Phylogeny and Coevolution of Actinorhizal Rosaceae and their Nitrogen-Fixing Symbionts

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Species in eight families of flowering plants form symbiotic relationships with nitrogen-fixing bacteria of the actinomycete genus Frankia, in which the prokaryotic symbionts inhabit nodules on the plants' roots. Phylogenetic analyses have suggested that such symbiotic relationships, known as actinorhizal associations, have evolved and/or been lost numerous times over the course of evolution, both within Frankia and among the plant families involved. One of these plant families is the rose family, Rosaceae, a large group with considerable economic importance and evolutionary interest. Within Rosaceae, members of five genera (Cercocarpus, Chamaebatia, Cowania, Dryas, and Purshia) are known to form actinorhizal associations. Recent molecular evidence has shown that these five genera, although traditionally classified in different tribes or subfamilies, instead form a monophyletic group within the family. Nucleotide sequence data from the actinomcyete strains infecting several of these species have shown that they all belong to one of the three known monophyletic groups of infective Frankia. Neither the actinorhizal Rosaceae nor their Frankia symbionts, however, have been the subject of thorough phylogenetic study. The relatively small number of plant species (total of about 30) in this group makes it an ideal system in which to study patterns of coevolution between the two participants in the actinorhizal symbiosis. Dr. Daniel Potter of the University of California-Davis and his colleague at University of Connecticut, Dr. David Benson, and their associates will use macromolecular data to evaluate genetic diversity and generate hypotheses of phylogenetic relationship within and among taxa of actinorhizal Rosaceae and their Frankia symbionts. Fieldwork in the western U.S. and by colleagues in Mexico will be devoted to the collection of leaves and root nodules from all species of the five actinorhizal genera of Rosaceae and leaf material from several outgroups. Wherever possible, multiple populations, and multiple individuals per population, will be sampled from throughout the geographic range of each species. Nodules from non-rosaceous actinorhizal plant species occurring at the same localities will also be collected. Nucleotide sequence data will be obtained from appropriately variable regions of the genomes (both chloroplast and nuclear in the case of the plants) of the host plants (from DNA extracted from leaf material) and their symbionts (from DNA extracted from nodules). Broad geographical sampling of multiple populations will enable investigation of host specificity of Frankia strains and promiscuity of the plant species with respect to their symbionts. The data will be analyzed phylogenetically and the resulting phylogenetic trees will be compared in order to examine the extent and patterns of coevolution and to address the question of the taxonomic level at which coevolution has occurred. What are the relative contributions of geography and phylogeny in determining the symbiont strain(s) infecting a particular host at a particular locality? The data from the plant genomes will also be used to evaluate taxonomic delimitation of species and genera in this group. The primary significance of this study will be in improving our understanding of the evolution of symbiosis in general and of ecologically important nitrogen-fixing symbioses in particular. The study will also extend our knowledge of the taxonomic and geographic distributions of actinorhizal associations and will contribute to clarifying the classification of this group of plants, most of which occur in western North America.