That is what Platypus do, even as mammals . Two days ago the Platypus genome was published in Nature. So if you haven’t already read it, might be worth a skim at least. So, they aren’t model organisms and not a medically or agriculturally important animals (unless someone starts figuring out how to cook platypus omelets), but platypus (platypi?) do hold a fascinating place in evolutionary history having mammalian (milk, fur) and reptilian (eggs, venom) characteristics. Their genome obviously holds clues to some of this unique history.
For example, the eggs and fertilization genes can tell us a bit of how that evolved…
The platypus shares with other mammals four genes associated with the zona pellucida, a gel-like coating that facilitates fertilization of the egg. But it also has two matches for ZPAX genes that had previously been found only in birds, amphibians and fish. And it shares with the chicken a gene for a type of egg-yolk protein called a vitellogenin. That suggests that vitellogenins, which are found in birds and fish, predate the split from the sauropsids, although the platypus retains only one vitellogenin gene, whereas the chicken has three.
And as the article states, some of these characteristics, which seem reptilian, such as the ability of the male platypus to inject venom from a spike, evolved independently from reptilian venom:
Other characteristics that seem purely reptilian turn out to have evolved independently, the analysis suggests. Male platypuses have spurs on their hind legs that are loaded with a venom so potent it can kill a dog. Like the venom of reptiles, the poison is a cocktail of variations on at least three kinds of peptide. But the variations arose from duplications of different genes in platypuses than in modern reptiles. The similarity in venom is an example of convergent evolution between the two tetrapods.
Interesting stuff if you, like me, were fascinated by platypus and marsupials when you were a kid.. or of course if you are an evolutionary biologist, researching immune system genetics, or studying the agricultural potential of platypus egg farms.
In all seriousness, this is a fascinating genome and will bring a pile (technical term) of new knowledge to our understanding of vertebrate/ mammalian genetics and evolution (see the graphic of genomes in the vertebrate evolutionary tree that have been or are being sequenced, it’s interesting). The UCSC Genome Browser has had the draft assembly available since almost exactly one year ago today, but I think the publication and further study is going to help answer a lot of questions.