement of DNA (its genome) decoded because of its current importance to science.

Its small size, ease of breeding and large litters have made the creature the predominant marsupial for laboratory study worldwide.

The double-stranded DNA molecule is held together by four chemical components scientists refer to as bases
Adenine (A) bonds with thymine (T); cytosine(C) bonds with guanine (G)
Groupings of these "letters" form the "code of life"; there are about 3.4 billion base-pairs in the opossum genome
Written in this DNA sequence are about 18-20,000 genes
Genes are starting templates used by cells to make proteins; the sophisticated molecules that build and maintain the body
Marsupial mammals (kangaroos, wallabies, etc) are closely related to the placental mammals (humans, mice, etc) - but not that close. They last shared a common ancestor about 180 million years ago and have arrived at very different reproductive solutions - with marsupials rearing their young externally, sometimes in a 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.

This open arrangement makes the study of early developmental processes much simpler, and should give scientists clues as to how they perform a number of very clever biological tricks.

For example, when newborns crawl to their mother's teats, they are little more than a mouth and gut. They have no functioning immune system but are still able to survive in an open, "dirty" environment. Looking at the genes should help scientists work out how this is possible.

Research has also shown that newborns can regrow their spinal cords, even if they are completely severed.

"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.

New jumps

The decoding work, led by the Broad Institute of MIT and Harvard in Cambridge, Massachusetts, reveals the opossum to have between 18,000 and 20,000 protein-coding genes.

The number is a little short of what is seen in a placental mammal like a human - but broadly similar.

The big difference, scientists say, is in the DNA sequences that are responsible for regulation of the genes - controlling when and where in cell processes they become active.

"Twenty percent of all the regulatory instructions in the human genome were invented after we parted ways with the marsupial," explained Eric Lander, the director of the Broad Institute.

"Evolution is tinkering much more with the controls than with the genes themselves."

Many of these innovative DNA instructions were derived from so-called "jumping genes", or transposons - small pieces of genetic code that hop around the genome and were previously thought to have no function.

"Transposons have a restless lifestyle, often shuttling themselves from one chromosome to another," said Broad researcher Tarjei Mikkelsen.

"It is now clear that in their travels, they are disseminating crucial genetic innovations around the genome."

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