Scientists have sequenced the genetic code of one of nature's strangest animals.
Piecing together the platypus puzzle. I'm Bob Hirshon and this is Science Update.
The duck-billed platypus looks like it's been stitched together from spare parts, which has baffled scientists for centuries. Now, they've finally cracked its genetic code, which could help them understand other mammals too.
Geneticist Wes Warren of the Washington University School of Medicine explains that the platypus is one of only five surviving species of ancient egg-laying mammals called monotremes. They were the first mammals to branch off from their reptile-like ancestors.
And we really needed to fill that hole in our phylogenetic tree to better understand the emergence of mammal-specific traits.
Those include lactation, sweat glands, and a sophisticated brain area called the neocortex. By studying primitive versions of those genes in the platypus, researchers hope to uncover new insights into gene function in all mammals. I'm Bob Hirshon for AAAS, the science society.
Making Sense of the Research
Even though it lives only in Australia, most people learn about the platypus from an early age, because it's such a funny-looking animal. It also makes a good trivia question, since it's a mammal with a bill like a duck that lays eggs. Like all mammals, though, the platypus has hair and sweat glands, is “warm-blooded” (in other words, it regulates its body temperature internally), and produces milk to feed its young. It also has two lesser-known but key mammalian traits: a single lower jawbone and three small bones in its inner ear.
The platypus belongs to a small sub-group, or order, of mammals called monotremes; all four other monotremes are species of the spiny anteater, or echidna. Monotremes are the oldest surviving order of mammals. While other mammals, including marsupials like kangaroos and placental mammals like humans, give birth to live young, monotremes lay eggs, like birds and reptiles. Their name comes from the Greek for “single opening,” because monotremes have one body opening called a cloaca that's used for reproduction, urination, and defecation. Other mammals have a separate opening for at least one of these functions, if not all three, while most birds, reptiles, and amphibians have a similar all-purpose cloaca. The platypus is also one of only a few venomous mammals; its hind legs have sharp poisonous spurs on them.
In short, the platypus wears its evolutionary history on its sleeve: it's a mammal in all the most important ways, but it shares more characteristics with birds and reptiles than other mammals do. What's more, some of its mammalian traits are less sophisticated than those of marsupials and placental mammals. That's because the monotremes split off from our common mammalian ancestor much earlier than other groups did. Understanding the platypus genome could help scientists fill in the genetic timeline that led to modern humans.
So far, the researchers have compared the platypus genome to that of the human, mouse, dog, opossum, and chicken, looking for particular genes that are linked to both mammalian and non-mammalian traits. They're also looking more closely at the genes to understand how they differ from those of other mammals, and why they produce more primitive mammalian traits: for example, although monotremes make milk for their young, they have no nipples. Instead, the milk simply leaks onto the mother's belly, and the babies lick it off. Breaking down mammalian traits like milk production into evolutionary steps may ultimately help scientists understand how all mammals, including people, function.
Now try and answer these questions:
- What are monotremes? What makes them mammals? How are they different from other mammals?
- Why might the platypus share more genes with reptiles and birds than other mammals do?
- Why is the platypus an important animal to understand genetically? Why sequence the genomes of animals at all?
You may want to check out the May 31, 2008, Science Update Podcast to hear further information about this Science Update and the other programs for that week. This podcast's topics include: how Rhesus monkey social relationships reveal a link between stress and overeating, what the platypus genome can tell us about being a mammal, how an antidepressant restores vision in rats, negligent mother mice that have abnormal brain chemistry, and fruit flies uncovering secrets of aging and pain.
National Geographic's Platypus Profile page offers resources related to the platypus, including an animal profile, fast facts, a map depicting the animal's range and a printable factsheet. Links to additional animal features and photos are provided.
In the National Geographic News article Scientists Recreate Genome of Ancient Human Ancestor, read about the reconstruction of part of the genetic code of a shrew-like species thought to have been the most recent common ancestor of most placental mammals, including humans. Scientists say their goal was to better understand human biology and evolution.