Allorecognition responses in the soft coral Parerythropodium fulvum fulvum from the Red Sea

Allogeneic encounters were experimentally arranged in Eilat, Red Sea for the first time in the Alcyonacea with the soft coral Parerythropodium fulvum fulvum. All possible pairwise combination assays within two groups of five colonies each and one group of three colonies were setup in situ in 2-4 replicates each (a total of 76 assays). Control isogeneic encounters always resulted in complete tissue fusion. Two types of allogeneic responses were documented following tissue-to-tissue contacts. The first was retreat growth, in which contacting allografts started to retreat from each other a few days following direct contact until a bare area of a few mm separated them. In several assays the colonies repeatedly grew into contact and retreated again. The second allogeneic response was unilateral or reciprocal tissue overgrowth. In this type of response one colony overgrew the conspecific partner by several mm and then stopped. The underlying tissue of the overgrown partner died. No cytotoxicity was observed in allogeneic contacts either in growing parts or when assays were established between cut surface areas. Repeated assays of the same pair-combination were not consistent in terms of type and directionality of responses. We propose that effector mechanisms elicited following allogeneic encounters in P. f. fulvum may be affected by biological as well as non-biological parameters and are not specific to the type of allogeneic challenge. However, colony specificity in this species is restricted only to the level of self- and non-self discrimination.