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Tunic Phagocytes Are Involved in Allorejection Reaction in the Colonial Tunicate Aplidium yamazii (Polyclinidae, Ascidiacea)

¹ Division of Biology, Department of Natural and Environmental Sciences, Faculty of Education and Human Studies, Akita University, Akita 010-8502, Japan
² Department of Chemistry, Biology and Marine Science, Faculty of Science, University of the Ryukyus, Nishihara, Okinawa 903-0213, Japan
³ Division of Science Education, Faculty of Education and Human Sciences, Yokohama National University, Yokohama 240-8501, Japan

^* To whom correspondence should be addressed. E-mail: tishii{at}ipc.akita-u.ac.jp

The colonial ascidian Aplidium yamazii exhibited an allorejection reaction when two allogeneic colonies were brought into contact at their growing edges or at artificial cut surfaces. This species has no vascular network in the tunic, unlike the botryllid ascidians, which have a vascular network throughout the colony's common tunic. In the allorejection reaction induced by contact at the growing edges, some small, hard-packed tunic masses were formed at the contact points. Histological and electron microscopic investigation of these tunic masses revealed that they contained aggregates of tunic cells, with tunic phagocytes being the major cell type present. Some of the tunic phagocytes in these tunic masses appeared to be disintegrating. When allogeneic colonies were placed in contact at their artificial cut surfaces, the colonies partially fused, then separated. In this allorejection reaction, some loosely packed tunic masses remained in the gap between the two withdrawn colonies. These results strongly suggest that the tunic phagocytes are likely to be the major effector cells in the allorejection reaction. We also propose that the tunic phagocytes are not only the effector cells in the allorejection reaction but also bear the sites of allorecognition.