Allorecognition in colonial tunicates: protection against predatory cell lineages?

The MHC molecules have been historically perceived as transplantation antig ens, though it is now recognized that their primary, if not sole, role is i n eliminating parasites and in surveillance and clearance of aberrant self. Indeed, pregnancy in mammals would represent the closest to a natural tran splantation process that occurs in vertebrates. However, among the immediat e ancestors to the vertebrates, natural intraspecific allorecognition proce sses are common. Among members of the colonial tunicate Botryllus schlosser i, true individuals that share a single allele of the highly polymorphic fu sibility/histocompatibility (Fu/HC) locus are able to fuse with one another Could this Fu/HC be related to the MHC such that the MHC really did have i ts origins as a transplantation antigen? Presently we review the genetics and biology of natural transplantation pro cesses in colonial tunicates, comparing it with allorecognition as mediated through the vertebrate T-cell receptor, killer cell inhibitory receptor/Ly 49, and MHC. Experimental approaches to determining if the molecules regula ting allorecognition in tunicates have any ancestral relationship to the ve rtebrate MHC are discussed, as is a genomic approach to isolating novel med iators of allorecognition. We also explore the biological basis for allorec ognition in colonial tunicates and recent work that highlights the costs of not maintaining a system for allorecognition.