The platypus, an odd animal with a mole's fur, a duck's bill and venomous spurs on its rear legs, contains genes that are mammalian, avian and reptilian, said an international team of scientists that mapped its genetic code.
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. And there are five times as many sex-determining chromosomes as normal.
"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 effort, described online Wednesday in the journal Nature.
Yet, 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 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.
Platypuses live on a relative sliver of Earth along Australia's east coast, Tasmania and Papua New Guinea. 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.
One of the more surprising elements was the animal's system for sex determination. Most mammals have two sex chromosomes, either two X chromosomes (to make a female) or an X and a Y (to make a male). Not only do platypuses have 10 instead of two, but some of those resemble the Z and W chromosomes of birds more than standard-issue Xs and Ys.
The platypus is the only mammal to make venom, and the chemicals in it are almost identical to those in some snake venoms. Yet the new analysis shows plainly that the two classes of animals came up with the innovation independently and by different evolutionary routes, though both built their poisons from the same starter molecule, an immune system chemical.