Allorecognition/xenorecognition responses in Botrylloides (ascidiacea) subpopulations from the mediterranean coast of Israel

Examinations of Botrylloides colonies along the Mediterranean coast of Israel have revealed the existence of three distinct morphs (subpopulations, SPs) which differ in some morphological characteristics and life history patterns. Here we study allorecognition and xenorecognition responses expressed by colonies of three SPs, by employing >400 colony allorecognition and xenorecognition assays. SP1 colonies exhibited a new type of nonself response by the formation of giant ampullae, the megaloampullae. These special structures appeared 1–5 days after first ampullae‐to‐ampullae contacts were established, in assays either within SPs or between SPs, and in xenogeneic challenges with Botryllus schlosseri but not with Didemnum. The megaloampullae developed from regular ampullae, which dilated up to 7 times their normal width and 1.5 times their length, but were usually not involved in cytotoxic reactions. SP2 and SP3 colonies produced “fusion‐rejection” responses in within‐SP assays; however, they may develop hemorrhages only when challenged between SPs (also with SP1 colonies) or xenogenetically with B. schlosseri. No xenogeneic response was recorded when the colonies were assayed with Didemnum. The fusion‐rejection response is characterized by reciprocal penetration of ampullae, develops to short‐term blood vessel anastomosis with increased numbers of amebocytes and macrophages, followed by a limited exchange of blood cells, and subsequently terminates in ampullae amputation and cell death. Hemorrhages developed without ampullae penetration. We studied fusion frequencies in SP1 and SP3 colonies collected from an area of 20 × 30 m. Both combinations of within‐SP assays revealed high rates of fusion (12.5 and 35.0%, revealing 30 and 10 allorecognition alleles on the fusibility locus, respectively), but no fusion was recorded in 50 between‐SP assays. The types of allogeneic responses may indicate that SP2 and SP3 are taxonomically closer to one another than to SP1, a conclusion which is not supported by analyzing morphological, ecological, and developmental features characteristic of the three Botrylloides spp. We also discuss here the phenomenon that some invertebrates express repeatability and specifically more than one distinctive nonself reaction, which indicates the complexity of invertebrate immune responses. © 1994 Wiley‐Liss, Inc.