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Researchers Say Opossum Genome Opens Window on Human Evolution

NEW YORK (GenomeWeb News) - A team of researchers led by the Broad Institute has published the genome sequence of the short-tailed opossum — the first marsupial genome — in today's issue of Nature.
According to the researchers, the “unique” evolutionary position of the opossum (Monodelphis domestica) could shed light not only on mammalian evolution, but on the mechanisms of evolution in general.
The National Human Genome Research Institute, which funded the project, said the opossum sequencing project began in 2003 and cost around $25 million.
The researchers compared the opossum sequence data to that of non-marsupial genomes, including the human genome. Because the marsupial family, or metatherians, separated from the ancestors of placental mammals, the eutherians, 180 million years ago, the data may be used to highlight genetic divergences that occurred since that time. 
The researchers believe that around one-fifth of the key functional elements in the human genome arose during the 180 million years since the eutherians and metatherians diverged.
NHGRI director Francis Collins said in a statement that the opossum genome "occupies a unique position in the tree of life." He noted that the sequence data should fill “a crucial gap in our understanding of how mammalian genomes, including our own, have evolved over millions of years.”
The researchers found the M. domestica genome has an unusual structure, in that it has only nine pairs of chromosomes. But at 3.4 billion base pairs, compared with 3 billion in humans, it is surprisingly long.
By comparing the opossum and human genomes, researchers found penetrating new insights into evolution and the immune system.
For example, the researchers said that about 95 percent of recent genetic innovation lies in regions of the genome that do not contain genes, which had until recently been referred to as junk DNA.
Tarjei Mikkelson of the Broad Institute said that many of the new DNA instructions come from transposons, or “jumping genes,” that shuttle between chromosomes. Mikkelson said it is clear now that as these transposons travel "they are disseminating crucial genetic innovations around the genome." 
The opossum also was found to have many genes involved in immunity, which NHGRI said "challenged the notion that marsupials possess only primitive immune systems.”
The researchers write in Nature that M. domestica is used as a model organism for studying immunogenetics, neurobiology, neoplasia, and developmental biology.
In addition, the opossum is the only known mammal besides humans "in which ultraviolet radiation is a complete carcinogen for malignant melanoma," the researchers said.
The report concludes with a tantalizing notion: Sequencing the genomes of other American metatherians and those in Australia, which includes kangaroos, koalas, and others, "would allow the reconstruction of the genome of their common ancestor," which might bring researchers one step closer to envisioning the common mammalian ancestor as well.

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