Mammalian Mash-up: Platypus genome shows three animals in one
An international team of scientists has sequenced the genome of a duck-billed platypus. Part bird, part reptile and part mammal, the platypus genome sheds light on the evolution of mammals, including humans, and on the genetics of disease. Led by Richard K. Wilson, director of the Genome Sequencing Center at Washington University School of Medicine, the scientific team found that this mammal's DNA is as unusual as its duck-bill.
An animal that lays eggs like a bird, makes milk like a mammal and shoots venom like a reptile might sound like the stuff of Saturday morning cartoons. But the duck-billed platypus is as real as it is strange. So strange, in fact, that the first British scientists to receive specimens from Australia thought the platypus was a hoax, believing a duck's bill had been sewn onto a mole. Now, centuries later, scientists see the platypus and its genome as a key piece of the evolutionary puzzle. It is a living transitional fossil, giving clues about the evolution of reptiles, birds and mammals.
Producing milk, the platypus is classified as a mammal, even though it lacks nipples and lays eggs like a bird or reptile. The male platypus produces venom like a reptile but shoots it through spurs on its hind legs, a unique method of venom-delivery. Also unique to the platypus, its duck-like bill contains an electro-sensory system for underwater navigation -- an important trait as it swims, searching for food, with its eyes, ears and nostrils shut.
Until the genome's publication in the May 8 issue of "Nature," the platypus' odd appearance was the main evidence of its close relation to birds and reptiles. Now the DNA sequence of Glennie, a female platypus from New South Wales, Australia, gives an in-depth look at this relationship and a few surprises as well.
About 166 million years ago, primitive mammals branched into two main groups: therian and prototherian mammals. The therian mammals branched again and eventually led to both marsupials, such as the kangaroo whose young develop in a pouch and to placental mammals, such as humans. Over millions of years, marsupials and placental mammals lost their reptilian features.
The prototherian mammals did not branch further, keeping many older traits like egg-laying and evolving into today's monotremes. Only two monotremes remain -- the spiny anteater and, of course, the platypus. The platypus, therefore, is a rare window to ancient mammals, retaining many traits from its reptilian ancestors. This branching timeline along with the platypus genome sequence can help scientists date the appearance of mammal-specific genes.
Shifting Evolution's Timeline and Other Oddities
In fact, the newly mapped platypus genome has already shuffled the timeline for the first appearance of the mammalian X and Y chromosomes. Humans and most other mammals have two sex chromosomes: XX for female and XY for male. The platypus, bizarrely, has 10: 5 pairs of Xs for female and 5Xs and 5Ys for male. Stranger still, this study found that the X chromosome of the platypus looks more like the Z chromosome of a bird than the X chromosome of a mammal. Therefore, the X and Y sex chromosomes must have evolved after monotremes split from the other mammal lineages. Contradicting previous hypotheses, these scientists claim the familiar X and Y chromosomes of mammals appeared 145 million years later than previously thought.
In addition to shifting the timeline of sex chromosome evolution, this study reveals other evolutionary quirks. Wes Warren, genetics professor at Washington University in St. Louis and a lead investigator of this project, called the development of platypus venom "a beautiful story of convergent evolution."
Convergent evolution occurs when two unrelated animals evolve similar traits. In this case, the scientists found evidence that platypus venom is not a remnant from reptiles. Rather, the platypus and reptile venom systems evolved in parallel -- from changes in the same set of immune genes but independent of one another. In snakes, for example, venom glands developed from salivary glands. The platypus, in contrast, produces venom from modified sweat glands.
Another surprise came in examining odor receptor genes, similar to those found in dogs or mice. Scientists did not expect the platypus to have a good sense of smell since it spends much of its life underwater, using electro-location to search for food.
"Spending 80 to 90 percent of its life underwater, we thought it wouldn't have many odorant receptors," said Warren, "but we saw the most abundant repertoire yet of any mammal, 50 percent more than a mouse, which is dependent on them."
In this trait, the platypus is more like a mammal than a bird or reptile, which have few odor receptors. Scientists speculate the animal may use its olfactory receptors to communicate using pheromones or to detect water-soluble smells.
Beyond revealing secrets about evolution, the platypus genome is a tool to investigate the origins and treatments of disease. Scientists hope the newly mapped genome will help understand the mammalian immune, nervous and reproductive systems. "The end game for studying animal genomes," said Warren, "is to better understand human biology."
Understanding the immune systems of primitive mammals could give clues to why humans are immune to one infectious agent but not another. Furthermore, reptile venom is already used to design therapeutic drugs and platypus venom could lead to new ones.
The platypus' primitive reproductive system could help explain sexual disorders in humans. A related paper in "Genome Research" already uses the platypus genome to investigate the evolution of testicular descent in mammals. Platypus testes are located inside the body, unlike most mammals. Understanding the genes governing testicular descent could help those premature baby boys whose testicles fail to descend properly.
The sequencing of the platypus genome is a milestone in the understanding of mammalian and human evolution. "As we learn more about things like platypuses," Wilson told the Washington Post, "we also learn more about ourselves and where we came from and how we work."
Julia Evangelou Strait is a freelance science writer based in St. Louis. She has a master's degree in biomedical engineering and works in hospital epidemiology for BJC HealthCare.