Platypus genetic blueprint reveals the early history of mammals
UK-based researchers at the Medical Research Council Functional Genomics Unit in Oxford and the European Molecular Biology Laboratory’s European Bioinformatics Institute in Cambridge have revealed the genetic makeup of the one of the world’s strangest mammals. They have analysed the DNA blueprint of the platypus, one of only a few surviving monotremes which, of all mammals, are the most distantly-related to humans. The platypus, a female nicknamed Glennie, was sequenced by scientists at the Genome Sequencing Centre of Washington University School of Medicine, USA, as part of an international research collaboration including scientists from the UK and Australia. The analysis is published in the 8 May issue of Nature.
The platypus is thought to have diverged from a common ancestor shared with humans approximately 170 million years ago. The species has many features that are unique to mammals; for example it has fur and rears its young on milk. However, it also shows reptile-like characteristics; the females lay eggs and the males produce venom. Some features, such as a specialised system in the platypus bill that uses electricity to detect food under water [electro-reception], are unique to monotremes. The researchers found that these diverse characteristics are mirrored by a patchwork of genes resembling those from reptiles, birds and other mammals. Lead researcher Chris Ponting from the MRC Functional Genomics Unit at the University of Oxford said: “The platypus genome is extremely important because it is the missing link in our understanding of how we and other mammals first evolved. This is our ticket back in time to when all mammals laid eggs while suckling their young on milk. It also provides an essential background to future advances in understanding mammalian biology and evolution.”
The researchers searched the genome for DNA sequences that are unique to the monotremes, as well as those known to be involved in venom production, electro-reception and milk production in other species. They discovered that platypus venom is a cocktail of proteins that originally had very different functions. Amazingly, the same proteins are found in reptile venom even though platypus and snake venom evolved independently. They also found that the platypus has many more sex chromosomes – the organised structures into which DNA is packed that determine sex – than do humans. The platypus has ten sex chromosomes, compared with our two. Furthermore, the gene sequences responsible for determining sex are more similar to those in birds than in mammals. Ewan Birney, who led the genome analysis performed at the European Bioinformatics Institute, commented: “The platypus looks like such a strange blend of mammalian, bird-like and reptilian features and now we know that the genome is an equally bizarre mix of all of these. It’s much more of a mélange than anyone expected.”