Modern mammals, including humans, evolved not so much by inventing new genes, but by redeploying existing ones - with a surprising assist from genetic elements usually derided as mere junk.
Genome biologists had speculated for several years that this was true, but now they know for sure, thanks to the grey, short-tailed opossum (Monodelphis domestica) - the latest in a growing list of organisms to have its genome completely sequenced.
The opossum genome, sequenced by an international consortium, is the first complete genome for a marsupial, which split from the placental mammal lineage about 180 million years ago. This makes it an ideal tool for understanding the changes that placentals have undergone in their evolution.
Staying in control
From the genome sequences of placental mammals such as humans, mice and chimpanzees, the researchers identified a set of sequences that are relatively unchanged (conserved) in all placental mammals and are therefore likely to be of some functional significance.
About one-third of these sequences lay within genes, and the rest were in noncoding regions that presumably help regulate the activity of genes.
The team then analysed the opossum genome to see if any of these sequences were unique to placentals - representing evolutionary innovations that arose after the lineages diverged.
Just 1.1% of the conserved sequences within genes were unique to placentals, they found. In contrast, a massive 20.5% of the noncoding sequences were unique to placental mammals. "Evolution is tinkering much more with the controls than with the genes themselves," says Eric Lander of the Broad Institute in Cambridge, Massachusetts, who was one of the leaders of the consortium.
About one in six of these evolutionary innovations corresponds to transposable elements, or "jumping genes" - semiautonomous bits of "selfish" DNA that can copy and reinsert themselves at new places in the genome. The new stretches of genome start out as useless junk, but can later be fashioned through evolution into new on-off switches for genes.
Ironically, the first marsupial genome reveals little about the biology or evolution of marsupials themselves, apart from a few surprises such as an unexpectedly robust set of immune-system genes. This is unexpected since marsupials are born without a functioning immune system - they develop immunological tissues, organs, and the ability to produce antibodies outside the shelter of the mother's body.
To really understand the evolutionary innovations that make marsupials what they are, researchers will need more marsupial genomes. The next one, that of the Tammar wallaby (Macropus eugenii), is expected to be completed by the end of 2007, says Jennifer Graves of the Centre for Kangaroo Genomics at the Australian National University in Canberra.
Journal references: Nature (vol 447, p 167), and Genome Research (DOI: 10.1101/gr.6121807)
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