Abstract
Egg-laying mammals (monotremes) are the only extant mammalian outgroup to therians (marsupial and eutherian animals) and provide key insights into mammalian evolution1,2. Here we generate and analyse reference genomes of the platypus (Ornithorhynchus anatinus) and echidna (Tachyglossus aculeatus), which represent the only two extant monotreme lineages. The nearly complete platypus genome assembly has anchored almost the entire genome onto chromosomes, markedly improving the genome continuity and gene annotation. 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. We provide evidence that the monotreme sex chromosome complex originated from an ancestral chromosome ring configuration. 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. 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.
Highlights
We provide evidence that the monotreme sex chromosome complex originated from an ancestral chromosome ring configuration
The short pseudoautosomal regions (PARs) of platypus chromosomes X2–X5 have a significantly higher GC content than the SDRs or the longer PARs (Extended Data Fig. 4c), which probably reflects strong GC-biased gene conversion that is caused by a high recombination rate[10]
Chicken orthologous sequences of these monotreme PARs are all located on the microchromosomes, which have a high GC content[12] (Extended Data Fig. 4c, d)
Summary
Complete and accurate reference genomes and annotations are critical for evolutionary and functional analyses. We have produced a high-quality platypus genome using a combination of single-molecule sequencing technology and multiple sources of physical mapping methods to assign most of the sequences to a chromosome-scale assembly. This permits better-resolved analyses of the origin and diversification of the complex sex chromosome system that evolved in monotremes. 3. Grützner, F. et al In the platypus a meiotic chain of ten sex chromosomes shares genes with the bird Z and mammal X chromosomes. D. et al Origins and functional evolution of Y chromosomes across mammals. E. et al Comparative genomic analysis of the pheromone receptor class 1 family (V1R) reveals extreme complexity in mouse lemurs (genus, Microcebus) and a chromosomal hotspot across mammals. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
