SPECIFICITY AND EARLY ONTOGENY OF HISTORECOGNITION IN THE HYDROID HYDRACTINIA

The marine hydroid Hydractinia is a model system for study of historecognition in invertebrates. Present knowledge of this system comes from fusibility studies involving intraspecific (i.e., isogeneic and allogeneic) encounters of adult-stage stolonal tissues. Little is known about the ability of Hydractinia to recognize and respond to xenogeneic tissue, about allorecognition by tissues other than stolonal, and about the ontogeny of historecognition. We have investigated these questions with stolonal-encounter and grafting experiments, using the species H. echinata and H. symbiolongicarpus. We find that (a) Hydractinia employs the nematocyst effector system against the closely related hydroid Podocoryne and the more distantly related Eirene; (b) unlike intraspecific encounters, these interspecific encounters elicit nematocyst discharge by Hydractinia over a distance of up to 50-mu-m prior to stolonal contact; (c) the capability for historecognition is not restricted to stolonal tissues; (d) the capacity to display incompatibility, or rejection, arises between the late larval stage and early post-metamorphosis; and (e) embryo grafts of H. symbiolongicarpus, but not those of H. echinata, appeared to show limited tolerization.