A central tenet of classical biogeography is that taxonomic similarity among ecological communities declines with increasing geographical distance due to both historical factors and contemporary environmental constraints. While this tenet has been well demonstrated for many plants and animals, there has been a lack of agreement as to whether it holds true for microorganisms. The question of microbial biogeography is further complicated for microorganisms that form symbioses, because the distributions of their hosts may themselves be limited by the aforementioned factors. To examine the geographic patterns in Alnus (alder)-associated microbial assemblages and determine how they may relate to host plant biogeography, we studied ectomycorrhizal (ECM) fungal and N-fixing Frankia bacterial assemblages associated with two Alnus species in montane central Mexico and compared them with Alnus-associated ECM and Frankia assemblages located throughout the Pacific Northwestern United States and beyond. ECM root samples and Frankia root nodules were collected from four sites in Mexico. ECM samples were identified with ITS and LSU rRNA gene sequences, while Frankia samples were identified using nifH (nitrogenase) gene sequences. Each microbial assemblage was assessed using both taxon- (richness, diversity, evenness indices) and sequence divergence-based (UniFrac clustering and significance) analyses to facilitate comparison to Alnus-associated ECM and Frankia assemblages found elsewhere.
Among the ECM fungal samples collected in Mexican forests, only 23 taxa were encountered, reflecting the relatively low richness and distinctive composition of Alnus-associated ECM assemblages found elsewhere in the Americas and in Europe. Clavulina, an ECM lineage never before reported with Alnus, contained the dominant taxon overall. However, there was a strikingly high sequence similarity among many of the other ECM taxa found in Mexico and those in Alnus forests in the United States, supporting a host-ECM fungi co-migration hypothesis from a common north temperate origin. Likewise, although two novel Frankia genotypes were found among the Mexican samples, the Frankia assemblages were overall highly similar to those associated with Alnus species in the Pacific Northwest. Genotype- and phylogenetic distance-based community analyses, however, indicated that neither geographic location nor the phylogenetic relationships among hosts is a strong predictor of Frankia assemblage structure at the continental scale. Thus, in contrast to ECM assemblages, our results suggest that factors other than classical biogeography are more influential in determining the continental-scale distribution and diversity of Alnus-associated Frankia bacteria.