THE EVOLUTION OF THE ACTINORHIZAL SYMBIOSIS THROUGH PHYLOGENETIC ANALYSIS OF HOST PLANTS

Phylogenetic analysis of actinorhizal plant families is now possible d ue to the availability of numerous angiosperm sequences of the plastid gene rbcL. A sampling of a 100 rbcL sequences, including representati ves of all eight actinorhizal families, was conducted to estimate thei r phylogenetic relationships. The phylogeny was estimated by neighbor- joining analysis of a matrix of pairwise substitution rates. Bootstrap and double-bootstrap values were estimated. In order to assess the di vergent hypotheses of unique or recurrent evolution of actinorhizal sy mbiosis, character state changes were traced along the branches of the phylogenetic consensus trees obtained, and the number of character ch anges for each of the two scenarios was calculated. The most parsimoni ous scenario favored the recurrent evolution of actinorhizal symbiosis . This scenario is supported by the morphophysiological diversity of a ctinorhizal symbioses and by the relaxed coevolutionary relationships between the host plants and the microsymbiont.