Alloimmune maturation in the coralStylophora pistillatais achieved through three distinctive stages, 4 months post–metamorphosis

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Abstract
Adult colonies of the reef–building coral Stylophora pistillata discriminate precisely between ‘self’ and ‘non–self’ attributes and respond selectively against specific allogeneic challenges. We studied the ontogeny of these allospecific responses on newly settled polyps by establishing allogeneic contacts within groups of 2–6 siblings or non–related offsprings. Interactions were observed for up to 8 months. Three types of responses, depending on the age of the interacting partners, were documented. The first was tissue fusion and the formation of a stable chimera, observed in partners less than 2 months old. The second was observed in contacts of partners 2–4 months old. It started with tissue fusion and transitory chimera since separation of the chimera–partners or polyp death resulted when the oldest partner in the chimera reached the age of 4 months. The third type was the regular histoincompatibility response, as documented in allogeneic interactions of adult colonies, recorded here in all encounters with S. pistillata partners over 4 months old. Maturation of allorecognition in this species was therefore achieved through three time–dependent stages, 4 months following metamorphosis. Combinations of siblings or genetically unrelated partners did not affect the results. We propose that the coral alloimmune maturation system may be used as a new evolutionary model scheme for studying tissue transplantation and tolerance.