A complex allorecognition system in a reef-building coral: delayed responses, reversals and nontransitive hierarchies

A highly polymorphic and complex allorecognition system in the coral Stylophora pistillata was revealed in the field by assaying branch pair combinations among 11 colonies (181 assays) for 24 months. Replicates of between-colony combinations exhibited consistent outcomes, in both time scale and type of response. Different allogeneic combinations exhibited one of two main outcomes, either unilateral rejection, or an array of other incompatible reactions following a state of "non-fusion". These responses were partially linked with color morphs (purple dominated yellow). An additional 22 isogeneic grafts resulted in complete fusion. Unilateral rejection occurred 1-7 months following initial contact. Nonfusion usually developed into skeletal suture barriers after 3-9 months, and then into unilateral colony-specific overgrowths at 6-23 months with some reversals in direction at 18-22 months. During this process, small lesions usually developed on the tissue of the subordinate partner, which were either overgrown by the dominant partner or healed. After two years, a network of overgrowths among colonies was established with essentially hierarchial properties, but some nontransitive interactions also occurred. The colonies segregated into three distinct histocompatibility groups; within each group, colonies engaged in nonfusion. Between groups, colonies exhibited nonfusion or rejected each other in a group-specific manner. Based on the results, we discuss the terminology used for fusion versus rejection phenomena in scleractinian corals, the possible genetic background for self-nonself recognition in Stylophora, and the methodological artifacts associated with the use of short-term allorecognition assays.