Is Botryllus a natural killer?

Natural killer (NK) cells in mammals attack virus-infected and tumor cells that stop making major histocompatibility complex (MHC) proteins. But the discovery by Konstantin Khalturin, Thomas Bosch (University Kiel, Kiel, Germany), and colleagues of a protein in 1050 squirts related to a key NK cell receptor suggests that the evolutionary predecessors of NK cells may have targeted genetic interlopers of their own species.

The sea squirt in question, Botryllus schlosseri, exists as a flower-shaped accumulation of petals, or zooids. When two clusters of zooids meet one another they either fuse or reject one another. For fusion to occur, the two must share at least one allele of the Fu/HC locus.

The molecular identity of Fu/HC remains unknown, and the search for direct relatives of MHC proteins, which determine transplant...