The genome of the platypus—our most distant mammal relative—has been decoded and analyzed, researchers reported today.
Decoding the platypus genome has long been an important goal for biologists seeking to understand the origins of mammal evolution.
The study, appearing in today's edition of the journal Nature, gives scientists a new window into the genetic architecture of the earliest mammals.
"The platypus genome, like the animal itself, is an amazing amalgam of reptile-like and mammal-like features," said project co-leader Jennifer Graves, of the Australian National University in Canberra.
The analysis confirms that the platypus was the earliest offshoot of the mammalian family tree, Graves noted.
The group of animals called monotremes—which includes the platypus and the closely related echidna—is thought to have split from other mammals at least 166 million years ago.
That early divergence means platypus genes carry information from a transitional point on the evolutionary time line leading from reptiles to mammals, said project leader Wesley Warren of Washington University in St. Louis, Missouri.
"The platypus is critical to helping us understand what genes were present in the ancestral reptilian lineage and how mammals evolved their particular traits," Warren said.
Eggs and Milk
Aquatic animals native to eastern Australia, platypuses have long perplexed biologists.
Although classified as mammals, they retain a number of primitive characteristics—including egg-laying—that are thought to have been passed down from mammal-like reptiles that lived over 300 million years ago.
(Read related story: "Platypus Much Older Than Thought, Lived with Dinos" [January 22, 2008].)
As part of the new study, researchers compared the platypus's genetic makeup with other fully sequenced genomes, including those of chickens, mice, and humans.
A central goal was to determine which platypus features may have been inherited from ancient reptilian ancestors and which evolved independently in the monotreme lineage.
As expected, platypus DNA was found to include a number of genes not found in other mammals. For example, the researchers identified genes for egg yolk proteins shared only with reptiles and fish.
"The new genomic data make a water-tight case for [platypus] egg-laying truly being a primitive retention from reptilian ancestors," said Matt Phillips, an Australian National University scientist who was not involved in the genome project.
On the other hand, the set of mammalian genes responsible for lactation—or milk production—was also found in the platypus genome.
While milk production and giving birth to live young would seem to go hand in hand, the platypus genome shows that the two common mammalian traits evolved at very different points in evolutionary time.
"The presence of the full repertoire of milk genes confirms that lactation evolved at least 166 million years ago, way before live-bearing," Graves said.
The genomic analysis also uncovered new information about the platypus's basic biology and the origin of some of its particularly unusual features.
One big surprise was the discovery of a number of specialized genes associated with odor detection, said lead author Warren.
The finding suggests that, in addition to electroreceptors in their bills, platypuses also rely on chemical cues when navigating and locating prey underwater.
Scientists also identified a set of genes associated with platypus venom production.
Male platypuses produce a pain-inducing, snake-like venom, composed of at least 19 different substances, which is delivered to enemies or rival males through spurs on the males' hind legs.
Previous studies of venomous snakes had found that poison production is the result of evolutionary modifications to genes that once served other functions.
Some of the same genes appear to have modified in the platypus, but in a process entirely separate from the evolution of venom in snakes, according to the new study.
Elliott Margulies, a scientist at the National Human Genome Research Institute in Bethesda, Maryland, said the significance of the platypus genome extends well beyond the field of early animal evolution.
"In the human genome, we're always trying to find sequences that are functional and doing something important, but it's a very tedious and difficult process," Margulies said.
Comparing the human and platypus genomes, he said, will enable researchers to locate functional DNA sequences more easily.
"Because the platypus has evolved independently for such a long period of time, any sequences it still has in common with humans are probably important," Margulies said.