Symbiotic Frankia bacteria in Alnus forests in Mexico and the United States of America: Is geographic location a good predictor of assemblage structure?

While the biogeography of Alnus species is well characterized, that of their microbial symbionts remains less well understood. Little is known, for example, about how the genotypic richness of Alnus-associated Frankia bacteria varies at the continental scale, and the richness of Alnus-associated Frankia at tropical latitudes has yet to be explored. In this study, we conducted sequence-based analyses of the nifH gene comparing Frankia found in root nodules of two Alnus species in central Mexico with those associated with two Alnus species in the northwestern United States of America (USA). Similar to Frankia assemblages in northwestern USA and other geographic locations, genotypic richness within the Mexican samples was low, with five genotypes total using a ≥97% nifH sequence similarity cutoff. The vast majority of Mexican sequences belonged to genotypes also very common in northwestern USA Alnus forests, although two novel Mexican genotypes were identified. Phylogenetic analyses confirmed that all of the genotypes present in Mexico belong to larger clades of Alnus-associated Frankia. Genotype- and distance-based community analyses indicated that neither geographic location nor the phylogenetic relationships among hosts are strong predictors of Frankia assemblage structure. Our results suggest that factors other than classic biogeography are more influential in determining the continental-scale distribution and diversity of Alnus-associated Frankia.