THE EVOLUTION OF ACTINORHIZAL SYMBIOSES - EVIDENCE FOR MULTIPLE ORIGINS OF THE SYMBIOTIC ASSOCIATION

According to morphologically based classification systems, actinorhiza l plants, engaged in nitrogen-fixing symbioses with Frankia bacteria, are considered to be only distantly related. However, recent phylogene tic analyses of seed plants based on chloroplast rbcL gene sequences h ave suggested closer relationships among actinorhizal plants. A more t horough sampling of chloroplast rbcL gene sequences from actinorhizal plants and their nonsymbiotic close relatives was conducted in an effo rt to better understand the phylogenetic relationships of these plants , and ultimately, to assess the homology of the different actinorhizal symbioses. Sequence data from 70 taxa were analyzed using parsimony a nalysis. Strict consensus trees based on 24 equally parsimonious trees revealed evolutionary divergence between groups of actinorhizal speci es suggesting that not all symbioses are homologous. The arrangement o f actinorhizal species, interspersed with nonactinorhizal taxa, is sug gestive of multiple origins of the actinorhizal symbiosis. Morphologic al and anatomical characteristics of nodules from different actinorhiz al hosts were mapped onto the rbcl-based consensus tree to further ass ess homology among rbcl-based actinorhizal groups. The morphological a nd anatomical features provide additional support for the rbcl-based g roupings, and thus, together, suggest that actinorhizal symbioses have originated more than once in evolutionary history.