#rip butler... you would have loved allograft theory...

[laurelnose]

here’s a speculative idea that has me in a HEADLOCK. first, infectious cancers are a thing: clonal cancers which can be transmitted from individual to individual without the aid of an infectious agent like a virus. best known are the canine venereal tumor and the Tasmanian devil facial tumors, both of which spread through direct contact with mucosal tissue. in the case of dogs, fucking, and in the case of Tasmanian devils, biting the shit out of each other’s faces. a handful more exist in clams, and the clam cancer cells are known to be able to survive in seawater and spread without direct contact.

modern bony fishes (teleosts) evolved in and still mostly live in seawater; they’re entirely covered in mucosal tissues, their skin having no layer of dead epithelial cells on top; and many species live in groups and engage in cannibalism. the major histocompatibility complex is what mediates tissue/allograft rejection. It’s possible that the ancestral condition of hugely diversified MHC class I alleles in teleosts is because an elevated risk of transmissible cancers exerted a selection pressure that’s just absent in mammals.

we’ve never found a transmissible cancer in teleosts so this remains fully hypothetical (but have we looked? I do not know) but god damn is it cool. bony fishes might have rearranged their whole fucking immune systems against the threat of infectious cancers. I really do need to reread Xenogenesis sooo bad