Study
Genome analysis of the platypus reveals unique signatures of evolution
Warren, Wesley C.; Hillier, LaDeana W.; Marshall Graves, Jennifer A.; Birney, Ewan; Wilson, Richard K.
Nature, 2008
The platypus genome was sequenced and assembled, then compared against other mammals, birds and reptiles to identify which features are shared with which lineage — including the genes behind egg-laying, lactation, venom and the sex-determination system.
- Studied in
- Ornithorhynchus anatinus
- Sample size
- whole-genome assembly with comparative analysis
The genome carries a mixture of features found separately in reptiles, birds and other mammals. Egg-yolk protein genes are retained alongside a full complement of casein milk-protein genes. Venom genes arose by duplication of genes with ordinary immune functions, independently of the reptile venom genes they resemble. The sex chromosome system involves ten chromosomes and is more like a bird's than a mammal's.
What it means
The authors' reading, and ours. Where they differ, that difference is the point.
How the authors put it
The platypus genome reflects a lineage that diverged early from other mammals and retained ancestral features rather than one that is transitional or primitive.
How NatureHQ reads it
The word to avoid here is "primitive", and the genome is the reason. A platypus is not an unfinished mammal or a stage on the way to better ones — it is the surviving branch of a lineage that split from ours around 170 million years ago and has been evolving independently ever since. Some things it kept and we lost; some things it invented that we never had. Egg yolk and milk in the same animal is not a halfway house, it is two working systems side by side.
- A single individual as the reference for a species with regional population structure.
- Assembly quality in repetitive regions was limited by the sequencing of the period, and later versions revised details.
- Gene presence indicates capability rather than how a gene is used.
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