Genome analysis of the platypus reveals unique signatures of evolution

https://www.nature.com/articles/nature06936

Abstract
We present a draft genome sequence of the platypus, Ornithorhynchus anatinus. This monotreme exhibits a fascinating combination of reptilian and mammalian characters. For example, platypuses have a coat of fur adapted to an aquatic lifestyle; platypus
females lactate, yet lay eggs; and males are equipped with venom similar to that of reptiles. Analysis of the first monotreme genome aligned these features with genetic innovations. We find that reptile and platypus venom proteins have been co-opted
independently from the same gene families; milk protein genes are conserved despite platypuses laying eggs; and immune gene family expansions are directly related to platypus biology. Expansions of protein, non-protein-coding RNA and microRNA families,
as well as repeat elements, are identified. Sequencing of this genome now provides a valuable resource for deep mammalian comparative analyses, as well as for monotreme biology and conservation.



The platypus genome, as well as the animal, is an amalgam of ancestral reptilian and derived mammalian characteristics. The platypus karyotype comprises 52 chromosomes in both sexes14,15, with a few large and many small chromosomes, reminiscent of
reptilian macro- and microchromosomes. Platypuses have multiple sex chromosomes with some homology to the bird Z chromosome16. Males have five X and five Y chromosomes, which form a chain at meiosis and segregate into 5X and 5Y sperm17,18. Sex
determination and sex chromosome dosage compensation remain unclear.

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You can't draw that conclusion from the study you cite. If you want, you could coin a term "reptile-like mammal", but that isn't any more use
than the now-deprecated "mammal-like reptile".  I'm surprised the nature article uses it at all.

You know I am just armchair speculating, but the evidence shows that it
has broken from all mainstream mammals maybe even before the Placental
split? That is pretty amazing and puts the platapus into its own classification.

It would be like a last surviving non-Avian therapod making it to the
present?

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On 1/15/24 21:58, Peter Nyikos wrote:

opping Mad has the late Isaac Asimov on his side: he wanted desperately for monotremes to be non-mammalian therapsids.

If I am reading the genetics hear correctly, Platpus is not related to
any modern monotremes.

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