PHYLOGENETIC-RELATIONSHIPS AMONG ACTINORHIZAL PLANTS - THE IMPACT OF MOLECULAR SYSTEMATICS AND IMPLICATIONS FOR THE EVOLUTION OF ACTINORHIZAL SYMBIOSES

A review of recent molecular systematic studies of actinorhizal plants and their Frankia endosymbionts is presented. For comparative purpose s, a discussion of recent studies pertaining to the evolution of nodul ation in the legume-rhizobium system is included. Molecular systematic studies have revealed that actinorhizal plants are more closely relat ed than current taxonomic schemes imply. Broad-based analyses of the c hloroplast gene rbcL indicate that all symbiotic root-nodulating highe r plants belong to a single large clade. More focused molecular analys es of both legume and actinorhizal hosts within this large clade indic ate that symbioses have probably arisen more than once. By comparing h ost phylogenies and recently published bacterial phylogenies, we consi der the coevolution of bacterial symbionts with their actinorhizal hos ts.