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Platypus unlocks evolution's secrets.

By any account, the platypus is an odd creature. It has a broad, rubbery bill that brings to mind a duck, but it swims mere like a beaver; yet, it lays eggs and can inject poisonous venom like a reptile. No wonder it is considered an elaborate hoax by scientists who examined the first specimen pelt shipped to England from the colony of New South Wales in 1799.

A consortium of scientists, including Cold Spring Harbor (N.Y.) Laboratory's Gregory J. Harmon, published results of an international effort to sequence the platypus genome. It reveals evolutionary secrets that go far beyond the obvious fact that the creature, found exclusively in eastern and southern Australia--including on the island-state of Tasmania--neither is a typical mammal nor reptile. In fact, the platypus is a member of a mammalian species called the menotremes, which includes only four other subspecies (those being echidnas, varieties of spiny anteaters). The monotremes diverged from other primitive mammals about 166,000,000 years ago, in the late Jurassic period--a fact corroborated indirectly in the newly sequenced genome.


The platypus has a distinct mode of rearing its offspring. All mammals, in addition to being warm-blooded, can be grouped according to their modes of gestation. Marsupials like the kangaroo rear profoundly immature young in external pouches for extended periods of time. Eutherian mammals like mice and humans protect their progeny for lengthy periods in an inner womb prior to giving birth.

The platypus and other menotremes, in sharp contrast, retain the reptilian mode of gestation, being egg-layers; yet, like other mammals, the female platypus feeds her newborn with milk, albeit secreted through broad patches of skin rather than teats.

"The fact that monotremes lactate yet lay eggs is only one instance of the fascinating combination they represent of reptilian and mammalian characteristics," notes Hannon. Another legacy of the platypus' reptilian heritage is the fact that males of the species are equipped with spikes on their hind legs through which they can deliver venom. Reptile and platypus venom proteins "have been coopted from the same gene families." The scientists also deduced from genomic data that the platypus, despite being an egg-layer, shares milk protein genes with all other mammals.

As a genome scientist, Hannon was interested to observe that the complement of chromosomes possessed by the platypus--its karyotype--is most unusual for a mammal. "In all, there are 52 chromosomes, many of them very small, much like reptilian microchromosomes. Like reptiles, they also have multiple sex chromosomes," he reveals.
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Title Annotation:Genome Sequencing
Publication:USA Today (Magazine)
Geographic Code:1USA
Date:Jun 1, 2009
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