Evidence for a species-specific cell fusion hormone in red algae.

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In plants of the red algal genus,Griffithsia, dead intercalary cells are often replaced by the process of cell repair. During this process two cells are produced: one a rhizoid, induces the formation of, stimulates the elongation of and attracts the second, a repair shoot cell. The two cells fuse to form a single shoot cell. The interactions between the rhizoid and the repair shoot appear to be mediated through a diffusible hormone,rhodomorphin. It is possible to induce cell repair, including cell fusion, between two different plants by juxtaposing a rhizoid and a freshly decapitated filament (in vitro cell repair). Rhizoids of three different species ofGriffithsia (G. pacifica, G. tenuis, andG. globulifera) were tested in thein vitro cell repair system to see if rhizoids of one species could induce repair shoot formation and cell fusion in filaments of the other two species. Cell fusion was only induced between plants of the same species; there was no interspecific cell fusion. This suggests that the cell fusion hormone of each species is specific for that species and cannot induce cell repair in other species. When three strains of the speciesG. pacifica were tested in thein vitro repair system, rhizoids of each strain induced repair shoots in decapitated filaments of all three strains and successful fusions occurred in each case. Thus hybrid cells were produced which contained nuclei of different genetic content. However there was evidence of cytoplasmic incompatibility in hybrid cells of some of the crosses.