A SYMBIOGENETIC THEORY FOR THE ORIGINS OF CNIDOCYSTS IN CNIDARIA

Did cnidarian cnidocysts originate from cnidocyst-bearing protoctistan s living as symbiotic partners with an epithelial placula? If an incre ase in the fitness of symbiotic partners was ''locked in'' by an evolu tionary stable strategy, co-evolution and compartmentalization could h ave led phyletically separate, eukaryotic symbionts to fuse and underg o nuclear merger. Traits originating in the symbiotic partners would h ave been brought to the ''synthetic'' organism and reworked through ev olution into the development of an integrated organism. Support for th e theory of symbiogenetic origins of Cnidaria rests on traces of symbi osis detected in the relationship of cnidarian epithelium to interstit ial cells (I-cells), the precursors of cnidocyst-producing cnidoblasts : (1) epithelium and I-cell are autonomous and differ in morphology, c ellular dynamics, the relationship of differentiation to proliferation and the variety of cell types formed; (2) hydras and planulas can be ''cured'' of I-cells and their derivatives, thereby creating ''epithel ial'' animals which lack responsiveness but retain vegetative properti es. (3) The reintroduction of 1-cells into ''epithelial'' animals rest ores missing differentiated cell and organismic characteristics. Symbi ogenesis as a source of metazoan species has consequences for concepts of development, from the origins of cell lines to the evolution of di fferentiation.