Alloimmune memory is absent in the Red Sea hydrocoral Millepora dichotoma

When two allogeneic colonies of the Red Sea hydrocoral Millepora dichotoma (Cnidaria, Hydrozoa) come into tissue contact, one of the genotypes is usually overgrown by the other. The directionality and pace of this alloresponse are thought to be genetically determined. We established tissue contacts between allogeneic colonies in situ in order to elucidate a possible memory component in this response. First-set interactions were established from all possible pairwise combinations between three colonies in eight replicates per combination. Interactions were followed up for 8 weeks. Thereafter, interacting pairs were detached and either regrafted near the original contact area to form second-set assays or challenged by third party grafts. Additional delayed first-set assays was also established. Overgrowth of delayed first-set, second-set, and third-party grafts was followed again for 8 weeks. The mean overgrowths recorded in the second set of the interactions were indistinguishable from the first sets in all three colony combinations. A specific alloimmune memory has not been found in this cnidarian system as opposed to other cases within the phylum.