May 12, 2008

Duck-Billed Platypus Genome Sequence Published

Photo of a platypus

The first analysis of the genome sequence of the duck-billed platypus, whose ancestors split from the rest of mammalian lineage some 166 million years ago, revealed clues about how genomes were organized during the early evolution of mammals.

The duck-billed platypus, native to Australia, is one of the few mammals that lays eggs. It has other peculiarities, too—for example, it boasts what looks like a duck's bill and houses an electrosensory system used when foraging for food underwater. Males also have venomous hind leg spurs.

"At first glance, the platypus appears as if it was the result of an evolutionary accident," said Dr. Francis S. Collins, director of NIH's National Human Genome Research Institute (NHGRI). "But as weird as this animal looks, its genome sequence is priceless for understanding how fundamental mammalian biological processes have evolved."

In a paper published in the May 8, 2008, issue of the journal Nature, an international team of researchers supported in part by NHGRI analyzed a high-quality draft genome sequence of Glennie, a female platypus from Australia. The sequencing was led by the Genome Sequencing Center at Washington University School of Medicine in St. Louis, part of NHGRI's Large-Scale Sequencing Research Network. The researchers then compared the genome with the well-characterized genomes of the human, mouse, dog, opossum and chicken, as well as the draft genome sequence of the green anole lizard.

The team found that the platypus genome contains about the same number of protein-coding genes as other mammals—approximately 18,500. The platypus shares more than 80% of its genes with other mammals whose genomes have been sequenced.

Like other mammals, the platypus genome contains a tightly clustered set of genes that produce the casein proteins that make up milk. However, the platypus genome harbors both reptilian and mammalian genes associated with the fertilization of eggs—particularly interestingly, since the female platypus lays eggs.

The researchers found several other unique characteristics to this odd mammal's genome. For example, the platypus has about half as many odor receptor genes as mice and other mammals, but more genes that code for a particular type of odor receptor called a vomeronasal receptor. Researchers think that this particular set of genes may be involved in detecting odors during underwater foraging.

The genes related to the immune system of the platypus are very similar to those of other mammals. However, the platypus genome has a few key differences that the researchers believe may help boost the immune systems of their offspring, which hatch from the egg at a less developmentally mature stage than other mammals when they are born.

The platypus genome sequence is now freely available online for other researchers to use. “This genome provides a unique perspective on what the genomes of our earliest mammalian ancestors may have looked like,” said Dr. Adam Felsenfeld, who heads NHGRI's Comparative and Sequencing Analysis Program. “It is fascinating that what we think of as being reptile-like and mammal-like features can co-exist in the same genome.”

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