Arnie is a very happy dog (see photo in upper left). Its always a party when Arnie is around– He loves everyone and everything…
… Everything, that is, except cats.
It didnt used to be like that. Arnie used to like cats just fine. But then one day, when he was a puppy…
Neighbor: Hey can I introduce my new kitten to Arnie? I want to get a dog later, so I want her to be used to dogs.
Arnie: *fightsbackpuppytears*… NOMNOMNOMNOM!!!!!! NOMNOMNOMNOM!!!!!!
Aaaaaand now whenever Arnie sees a cat, he goes full on ‘NOMNOM’.
Im not looking forward to breaking this news to him– Incontrovertible proof that cats and dogs shared a common ancestor 60-85 million years ago:
There is a protein necessary for the formation of placentas in mammals, syncytin. Syncytin is a retroviral protein, envelope, that has been coopted for the hosts purposes from endogenous retroviruses. This didnt happen just once, at the origin of mammals– It might have happened then, but then as different species evolved, they selected different ERV Envs that worked ‘best’ for them, meaning that different mammals use different ERV Envs for the same purpose. Simians have syncytin-1 and syncytin-2. Mice have syncytin-A and syncytin-B. A species of rabbit has syncytin-Ory1.
As these authors indicate, domesticating retroviral genes for other purposes isnt rocket science:
… on several occasions in the course of mammalian evolution, env genes from endogenous retroviruses have been co-opted by their host to participate in the formation of the placenta.
Interestingly, this stochastic acquisition of genes of exogenous origin might be related to the unexpectedly large diversity observed in placental structures and the physiology of placentation among eutherian mammals.
When the dog (Canis lupus familiaris) and cat (Felis catus) genomes were sequenced, they identified several envelope genes that *might* be playing the syncytin role in these organisms. They then looked for mRNA from these genes in dog and cat placenta, indicating which of the genes was actually active in that location.
They found a hit, which they named syncytin-Car1. ‘Car‘ because that envelope gene is found in orthologous genomic locations in 26 species of carnivores (including Giant Pandas!)– nestled inside of an intron of some GTPase gene (TBC1D19):
Moreover, high sequence homology of the sequences flanking the proviruses (pairwise percentage of identity comprised between 78% and 82% over 1 kb of the 5′ flanking region and between 59% and 67% over 1 kb of the 3′ flanking region for the three proviruses) as well as the presence of a SUMO-like pseudogene inserted at the same position within all three proviruses (Fig. 6B) confirm that they are strict orthologous copies of the same gene in the dog, cat, and giant panda genomes.
Common descent FTW!!
But poor, poor Arnie…