Histocompatibility bioassays of population structure in marine sponges

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Abstract
Clonal population structure in two marine sponges, Verongia longissimia and lotrochota birotulata, was examined with a self-recognition bioassay. The bioassay consists of grafts of branch segments between conspecific individuals. Results were consistent with the operational properties expected of a precise histocompatibility system. Autografts exhibited acceptance responses; grafts between individuals separated by large distances exhibited rejection responses; Individuals were not limited to a single mode of response at one time; and all identity relationships were transitive. Clonal population structure was assessed by examining the relationship between graft response and donor-to-recipient distance, and by actually mapping the distributions of particular clones. Clones of lotrochota birotulata were usually restricted to single coral heads or small patch reefs (1–3 m diameter). For Verongla longissima, which can grow directly upon the coral rubble surrounding coral heads and patch reefs, individual clones often occupied larger areas (up to 10 m diameter). The spatial patterns of clonal distributions are readily interpreted as consequences of the particular demographies and habitat specificitles of these two species.