Recent advances in the biogeography and genecology of symbiotic Frankia and its host plants

Molecular phylogenetic approaches have begun to outline the origin, distribution and diversity of actinorhizal partners. Geographic isolation of Frankia and its host plants resulting from shifting continents and dispersal patterns have apparently led to the development of Frankia genotypes with differing affinities for host genera, even within the same plant family. Actinorhizal plant genera of widespread global distribution tend to nodulate readily even outside their native ranges. These taxa may maintain infective Frankia populations of considerable diversity on a broad scale. Arid environments seem to have distinctive actinorhizal partnerships, with smaller and more specific sets of Frankia symbionts. This has led to the hypothesis that some host families have taxa that are evolving towards narrow strain specificity, perhaps because of drier habitats where fewer Frankia strains would be able to survive. Harsh conditions such as water-saturated soils near lakes, swamps or bogs that are typically acidic and low in oxygen may similarly lessen the diversity of Frankia strains present in the soil, perhaps limiting the pool of frankiae available for infection locally and, at a larger scale, for natural selection of symbiotic partnerships with host plants. Recent molecular ecological studies have also provided examples of Frankia strain sorting by soil environment within higher order cluster groupings of Frankia host specificity. Future frontiers for ecological research on Frankia and actinorhizal plants include the soil ecosystem and the genome of Frankia and its hosts.