Transplantation of Fu/HC-incompatible zooids in Botryllus schlosseri results in chimerism

The colonial urochordate Botryllus schlosseri undergoes a genetically defined, natural transplantation reaction that is controlled by a single Mendelian locus (called the Fu/HC). This Fu/HC-based allorecognition system is initiated when peripheral elements of the vasculature interact on the edges of two asexually expanding colonies. To better understand the spatial organization of the cellular elements responsible for Fu/HC-based allorecognition, we bypassed the normal site of interaction (the ampullae) and experimentally transplanted zooids between Fu/HC-noncompatible Botryllus schlosseri pairs. The results show that (1) instead of the expected rejections (tissue necroses) that develop after natural contacts between peripheral blood vessels, the transplanted organs are morphologically eliminated within a few days in conjunction with the normal blastogenic cycle; and (2) donor-recipient chimerism is established after complete morphological elimination of transplanted tissues. These results suggest that Fu/HC-based allorecognition responses in Botryllus schlosseri occur exclusively at the ampullae and that once cells have crossed this barrier, they are able to survive and proliferate in the new host colony.