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Vol 453|8 May 2008

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Top billing forplatypus at end of evolution tree
A draft sequence of the platypus genome with other mammals four genes associated reveals reptilian and mammalian elements with the zona pellucida, a gel-like coating that and provides more evidence for its place in the facilitates fertilization of the egg. But it also has ancestral line of animal evolution. two matches for ZPAX genes that had previously The platypus(Ornithorhynchus anatinus) been found only in birds, amphibians and fish. is endemic to Australia and one of nature’s And it shares with the chicken a gene for a type oddest creatures, seemingly assembled from of egg-yolk protein called a vitellogenin. That the spare parts of other animals. The semi- suggests that vitellogenins, which are found in aquatic monotreme is a venomous, duck- birds andfish, predate the split from the sauropbilled mammal that lays eggs, nurses its young sids, although the platypus retains only one viteland occupies a lonely twig at the end of a sparse logenin gene, whereas the chicken has three. branch of the vertebrate evolutionary tree. Other characteristics that seem purely reptilian turn out to have evolved Now, the structure of its genome independently, theanalysis sughas revealed new clues to how “There is nothing mammals evolved. “The analysis quite as enigmatic gests. Male platypuses have spurs is beginning to align these strange on their hind legs that are loaded features with genetic innovation,” as a platypus.” with a venom so potent it can kill says Wesley Warren of Washinga dog. Like the venom of reptiles, ton University in St Louis,Missouri, the lead the poison is a cocktail of variations on at least author of the genome analysis — a huge inter- three kinds of peptide. But the variations arose national project (see page 175). Comparisons from duplications of different genes in platywith the genomes of other mammals will help to puses than in modern reptiles. The similarity in date the emergence of the platypus’s distinguish- venomis an example of convergent evolution ing characteristics and reveal the genetic events between the two tetrapods. that underlie them. “There is nothing quite as enigmatic as a For example, mammals are defined by their platypus,” says Richard Gibbs, who directs the possession of mammary glands, which in Human Genome Sequencing Center at Baylor females can produce milk. Although the platy- Collegeof Medicine in Houston, Texas. “You pus doesn’t have nipples, it produces true milk have got these reptilian repeat patterns and these — full of fats, sugars and proteins — which more recently evolved milk genes and independthe young suck through a glandular patch on ent evolution of the venom. It all points to how its skin. The analysis shows that the platypus idiosyncratic evolution is.” hasgenes for the family of milk proteins called The sex of the platypus is determined by a set caseins, which map together in a cluster that of ten chromosomes, an oddity that sets it apart matches that of humans. This is a sign that from all other mammals and from birds. These one of the genetic innovations that led to the chromosomes link during meiosis to form a development of milk occurred more thanchain that ensures every sperm gets a set of all 166 million years ago, and after mammals Xs or all Ys. Despite the similar designations, first split from the lizard-like The platypus has genetic sauropsids that gave rise to modern reptiles and birds. characteristics of both The genes relating to the reptiles and mammals. platypus’s eggs offer further insight. The embryos develop within the...
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