Allogeneic resorption in colonial protochordates: consequences of nonself recognition

Colonial tunicates (protochordates of the subfamily Botryllinae) have the property of forming natural chimeras in the wild or in the laboratory with other members of the same species if both individuals share at least one allele at a single highly polymorphic locus, termed the fusibility/histocompatibility (Fu/HC) locus. Laboratory studies revealed that after the establishment of a common blood circulatory system between a fusible pair one of the colonies in the chimera is usually resorbed, a phenomenon that occurs after an interval of 1 week or extends to up to 8 months. In the present article, we review this allogeneic resorption in Botryllus schlosseri, a cosmopolitan protochordate. The studies on allogeneic resorption in B. schlosseri revealed 13 typical characteristics. However, several processes interrupt successful resorption, such as the retreat growth phenomenon, chimeric death, and others. It was also found that allogeneic resorption is not only genetically controlled but is also controlled by a polymorphic hierarchial phenomenon, including the Fu/HC locus and additional unrelated loci. One basic rule for this genetic system is that colonies heterozygotic on the resorption elements will resorb more homozygotic partners. This colony resorption, expressed by a morphological elimination of one partner, is probably manifested by cellular elements circulating in the tunicate blood system. However, it was also shown that some blood-borne cells of the inferior partner, such as the stem cells, may escape resorption, a phenomenon that raises the threat of germ/somatic cell parasitism.