Behavior of hemocytes in the allorejection reaction in two compound ascidians, Botryllus scalaris and Symplegma reptans

In botryllid ascidians, the type of allorejection reaction differs among sp ecies. Comparative studies of these different reactions contribute to our u nderstanding of the allorecognition and nonself-rejection system. We studie d the morphology of hemocyte behavior during allorejection reactions in two species, Botryllus scalaris and Symplegma reptans, which stand at importan t points in botryllid phylogeny. In B. scalaris, phagocytes mediated hemocy te aggregation, resulting in interruption of blood flow just after vascular fusion of incompatible colonies. Although previous studies indicate that m orula cells (MCs) play a central role in the rejection reaction, the MCs of B. scalaris did not participate in the rejection reaction. Colonies of S. reptans showed two types of allorejection reaction that started at differen t points in the process of vascular fusion between two colonies. In both ty pes of rejection reaction, the MCs played a central role and behaved simila rly to those of all botryllids except. B. scalaris and a botryllid from Isr ael. These observations suggest that the differences in hemocyte behavior a nd allorecognition site observed in this study reflect the variation in all orejection reactions among botryllids.