Platypus genome holds key to mammalian evolution

May 8th, 2008 - 11:29 am ICT by admin  

Washington, May 8 (IANS) Scientists have decoded the genome of one of the most unusual creatures in existence - the duck-billed platypus. And now they know why it is part bird, part reptile and part mammal. The platypus represents the earliest offshoot of mammalian lineage - a branch-out that occurred 166 million years ago from primitive ancestors with both mammalian and reptilian features.

“At first glance, the platypus appears as if it was the result of an evolutionary accident,” said Francis S. Collins of the National Human Genome Research Institute.

“But as weird as this animal looks, its genome sequence is priceless for understanding how mammalian biological processes evolved.”

A comparison of platypus genome with the DNA of humans and other mammals, which diverged later, and the genomes of birds, whose ancestors branched off some 315 million years ago, can help scientists piece together a more complete picture of mammalian evolution.

Sequencing and assembling the platypus genome proved far more daunting than sequencing any other mammalian genome to date. About 50 percent of the genome is composed of repetitive elements of DNA, which makes it a challenge to assemble properly.

“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,” said Wes Warren of Washington University, who led the project.

The fur-coated platypus, classified as a mammal because it produces milk, also possesses features of reptiles, birds and their common ancestors, along with some curious attributes of its own.

One of only two mammals that lays eggs, the platypus sports a duck-like bill that holds a sophisticated electro-sensory system for foraging food underwater.

At roughly 2.2 billion base pairs, the platypus genome is about two-thirds the size of human genome and contains about 18,500 genes, similar to other vertebrates.

The animal has 52 chromosomes, including an unusual number of sex chromosomes: 10. The platypus X chromosome bears resemblance to the sex chromosome of a bird, known as Z.

As part of the analysis, researchers compared the platypus genome with genomes of the human, mouse, dog, opossum and chicken. They found that the platypus shares 82 percent of its genes with these animals.

The chicken genome was chosen because it represents a group of egg-laying animals, including extinct reptiles, which passed on much of their DNA to the platypus and other mammals over the course of evolution.

The researchers also found genes that support egg laying - a feature of reptiles - as well as lactation, a characteristic of all mammals. Interestingly, the platypus lack nipples, so its young nurse through the abdominal skin.

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

Surprisingly, researchers found the genome contains an expansion of genes that code for a particular type of odour receptor.

“We were expecting very few of these odour receptor genes because the animals spend the majority of their life in the water,” said Warren.

An international consortium of scientists, led by Washington University School of Medicine in St. Louis, was involved in the project.

These findings were published Wednesday in the journal Nature.

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