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 species. Comparative studies of these different reactions contribute to our understanding of the allorecognition and nonself-rejection system. We studied the morphology of hemocyte behavior during allorejection reactions in two species, Botryllus scalaris and Symplegma reptans, which stand at important points in botryllid phylogeny. In B. scalaris, phagocytes mediated hemocyte aggregation, resulting in interruption of blood flow just after vascular fusion of incompatible colonies. Although previous studies indicate that morula 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 different points in the process of vascular fusion between two colonies. In both types of rejection reaction, the MCs played a central role and behaved similarly to those of all botryllids except B. scalaris and a botryllid from Israel. These observations suggest that the differences in hemocyte behavior and allorecognition site observed in this study reflect the variation in allorejection reactions among botryllids.