Platypus Looks Strange on the Inside, Too

From the N.Y. Times:

May 8, 2008
Platypus Looks Strange on the Inside, Too
By JOHN NOBLE WILFORD

If it has a bill and webbed feet like a duck, lays eggs like a bird or a reptile but also produces milk and has a coat of fur like a mammal, what could the genetics of the duck-billed platypus possibly be like? Well, just as peculiar: an amalgam of genes reflecting significant branching and transitions in evolution.



An international scientific team, which announced the first decoding of the platypus genome on Wednesday, said the findings provided “many clues to the function and evolution of all mammalian genomes,” including that of humans, and should “inspire rapid advances in other investigations of mammalian biology and evolution.”

The research is described in Thursday’s issue of the journal Nature by a group of almost 100 scientists led by Wesley C. Warren, a geneticist at Washington University School of Medicine in St. Louis. The single subject of the study was a female platypus named Glennie, a resident of Glenrock Station in New South Wales, Australia, whose DNA was collected and analyzed.

The platypus, native to Australia, is so odd that when the first specimens were sent to Europe in the 19th century, scientists suspected a hoax. It was classified as a mammal, one of only two monotremes (echidna is the other) living today that are offshoots of the main mammalian lineage. The divergence occurred some 166 million years ago from primitive ancestors combining features of both mammals and reptiles.

“What is unique about the platypus is that it has retained a large overlap between two very different classifications, while later mammals lost the features of reptiles,” Dr. Warren said in an interview.

In their investigation of the platypus genetic blueprint, the scientists found that its genome contains about 18,500 genes, similar to other vertebrates and about two-thirds the size of the human genome. The platypus shares 82 percent of its genes with the human, mouse, dog, opossum and chicken. Some repeated elements in the genome, the scientists noted, hold hints as to the chronology of changes in the platypus.

Of particular interest, the researchers reported, the analysis identified families of genes that link the platypus to reptiles (like those for egg-laying, vision and venom production), as well as to mammals (antibacterial proteins and lactation). The platypus lacks nipples; the young nurse through the abdominal skin.

One surprise was finding genes responsible for sensitive odor receptors. As a primarily aquatic animal, the platypus was already known to rely on electrosensory receptors in its bill to detect faint electric fields emitted by underwater prey. So why the considerable ability to sense odors? The scientists speculate that it may involve sexual communication or the use of water-soluble odorants in navigating and hunting underwater.

Richard K. Wilson, director of the Genome Sequencing Center at Washington University, said that the comparison of the platypus genes with those of other mammals was the beginning of an examination of how “genes have been conserved throughout evolution.”

The project, involving scientists from eight countries, was primarily financed by the National Human Genome Research Institute in the United States. Its director, Francis S. Collins, said, “As weird as this animal looks, its genome sequence is priceless for understanding how mammalian biological processes evolved.”