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We anchored 172 Mb (92% compared to 22% in OANA5) X-borne sequences to chromosomes (Supplementary Tables 4, 6). To study the origin and evolution of monotreme sex chromosomes, we greatly improved the assembly of the platypus sex chromosomes.
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We also produced a less-continuous assembly for the short-beaked echidna, which enables us to infer the genomic changes that occurred in the ancestral monotremes and other mammals. Here we combined PacBio long-read, 10× linked-read, chromatin conformation (Hi-C) and physical map data to produce a highly accurate chromosome-scale assembly of the platypus genome.
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The incomplete platypus assembly without Y chromosome sequences and lack of an echidna genome have limited the interpretation of the evolution of mammals and monotremes. However, only about 25% of its sequence was assigned to chromosomes 2. The previous female platypus genome assembly (OANA5) provided many important insights into monotreme biology and mammalian evolution. Of particular interest are their sex chromosomes, which originated independently from those of therian mammals through additions of autosomes onto an ancestral XY pair, resulting in a multiple sex chromosome system that assembles as a chain during meiosis 3. Owing to their distinct ecological, anatomical and physiological features, monotremes are interesting mammals well-suited for the study of the evolution of ecological adaptation. Platypuses and echidnas feature radical differences in diet (carnivorous compared with insectivorous), neurophysiology (electroreception-oriented compared with olfaction-oriented), as well as specific intraspecific conflict and defence adaptations 1. aculeatus and three Zaglossus spp.) are present in Australia and New Guinea ( Supplementary Information). At present, the single species of platypus has a restricted distribution in Eastern Australia, whereas four echidna species ( T. The monotreme lineage comprises two extant families, the semi-aquatic Ornithorhynchidae (platypus) and the terrestrial Tachyglossidae (echidna). The iconic egg-laying monotremes of Australasia represent one of the three major mammalian lineages. Further comparative genomic analyses unravel marked differences between monotremes and therians in haptoglobin genes, lactation genes and chemosensory receptor genes for smell and taste that underlie the ecological adaptation of monotremes. The formation of such a unique chromosome complex may have been facilitated by the unusually extensive interactions between the multi-X and multi-Y chromosomes that are shared by the autosomal homologues in humans. We provide evidence that the monotreme sex chromosome complex originated from an ancestral chromosome ring configuration. Together with our echidna sequence, the genomes of the two species allow us to detect the ancestral and lineage-specific genomic changes that shape both monotreme and mammalian evolution. The nearly complete platypus genome assembly has anchored almost the entire genome onto chromosomes, markedly improving the genome continuity and gene annotation. Here we generate and analyse reference genomes of the platypus ( Ornithorhynchus anatinus) and echidna ( Tachyglossus aculeatus), which represent the only two extant monotreme lineages. Nature volume 592, pages 756–762 ( 2021) Cite this articleĮgg-laying mammals (monotremes) are the only extant mammalian outgroup to therians (marsupial and eutherian animals) and provide key insights into mammalian evolution 1, 2.
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Platypus and echidna genomes reveal mammalian biology and evolution