Allorecognition histocompatibility in a protochordate species: is the relationship to MHC somatic or structural?

Colonial tunicates are complex marine invertebrates (in fact protochordates) that undergo a variety of histocompatibility reactions in their intraspecific competition for feeding surfaces. By means of these reactions colonies fuse with kin, extend domination over a feeding surface, while isolating unrelated conspecifics. The primary determinant of fusion (with kin) or rejection (of non-kin) is a single, highly polymorphic, histocompatibility gene locus (or haplotype), called Fu/HC. Following fusion with nonidentical kin sharing 1 or more Fu/HC allele(s), the fused pair expands both chimeric partners via an asexual budding process, further extending domination over a feeding surface. However, at some later time point an intense set of histoincompatibility reactions occurs between fused kin, resulting in the destruction of all individuals of one of the genotypes, ending the chimeric state. In this review we describe what is known of the genetics and several biological properties encoded by the Fu/HC, and the several independent gene loci that control the colony resorption phenomena that return the colony to the province of a single genotypic individual.