Studies on histocompatibility in natural populations of freshwater sponges

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Abstract
Histocompatibility in natural populations of individuals was examined for the following four Japanese species of freshwater sponges: Ephydatia fluviatilis, Ephydatia muelleri, Radiospongilla cerebellata, and Eunapius fragilis. Gemmules were obtained from field‐grown specimens. Then, using juvenile sponges hatched from gemmules, fusibility was tested in the laboratory. Every interspecific combination among these four species displayed non‐fusion with no cytotoxic interaction. For all species, the occurrence of allogeneic histoincompatibility was assured, but manifestation of incompatibility was different from species to species. Among 36 specimens of E. fluviatilis collected from three localities, two strains nonfusible with each other were distinguished. Each locality was populated by a single strain. Specimens of E. muelleri were obtained from two localities. All combinations of specimens coming from the same locality were compatibly fusible, but those between specimens coming from different localities displayed conspicuous cytotoxic rejection reaction. This reaction occurred asymmetrically, resulting in the destruction of one partner. In R. cerebellata, two types of reactions, fusion and rejection, were also distinguished. In rejection, cytotoxic reaction occurred bilaterally and symmetrically in and around the contact zone. A total of 40 specimens, collected from a single pond, could be divided into 16 groups by their fusibility. As to E. fragilis, some combinations of specimens were compatibly fusible; the others displayed various degrees of incompatibility. In completely incompatible combinations, the paired sponges simply showed nonfusion with no sign of cytotoxic reaction. In more weakly incompatible combinations, the paired sponges were separated after initial fusion, the degree of which was variable depending on combination. A total of 59 specimens, collected from a single pond, could be classed into 14 groups by their fusibility. In both R. cerebellata and E. fragilis, some intergroup combinations were compatible, giving rise to probable genetic chimeras. Such chimeric sponges persisted with no sign of segregation for more than 1 month in the field.