THEY may well have reached Europe from their native Asia in 1727 after swimming up the Volga river to escape an earthquake. Now, two centuries after being tamed, bred and adopted as one of scientists鈥� favourite laboratory animals, the brown Norway rat has become the latest mammal to have its genetic plan read.
The Rat Genome Sequencing Project Consortium, which consists of 20 institutions in six different countries led by the Baylor College of Medicine Human Genome Sequencing Center in Houston, Texas, this week published a detailed comparison of the rat, human and mouse genomes (Nature, vol 428, p 493). This has yielded tantalising insights into how each evolved.
鈥淩odent evolution is an order of magnitude faster than in humans,鈥� says project head Richard Gibbs. The genetic regions evolving fastest are those associated with the rat鈥檚 acute sense of smell and use of scents to detect danger, mark territory and choose mates. Rats have an estimated 2070 smell-receptor genes, about a third more than mice.
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As well as having more 鈥渟mell鈥� genes than both us and mice, they have also evolved more genes that help them detoxify chemicals. By disabling these, it may be possible to genetically engineer rats to make their detox machinery more like ours, improving the accuracy of toxicology and drug safety testing.
Mistakenly classified in 1769 as an 茅migr茅 from Norway rather than its native Asia, Rattus norvegicus turns out to have 2.75 billion pairs of DNA bases in its genome, slightly fewer than our 2.9 billion and slightly more than the mouse鈥檚 2.6 billion. Humans, rats and mice all have about the same number of genes.
The analysis also reveals that humans and rodents split from a common ancestor about 80 million years ago, with rats and mice diverging 12 to 24 million years ago. Even today, however, 280 large portions of the chromosomes of humans, rats and mice are virtually identical, suggesting that these regions are indispensable. These 1 billion or so bases form what the sequencers call an 鈥渁ncestral core鈥�, containing about 95 per cent of all the known genes in these three species.
The draft sequence, which covers 90 per cent of the rat genome, should accelerate research into inherited disorders in people. 鈥淭he sequencing of the rat genome constitutes another major milestone in our effort to expand our knowledge of the human genome,鈥� says Francis Collins, director of the National Human Genome Research Institute in Bethesda, Maryland.
Researchers will soon have many more mammal genomes to compare with ours. Last year, a draft version of our closest relative, the chimpanzee, was unveiled, along with a very rough draft of the poodle genome. In a year or so, drafts are due of the cow and the rhesus macaque monkey, as well as a high-quality sequence of the female boxer dog.