With a face only a mother could love—and a body to match—the duck-billed platypus is truly a freak of nature. As if designed by a committee, the animal's body comprises aspects of mammals, birds and reptiles. In fact, the Australian animal is so bizarre the British naturalists who first encountered it couldn't believe their eyes.
Indeed, to look at a platypus, it's easy to assume something went terribly wrong 166 million years ago, when birds and reptiles parted ways with primitive mammals. Although it's categorized as a mammal, the platypus doesn't fit neatly into any of the usual zoological classes. It lays eggs but also nurses the hatchlings; it has a duck's bill and webbed feet; males have spurs to deliver snake-like venom; and adults are toothless.
That nature stopped the clock on these ugly Betties of the Centaur genre—this animal was actually named Glennie—gives scientists a remarkable opportunity to trace the evolutionary origins of modern traits in several species, particularly mammals, with a single animal.
Genomic analysis published today in the journal Nature, shows platypus' 18,527 protein-encoding genes contain alive-and-well representatives from mammals, birds and reptiles.
For the most part, those species continued to adapt and specialize so that today they look nothing like each other and have developed unique genes. Biologists estimate some 9,500 different species of birds currently inhabit the Earth, along with nearly 9,000 species of reptiles and 4,000 species of mammals. Only three species of egg-laying mammals, known as monotremes, currently exist, including the platypus and its long- and short-nosed echidna cousins—small hedgehog-looking animals also known as spiny anteaters.
"What is unique about the platypus is that it has retained a large overlap between two very different classifications, while later mammals lost the features of reptiles," said Wes Warren, an assistant professor of genetics at Washington University in St. Louis, where much of the work was done.
"In other words," says Mark Batzer, a professor of biological sciences at Louisiana State University and member of the research team, the platypus sequence "provides the 'big picture' compared to the genomes of other animals more similar to humans that have a more focused window."
The scientists looked for genes associated with traits found in modern mammals—such as reproduction, lactation and immunity—and the chicken, which represents egg-laying animals, including extinct reptiles that passed on much of their DNA to the platypus and other mammals over the course of evolution.
"The genomic organization was strange and a little unexpected," said Batzer. "It appeared much more bird- and reptile-like than mammalian, even though it is indeed classified as a mammal. It's an ancient animal, too, and it has remained relatively primitive and unchanged, both in physical appearance and genetically."
The platypus shares more than 80 percent of its genes with other known mammalian genomes yet retains a number of genes found in reptiles. Genes associated with reptilian characteristics, such as venom production, appear to have arisen independently in the platypus and ancestral reptiles.
At roughly 2.2 billion base pairs, the platypus genome is about two-thirds the size of the human genome. The animal has 52 chromosomes, including 10 sex chromosomes—humans have two. The platypus X chromosome is similar to the sex chromosome of birds. Within those base pairs are arrangements of DNA letters the scientists hope will tell the story of not only when species differentiated throughout evolution, but how. For that, Batzer and his team looked for mobile DNA elements, small bits of DNA that copy themselves and then insert elsewhere in the genome. Unlike typical genes, mobile DNA elements do not encode for a proteins. About half of the platypus genome contains mobile elements similar to those found in mammals.
"Mobile elements were once thought to be so small that they had no function," said Batzer. "But, in reality, they cause insertions and deletions, which can lead to genetic diseases in humans as well as the creation of new genes and gene families in the genome."