To understand how humans and other mammals evolved to be so different from reptiles, scientists needed to study the genes of an intermediate species, and the platypus filled the bill.
Though it’s a true mammal with fur, milk and sweat, the waddling duck-billed platypus also retains reptilian features, like venom production and egg-laying. The catalog of its genes, or genome, published in the May 8 issue of Nature reflects those traits. It is the first venomous vertebrate genome cataloged.
“It’s retained many genes that other mammals have lost from a time when all mammals looked much like lizards,” says Chris Ponting, a geneticist on the project at the University of Oxford in England. The platypus has also gained and altered genes in ways that highlight how molecular evolution happens, the new report shows
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Take the venom genes — common in reptiles, but not in mammals. Spurs on platypus males’ hind legs inject venom like snake fangs do — venom that evolved as snake venom did, from tweaks in extra copies of the same types of ordinary nontoxic genes, the genome analysis shows.
Platypus venom genes, the study shows, are similar to reptilian venom genes but aren’t exactly the same. The genes are like skinny, blond, spoiled party-girls who grew up on different continents. Although they look alike, the genes became toxic independently. “Venom doesn’t come out of thin air,” explains Bryan Fry, a venom biologist at the University of Melbourne in Australia who was not involved with the project. “Knowing the genome will shed light on what types of proteins tend to be recruited for use as a toxin,” he says.
Other insights from the platypus genome relate to the evolution of milk-producing mammals from egg-laying ancestors. Platypuses are in an intermediate position — they lay eggs but the eggs are rather yolkless. They retain just one of the three yolk-related genes found in the chicken genome. Other mammals have none. Their infants hatch early and feed on milk rather than yolk. Yet for the platypus, nursing isn’t quite what it is in other mammals. For one, the platypus doesn’t have nipples and babies feed through pores in the skin. In other mammals with the whole milk-feeding system in place, yolk-making genes are obsolete.
The platypus genome offers a clue to how genes for milk production began to appear. Commonly in evolution, a new gene appears when an old gene is duplicated in a species’ DNA, allowing the extra copy to take on a new function. Milk genes appear to have arisen by such duplication. As in the dog, mouse and human genomes, genes for the milk protein beta-casein in the platypus are located beside tooth enamel genes. That position suggests that just before the time of the platypus’ origin, about 166 million years ago, a duplication in tooth enamel genes led to the origin of mother’s milk.
“These gene loss and gene gain stories fascinate us,” says Wes Warren of the GenomeSequencingCenter at WashingtonUniversity in St. Louis, lead geneticist on the project. “That is at the core in trying to understand what makes us mammals.”