A Comparison of Hemocytes and Their Phenoloxidase Activity among Botryllid Ascidians

Abstract

The colony specificity of colonial animals involves allorejection reactions, which are the defense reactions for allogeneic tissues that occur naturally. In colonial botryllid ascidians, all of the species already studied have colony specificity, and their allorejection modes differ from one another. However, in most of these botryllids, morula cells (MCs) always participate in the allorejection reactions, and the prophenoloxidase (proPO) system of MCs is considered to contribute to the allorejection reaction. The present study was performed using five botryllids and Symplegma reptans, which is closely related to botryllids, in an effort to clarify the relationship between the modes of allorejection and the characteristics of MCs, such as the ratio of MCs to total hemocytes and the phenoloxidase (PO) activity levels in the MCs. The MCs of these six ascidians resembled one another morphologically and the MCs of all species showed PO activity. In Botryllus scalaris, PO activity was also found in granular leukocytes, but the level of activity was much lower than that in MCs. The PO of these species resembles one another, at least in terms of their sensitivity to inhibition by common inhibitors of the proPO system. The PO activity per fixed number of hemocytes varied among these ascidians. This variation was due to a difference in the ratio of MCs to total hemocytes and/or a difference in PO activity per MC. In most ascidians, except for B. scalaris, the rejection reaction area showed a higher level of PO activity than the fusion area of the syngeneic colonies. These results suggest that the characteristics of MCs including their PO activity are closely correlated with the mode of the allorejection reaction.