Genetic relationship and maturity state of the allorecognition system affect contact reactions in juvenile Seriatopora corals

Adult stony corals are characterized by a well-developed allorecognition system. In most cases, genetically identical coral grafts fuse, whereas genetically different grafts (allografts and xenografts) show various histoincompatible responses. Very little is known, however, about the allorecognition system in juvenile corals. We studied contact reactions of juvenile pairs in 2 scleractinian coral species: Seriatopora caliendrum and S. hystrix (0 to 59 d after settlement). Juvenile colonies were experimentally grafted in combination groups of various genetic distances: sibling allografts of S. hystrix (4 pairs), non sibling allografts of S. caliendrum (12 pairs), non sibling allografts of S. hystrix (16 pairs) and xenografts of the 2 species (26 pairs). Three tissue contact reactions were detected in all of the genetic combination groups: nonfusion, fusion and incompatible fusion. The transition in contact reaction patterns was recorded over a 1 yr period. Contact reactions and their duration were largely consistent among pair replicates of each combination and within each of the 4 genetic combination groups. We suggest that in juvenile Seriatopora corals, the genetic combination of each graft is a primary determinant of the contact reaction and its endurance. Maturity state of the allorecognition system, affects both onset and duration of the fusion and incompatible fusion reactions in the juvenile corals. Onset of the fusion reaction was observed only when grafted colonies were younger than 4 mo. Throughout the experimental period, most fusion reactions transformed into histoincompatible reactions or disconnection and virtually none of the allografts or xenografts maintained stable chimeras. We suggest that as the alloimmune system matures, incompatible fusion reactions eventually transform into nonfusion or disconnection via tissue necrosis of the contact zone.