From: Reuters
Published May 8, 2008 12:42 AM

Australian platypus genome a link to evolution

By Michael Perry

SYDNEY (Reuters) - Australia's unique duck-billed platypus -- an egg-laying, furry animal with web feet that spends most of its time underwater -- is in fact part bird, part reptile and part mammal according to its gene map.

A team of international scientists released the platypus genome on Thursday, saying its complex sequence would aid the study of human evolution -- particularly the development of the immune, nervous and reproductive systems.

"Its probably the most eagerly awaited genome since the chimp genome because platypuses are so weird," said Jenny Graves, head the Comparative Genomics Group at the Australian National University.

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"Comparing us with the platypus means that we can say something about our common ancestor, which was one of the earliest mammals, so that means that we can ask questions about what happened to make us mammals," said Graves, after a briefing on the platypus genome, published in the journal Nature.

The platypus represents the earliest offshoot of the mammalian lineage, some 166 million years ago, from primitive ancestors that had features of both mammals and reptiles.

When the platypus was first discovered, English scientists regarded it to be an Australian joke, thinking someone had stuck a duck's bill and feet onto an otter-like animal.

The platypus is classified as a mammal because it produces milk, suckles its young and is covered in fur, but it also lays eggs like a bird or reptile and males have poisonous spurs on their hind legs like a reptile.

CLUES TO HUMAN EVOLUTION

The Nature paper analyzed the genome of a female platypus named Glennie in Australia. Its genome contained roughly 18,500 genes, similar to other vertebrates.

The researchers found genes that support lactation, and egg laying, and genes responsible for venom production, which evolved from ancestral reptile genomes.

"You see genes that look reptile-like, genes that look bird-like and genes that look mammal-like. Its a pretty amazing picture," Rick Wilson, director of The Genome Center at Washington University in St Louis, said in an interview. Wilson directed the platypus genome report.

The platypus swims with its eyes, ears and nostrils closed, relying on electrosensory receptors in its bill to detect faint electric fields emitted by underwater prey.

The researchers found the platypus had genes that allowed the platypus to detect odors underwater. Similar genes are found in dogs and rodents that use smell to forage.

The scientists compared the platypus genome with human, mouse, dog, opossum and chicken genomes and found that the platypus shares 82 percent of its genes with these animals.

"It teaches us a lot about some of the biology that some of our earliest common ancestors might have had, in terms of immune systems and early nervous systems functions," said Wilson.

"If you really want to understand why humans are or are not resistant to particular types of infectious agents...you really have to understand rudimentary systems like birds, reptiles and monotremes like the platypus."

Scientists said platypus sex chromosomes may help study sex determination in mammals and infertility in humans. It has 10 sex chromosomes, 5 male and 5 female. Humans only have one of each.

Researchers at Stanford University's School of Medicine in California said the genome of the platypus, which unlike other mammals carries its testicles internally, allowed them to study two genes that move testes into the scrotum in most mammals.

They said understanding this process may explain why testes of about 30 percent of premature boys fail to descend properly.

(Editing by Valerie Lee)

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