With its furry body and duck-billed face, the platypus is nothing if not bizarre looking. But scientists have confirmed that its strangeness is more than skin deep — after unraveling the animal's genome, they have discovered that even its genes are odd.
Most of the platypus's genes control characteristics typical of mammals, such as genes for making milk and fur. Unlike other mammals, however, the platypus lays eggs instead of birthing live young. Geneticist Richard Wilson of Washington University in St. Louis and his colleagues report in this week's issue of Nature that they have identified platypus genes that make hard-shelled eggs — much like the genes of birds.
And that's not the only quirk.
In addition to bird- and mammal-like genes, the platypus also has genes that trigger features associated with reptiles. Male platypuses produce a potent venom delivered through a spur on their hind legs.
The creatures also have very unusual sex chromosomes, Wilson notes. "A male platypus has five X chromosomes, no two alike, and five Y chromosomes," he says.
Scientists are eager to study its genes because the platypus is one of the most ancient mammals, appearing on Earth more than 150 million years ago.
"If we want to try to understand why nervous systems developed the way that they did or immune systems developed the way that they did, we can learn a tremendous amount by studying these lower mammals," says Wilson. "The genomes are the blueprints."
When the first platypus specimens were brought to England from Australia in the 18th century, Wilson says, "most of the naturalists who looked at them thought they were some kind of fake." People suspected that a taxidermist had gotten creative "and sewn the bill of a duck on some sort of beaver-like creature and added webbed feet for grins," he says.
Jerry Siegel, a neuroscientist at UCLA, says he became interested in the platypus because he believed it would help explain how sleep evolved in humans. One theory is that rapid eye movement (REM) sleep evolved recently in humans as our brains got bigger and more complex. It was initially thought that the platypus didn't have REM sleep cycles, so Siegel went to Australia with modern technology to do more testing.
"And what we saw is that in the platypus, the REM sleep is absolutely unequivocal," he says.
Siegel says he now believes that REM sleep may have evolved as part of a system for conserving energy in warm-blooded animals, like birds and mammals.
The platypus is just one of many species currently getting its genome probed. Wilson says this is partly because sequencing is becoming a bargain.
"It took a few million bucks to sequence the platypus genome with older technology, last year's technology, which is older," he says.
And there are plenty of gaps in the genetic record that scientists would like to fill.
"There's a couple hundred million years of evolution in between the platypus and the chicken," says Wilson. "So you'd like to sequence things in between."