Diversity and specificity of Frankia strains in nodules of sympatric Myrica gale, Alnus incana, and Shepherdia canadensis determined by rrs gene polymorphism

The identity of Frankia strains from nodules of Myrica gale, Alnus incana s ubsp, rugosa, and Shepherdia canadensis was determined for a natural stand on a Lake shore sand dune in Wisconsin, where the three actinorbizal plant species were growing in close proximity, and from two additional stands wit h M. gale as the sole actinorhizal component. Unisolated strains were compa red by their 16S ribosomal DNA (rDNA) restriction patterns using a direct P CR amplification protocol on nodules. Phylogenetic relationships among nodu lar Frankia strains were analyzed by comparing complete 16S rDNA sequences of study and reference strains. Where the three actinorhizal species occurr ed together, each host species was nodulated by a different phylogenetic gr oup of Frankia strains. M. gale strains from ail three sites belonged to an Alnus-Casuarina group, closely related to Frankia alni representative stra ins, and were low in diversity for a host genus considered promiscuous with respect to Frankia microsymbiont genotype. Frankia strains from it. incana nodules were also within the Alnus-Casuarina cluster, distinct from Franki a strains of M. gale nodules at the mixed actinorhizal site but not from Fr ankia strains from two M. gale nodules at a second site in Wisconsin. Frank ia strains from nodules of S. canadensis belonged to a divergent subset of a cluster of Elaeagnaceae-infective strains and exhibited a high degree of diversity. The three closely related local Frankia populations in Myrica no dules could be distinguished from one another using our approach. In additi on to geographic separation and host selectivity for Frankia microsymbionts , edaphic factors such as soil moisture and organic matter content, which v aried among locales, may account for differences in Frankia populations fou nd in;Myrica nodules.