A BAC-based physical map was developed in parallel with the sequence assembly and subsequently integrated with the WGS assembly to provide the primary means of scaffolding the assembly into larger ordered and oriented groupings (ultracontigs Supplementary Notes 2 and 3 and Supplementary Table 2). A draft assembly was produced from ∼6× coverage of whole-genome plasmid, fosmid and bacterial artificial chromosome (BAC) reads ( Supplementary Table 1) using the assembly program PCAP 20 ( Supplementary Notes 1). Here we describe the platypus genome sequence and compare it to the genomes of other mammals, and of the chicken.Īll sequencing libraries were prepared from DNA of a single female platypus (Glennie Glenrock Station, New South Wales, Australia) and were sequenced using established whole-genome shotgun (WGS) methods 19. Because the platypus has rarely bred in captivity and is the last of a long line of ornithorhynchid monotremes, their continued survival is of great importance. Water quality, erosion, destruction of habitat and food resources, and disease now threaten populations. Platypuses are still relatively common in the wild, but were recently reclassified as ‘vulnerable’ because of their reliance on an aquatic environment that is under stress from climate change and degradation by human activities. Its secretive lifestyle hampers understanding of its population dynamics and the social and family structure. Platypuses live in the waterways of eastern and southern Australia, including Tasmania. Sex determination and sex chromosome dosage compensation remain unclear. Males have five X and five Y chromosomes, which form a chain at meiosis and segregate into 5X and 5Y sperm 17, 18. Platypuses have multiple sex chromosomes with some homology to the bird Z chromosome 16. The platypus karyotype comprises 52 chromosomes in both sexes 14, 15, with a few large and many small chromosomes, reminiscent of reptilian macro- and microchromosomes.
The platypus genome, as well as the animal, is an amalgam of ancestral reptilian and derived mammalian characteristics.
Interestingly, adult monotremes lack teeth. With its eyes, ears and nostrils closed while foraging underwater, it uses an electro-sensory system in the bill to help locate aquatic invertebrates and other prey 12, 13. Platypus is an obligate aquatic feeder that relies on its thick pelage to maintain its low (31–32 ☌) body temperature during feeding in often icy waters. Other special features of the platypus are its gastrointestinal system, neuroanatomy (electro-reception) and a venom delivery system, unique among mammals 11. The testes synthesize testosterone and dihydrotestosterone, as in therians, but there is no scrotum and testes are abdominal 10. Chromosomes are arranged in defined order in sperm 8 as they are in therians, but not birds 9. Spermatozoa are filiform, like those of birds and reptiles, but, uniquely among amniotes, form bundles of 100 during passage through the epididymis. The anatomy of the monotreme reproductive system reflects its reptilian origins, but shows features typical of mammals 7, as well as unique specialized characteristics.
Platypus milk changes in protein composition during lactation (as it does in marsupials, but not in most eutherians 5). For about 4 months, when most organ systems differentiate, the young depend on milk sucked directly from the abdominal skin, as females lack nipples. The egg is laid in an earthen nesting burrow after about 21 days and hatches 11 days later 5, 6. In 1884, William Caldwell’s concise telegram to the British Association announced “Monotremes oviparous, ovum meroblastic”, not holoblastic as in the other two mammalian groups 3, 4. The most extraordinary and controversial aspect of platypus biology was initially whether or not they lay eggs like birds and reptiles. Estimates of the monotreme–theria divergence time range between 160 and 210 Myr ago here we will use 166 Myr ago, recently estimated from fossil and molecular data 2. The divergence of monotremes and therians falls into the large gap in the amniote phylogeny between the eutherian radiation about 90 million years (Myr) ago and the divergence of mammals from the sauropsid lineage around 315 Myr ago ( Fig. Traditionally, the Monotremata are considered to belong to the mammalian subclass Prototheria, which diverged from the therapsid line that led to the Theria and subsequently split into the marsupials (Marsupialia) and eutherians (Placentalia). The platypus was placed with the echidnas into a new taxon called the Monotremata (meaning ‘single hole’ because of their common external opening for urogenital and digestive systems). Some initially considered it to be a true mammal despite its duck-bill and webbed feet. The platypus ( Ornithorhynchus anatinus) has always elicited excitement and controversy in the zoological world 1.
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