Scientists Unravel Platypus DNA

Published: May 8, 2008

WASHINGTON - 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 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."

Now, more than 200 years later, a team of scientists has determined the platypus's entire genetic code.

Right down to its DNA, it turns out, the platypus continues to strain credulity, bearing genetic modules that are in turn mammalian, reptilian and avian.

There are genes for egg laying - evidence of the animals' 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.

There are also five times more sex-determining chromosomes than scientists know what to do with.

"It's such a wacky organism," said Richard Wilson, director of the genome center at Washington University in St. Louis, who led the two-year international effort, described online Wednesday 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 how early mammals learned to nurse their young; how they matched poisonous snakes at their own 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 live on a relative sliver of Earth along Australia's east coast, Tasmania and Papua New Guinea. But Ornithorhynchus anatinus has a global fan base, it seems, having been chosen as the mascot of 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 been sequenced.