A comparison of hemocytes and their phenoloxidase activity among botryllidascidians

The colony specificity of colonial animals involves allorejection reactions , which are the defense reactions for allogeneic tissues that occur natural ly. In colonial botryllid ascidians, all of the species already studied hav e colony specificity, and their allorejection modes differ from one another . However, in most of these botryllids, morula cells (MCs) always participa te in the allorejection reactions, and the prophenoloxidase (proPO) system of MCs is considered to contribute to the allorejection reaction. The prese nt study was performed using five botryllids and Symplegma reptans, which i s closely related to botryllids, in an effort to clarify the relationship b etween the modes of allorejection and the characteristics of MCs, such as t he ratio of MCs to total hemocytes and the phenoloxidase (PO) activity leve ls in the MCs. The MCs of these six ascidians resembled one another morphol ogically and the MCs of all species showed PO activity. In Botryllus scalar is, PO activity was also found in granular leukocytes, but the level of act ivity was much lower than that in MCs. The PO of these species resembles on e another, at least in terms of their sensitivity to inhibition by common i nhibitors of the proPO system. The PO activity per fixed number of hemocyte s varied among these ascidians. This variation was due to a difference in t he ratio of MCs to total hemocytes and/or a difference in PO activity per M C. In most ascidians, except for B. scalaris, the rejection reaction area s howed a higher level of PO activity than the fusion area of the syngeneic c olonies. These results suggest that the characteristics of MCs including th eir PO activity are closely correlated with the mode of the allorejection r eaction.