Ontogenetic Variation in Sponge Histocompatibility Responses

Grafting of adult sponge fragments (Chalinula sp.) led to isograft fusion and allograft nonfusion in both parabiotic and implant grafts. We conclude that adult Chalinula sp. individuals discriminate between self and nonself, and fuse only isogeneic fragments. In the laboratory, however, larvae and early juveniles fuse. Larvae used in the experiments were probably genetically different, even if they were asexually reproduced. These results indicate that the capacity for fusion between allogeneic individuals disappears during ontogenesis in this sponge. In some cases, multichimeras were formed when up to five larvae fused to yield a single sponge. All 37 chimeras metamorphosed and survived during 17 days of observation. Possible mechanisms for the formation of sponge chimeras during early development are discussed, as are the costs and benefits of chimera formation at juvenile versus adult stages. We propose that, if fusion exists in the field, it occurs between kin larvae.