Reference: Biol. Bull. 197: 188-197. (October 1999) Behavior of Hemocytes in the Allorejection Reaction in Two Compound Ascidians, Botryllus scalaris and Symplegma reptam MAKI SHIRAE 1 *, EUICHI HIROSE : , AND YASUNORI SAITO 1 1 Shimoda Marine Research Center, University ofTsukiiba. Shimoda 5-10-1. Shi-noka 415-0025. Japan: uiitl Department of Chemistry, Biology and Marine Science, Faculty of Science, University of the Ryiikyns, Nishihara, Okinawa 903-0213. Japan Abstract. 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 stud-ied the morphology of hemocyte behavior during allorejec-tion reactions in two species. Botryllus scalaris and Svm-p/egma 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 previ-ous 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 botryl-lids. Introduction Colony specificity is a type of self-nonself recognition and rejection reaction against conspeeifics that occurs in many colonial forms of animals; allogeneic colonies fuse to form a single mass or reject each other when the colonies come into contact at their growing edges, whereas synge-Received 30 September 1998; accepted 26 June 1999. Abbreviations: MCs, moriih cell-. * To whom correspondence should he addressed. Email: shirae@ kurofune.shimoda.tsukuba.ac.jp neic colonies always fuse with each other. The phenomenon has been studied in various species in different phyla (e.g., Porifera. Cnidaria, Bryoz.oa, and Urochordata), and the sig-nificance of the fusion and the rejection to survival has been discussed. However, it is difficult to reach a reasonable conclusion by comparing the features of colony specificity among a broad range of species because they differ in body plan and physiological condition. In botryllid ascidians, the morphological process of fu-sion and rejection reactions in colony specificity has been described in detail for five species: Botrvllus primigenus (Oka and Watanabe, 1957, 1960;Tanaka, 1973; Tanaka and Watanabe, 1973; Taneda and Watanabe. 1982a, 1982b); Botryllus schlosseri (Sabbadin, 1962; Boyd et ai, 1990); Botrylloides simodensis (Mukai and Watanabe, 1974; Hi-rose et ai, 1988. 1997); Botrylloides fuscus (Hirose et ai, 1994, 1997); and Botrylloides violaceus (Hirose et ai, 1988). In these botryllid ascidians, the processes of fusion are essentially the same. When rejection reactions occur, they interrupt the progress of fusion, and the beginning stages of rejection differ among species. That is, the varia-tion in allorejection types is thought to be caused by differ-ences in the allogeneic recognition stage in the colony fusion process (Taneda et ai, 1985; Saito et ai. 1994). Hemocytes play important roles in the allorejection reac-tions of botryllid ascidians (Taneda et ai, 1985; Saito et ai, 1994). An earlier comparative study on the ultrastructure of hemocytes in some botryllids, including Botryllus scalaris and Syniplc^ma reptans, showed that hemocytes could be commonly classified into five morphological types: hemo-blasts, morula cells (MCs), granular leukocytes, vacuolated cells, and phagocytes (Shirae. unpub. data). Transient cells in differentiation were also observed. We hypothesize that I 88
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