Low genetic diversity amongFrankiaspp. strains nodulating sympatric populations of actinorhizal species of Rosaceae,Ceanothus(Rhamnaceae) andDatisca glomerata(Datiscaceae) west of the Sierra Nevada (California)

Abstract

Frankia spp. strains typically induce N2-fixing root nodules on actinorhizal plants. The majority of host plant taxa associated with the uncultured Group 1 Frankia strains, i.e., Ceanothus of the Rhamnaceae, Datisca glomerata (Datiscaceae), and all actinorhizal members of the Rosaceae except Dryas, are found in California. A study was conducted to determine the distribution of Frankia strains among root nodules collected from both sympatric and solitary stands of hosts. Three DNA regions were examined, the 5' end of the 16S rRNA gene, the internal transcribed spacer region between the 16S and 23S rRNA genes, and a portion of the glutamine synthetase gene (glnA). The results suggest that a narrow range of Group 1 Frankia spp. strains dominate in root nodules collected over a large area of California west of the Sierra Nevada crest with no apparent host-specificity. Comparisons with Group 2 Frankia strain diversity from Alnus and Myrica within the study range suggest that the observed low diversity is peculiar to Group 1 Frankia strains only. Factors that may account for the observed lack of genetic variability and host specificity include strain dominance over a large geographical area, current environmental selection, and (or) a past evolutionary bottleneck.

FrankiaAssemblages Associated withAlnus rubraandAlnus viridisAre Strongly Influenced by Host Species Identity

“…More strikingly, studies have also found that distinct Frankia assemblages associate with different sympatric host species both within (Anderson et al 2009;Pokharel et al 2010) and across (Huguet et al 2001) genera. While some studies have found no evidence for a correlation between Frankia genotypes and host plant species (Vanden Huevel et al 2004), there appears to be general support for the importance of host species in influencing both the composition and diversity of Frankia assemblages (Oakley et al 2004;Mirza et al 2009). …”

Absence of Cospeciation between the UnculturedFrankiaMicrosymbionts and the Disjunct ActinorhizalCoriariaSpecies

“…Despite the high taxonomic diversity of host plants belonging to the cross-inoculation group of cluster 2 and its disjunct range, uncultured Frankia in root nodules of several host plants have so far shown a low level of diversity regardless of the typing locus used [6, 7, 11–16], suggesting a recent emergence, a strong and recent evolutionary bottleneck, or a nonrepresentative sampling. The time of emergence of all Frankia lineages is poorly documented as no convincing fossil remains.…”

Genome characteristics of facultatively symbiotic Frankia sp. strains reflect host range and host plant biogeography

“…Finally, the "Rosaceous strains" form Cluster 2, which is sister to the others; representatives of this cluster have not been isolated and grown in culture. Cluster 2 strains nodulate plants from four families in the Rosales and Cucurbitales Vanden Heuvel et al 2004).…”