Allorecognition and xenorecognition in reef corals: A decade of interactions

Xenorecognition phenomena in coral reefs are expressed by a striking array of morphological and cytological responses. Corals encountering conspecifics further elicit additional repertoires of effector mechanisms, specific to allogeneic challenges. Both inducible sets of antagonistic machineries of allo- and xenoresponses are highly specific. In many cases, they are predictable, reproducible, and reveal the hallmark of coral tissue capacity to distinguish between self and non-self. This essay summarizes a decade (1992-2002) of published results on reef coral immune features. While studies on xenorecognition uncovered the existence of established, non-transitive hierarchies and the importance of antibacterial/ cytotoxic compounds secreted by corals, allorecognition assays disclosed the presence of specific and complex non-transitive hierarchies dictated by the expression of a variety of effector mechanisms ('tailored' against different conspecifics), the existence of gradual maturation of alloresponses (important in the formation of natural chimeras), the debatable issue of allorecognition memory, and the appearance of delayed, second sets of alloresponses. A critical evaluation of historecognition reveals that expressed responses in different coral systems are phenotypically matched with counterpart outcomes recorded in the mammalian immune systems. Histocompatibility in corals, as in vertebrates, relies on recognition elements (not yet disclosed on the molecular level) as well as on effector mechanisms.