An Australian native platypus is seen in this May 15, 2002 handout photograph made available on May 7, 2008. (Xinhua/Reuters Photo) Photo Gallery>>>

   　WASHINGTON, May 7 (Xinhua) -- The genome sequence of the duck-billed platypus has been decoded, revealing clues about how genomes were organized during the early evolution of mammals, an international team of scientists reported Wednesday.

    In a paper published in the journal Nature, researchers analyzed a high-quality draft genome sequence of Glennie, a female platypus from Australia. The consortium included scientists from the United States, Australia, England, Germany, Israel, Japan, New Zealand and Spain.

    The platypus, native to Australia, is one of the few mammals that lays eggs. However, its peculiarity does not end there. For example, these odd animals boast what looks like a duck's bill, along with thick fur coats adapted for the icy waters in which it resides.

    "Its genome sequence is priceless for understanding how fundamental mammalian biological processes have evolved," said Francis Collins, director of the U.S. National Human Genome Research Institute.

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

    Once the sequence was produced, researchers began comparing the genome of the platypus, whose ancestors split from the rest of the mammalian lineage some 166 million years ago, with the well-characterized genomes of the human, mouse, dog, opossum and chicken.

    Scientists were particularly interested in finding features within the platypus genome that could explain the odd mix of characteristics seen in the platypus, those that were more like reptile and birds and those like mammals.

    The female platypus lays eggs, a reptilian characteristic, yet also produces milk to nourish its young, which is a mammalian characteristic. Interestingly, the platypus genome harbors both reptilian and mammalian genes associated with fertilization of eggs. However, researchers discovered that, like other mammals, the platypus genome contains a tightly clustered set of genes that produce the casein proteins that make up milk.

    A good sense of smell is something mammals have in common. Compared to most mammals, chickens and lizards have a relatively poor complement of odorant receptor genes responsible for their ability to detect smells. Interestingly, the platypus has about half as many odor receptors as do mice and other mammals. However, the researchers also found that the platypus genome has experienced an expansion in the genes that code for a particular type of odor receptor, called a vomeronasal receptor. Researchers think the expansion of this particular set of genes may be involved in the ability of the platypus to detect odors during underwater foraging.

    Researchers also found that genes related to the immune system of the platypus are very similar to those of other mammals, with a few key differences. For example, the platypus genome contains expansions in the gene family that codes for a microbe-fighting peptide called cathelicidin. Primates and rodents have only a single copy of the cathelicidin gene. Scientists think that the expansion of this gene family may be involved in boosting their immune system as platypuses are extremely immature when they hatch.