Urochordata: Botryllus – Natural Chimerism and Tolerance Induction in a Colonial Chordate

Chimerism is defined as the coexistence of two or more genomes of separate origin within an individual. In placental mammals such as humans, natural chimerism develops during pregnancy between a mother and fetus and has an important role in the induction of fetal tolerance to maternal tissues. Natural chimerism between kin also occurs in colonial ascidians, the closest extant ancestors of chordates. In the ascidian, Botryllus schlosseri, some colonies fuse to create lifelong chimeric entities of two allogeneic genomes. The decision to fuse in B. schlosseri is governed by a polymorphic histocompatibility gene called the Botryllus histocompatibility factor (BHF). Colonies that share at least one BHF allele fuse upon contact, whereas colonies without any BHF alleles in common ultimately reject. Following vasculature fusion, stem cells from each histocompatible B. schlosseri colony compete to overtake germline or somatic lineages. Stem cell competition may lead to elimination of the other colony’s genome, or it may produce a chimeric colony with mixed genotypes. In this way, chimerism in B. schlosseri represents a nexus between stem cell competition, genome parasitism, and allorecognition. Here we review studies conducted over six decades that led to the discoveries of the nature of the cells that mediate chimerism in colonial ascidians and the gene that controls it.