· A May 8 Page One article incorrectly said that the platypus is the only mammal that makes venom. It is one of very few mammals that do so.
Platypus Genome Found Fittingly Strange
Thursday, May 8, 2008
When the British naturalist George Shaw received a weird specimen from Australia in 1799 -- one with a mole's fur, a duck's bill and serpentlike spurs on its rear legs -- he did what any skeptical scientist would do: He looked for the stitching and glue that would reveal it to be a hoax.
"It was impossible not to entertain some distant doubts as to the genuine nature of the animal," Shaw wrote of the seemingly built-by-committee creature, which he eventually named "platypus," for its flat, webbed feet.
Now, more than 200 years later, a team of scientists has determined the platypus's entire genetic code. And right down to its DNA, it turns out, the animal continues to strain credulity, bearing genetic modules that are in turn mammalian, reptilian and avian.
There are genes for egg laying -- evidence of its reptilian roots. Genes for making milk, which the platypus does in mammalian style despite not having nipples. Genes for making snake venom, which the animal stores in its legs. And there are five times as many sex-determining chromosomes as scientists know what to do with.
"It's such a wacky organism," said Richard Wilson, director of the Genome Sequencing Center at Washington University in St. Louis, who with colleague Wesley Warren led the two-year effort, described today in the journal Nature.
Yet in its wackiness, Wilson said, the platypus genome offers an unprecedented glimpse of how evolution made its first stabs at producing mammals. It tells the tale of how early mammals learned to nurse their young; how they matched poisonous snakes at their venomous game; and how they struggled to build a system of fertilization and gestation that would eventually, through relatives that took a different tack, give rise to the first humans.
"As we learn more about things like platypuses," Wilson said, "we also learn more about ourselves and where we came from and how we work."
Platypuses (preferred over "platypi" in U.S. dictionaries) live on a sliver of Earth along Australia's east coast, in Tasmania and in Papua New Guinea. They are not endangered, but few people see them since they spend their days in burrows built into stream banks. But Ornithorhynchus anatinus has a global fan base, it seems, serving as the mascot for countless companies, products and events.
The animal's complete genetic code, or genome, turns out to have 2.2 billion molecular "letters" of DNA, or about two-thirds as many as the human genome, and contains 18,500 genes, about the same as humans.
Finding the order of all those letters was grueling, scientists said, because no similar animal has ever been sequenced. The platypus inhabits an isolated branch on the evolutionary tree with just one cousin, the echidna, also of Australia. That left researchers with no model to help them figure out how the platypus's DNA fits together.
"It was quite a difficult thing," said Jennifer Marshall Graves of Australian National University in Canberra, who led part of the analysis after the St. Louis team derived the basic sequence.
"The genome was completely unknown, and we knew it was going to be fairly weird," Graves said. "You'd look at some of these repetitive sequences and think, 'What on Earth is that?' "