When the first skin of a duck-billed platypus arrived in England in 1799, the keeper of natural history at the British Museum thought it must be an elaborate hoax; how else to explain an animal with the fur of a mammal and the beak of a bird?
But European naturalists were soon to realise that the hairy, egg-laying creature from Australia which suckles its young and hunts "blind", with its eyes, ears and nostrils all closed while swimming underwater, was very real. Zoologists studying the creature's anatomy and behaviour confirmed that the duck-billed platypus was one of the strangest anomalies in the animal kingdom, and now geneticists have confirmed just how weird it is with the first complete analysis of its fully decoded genome.
Scientists said yesterday that they have now deciphered the entire DNA of the duck-billed platypus in a study involving more than 100 scientists from eight countries. They found that the animal's genes are indeed an unusual amalgam derived from the disparate worlds of reptiles, birds and mammals.
The duck-billed platypus is one of just a few living species of mammals that lays eggs rather than giving birth to live young – the other egg layers being the echidnas. It is a member of the monotreme group of mammals, which refers to the "single hole" that serves as both anus and urino-genital opening.
Scientists were keen to explore its DNA because the platypus represents one of the few living species of mammals forming the monotremes, which split off from the rest of the mammals about 166 million years ago.
"It's probably the most eagerly awaited genome since the chimp genome because platypuses are so weird," said Professor Jenny Graves of the Australian National University in Canberra, one of the co-authors of the study published in the journal Nature.
The scientists found that the platypus has about the same number of functional genes as its mammalian cousins but that some of them bear a closer resemblance to the reptiles, particularly the genes involved in producing the venom used by male platypuses to defend their territories. The study found that the toxin delivered by the kicking back claws of irate male platypuses is the result of a duplication in a set of reptilian genes that has undergone the same sort of duplication independently in reptiles to produce snake venom.
It also found that the platypus shares about 82 per cent of its genes with other mammals, including the genes involved in lactation although the female platypus breast feeds through her skin rather than through nipples, which she lacks.
"The fascinating mix of features in the platypus genome provides many clues to the function and evolution of all mammalian genomes, said Richard Wilson of Washington University in St Louis, Missouri, one of the study's lead authors. "By comparing the platypus genome to other mammalian genomes, we'll be able to study genes that have been conserved throughout evolution."
Another surprising discovery was the nature of the chromosomes that determine sex in the platypus. In mammals, just two chromosomes, the X and the Y, are involved in sex determination.
"The platypus is exceptional in that females have five different pairs of X chromosomes, and males have five X chromosomes and five Y chromosomes. To our surprise we discovered that the platypus X and Y chromosomes are completely unrelated to the X chromosome of all other mammals," said Dr Paul Waters, of the Australian National University.
Instead, the sex chromosomes of the platypus share much in common with those of birds, suggesting that the original common ancestor of all mammals, including man, may have also had sex chromosomes more like modern-day birds, he said.