Disparate origins ofBradyrhizobiumsymbionts for invasive populations ofCytisus scoparius(Leguminosae) in North America

Abstract

To identify the geographic origin of nodule bacteria associated with invasion of the European legume Cytisus scoparius in the United States, isolates from 15 sites in six states were compared to > 200 Bradyrhizobium strains from indigenous legumes in the U.S., Mexico, Europe (six countries), Morocco, and Australia. Portions of five housekeeping loci (2849 bp) were sequenced, along with the nifD locus in the symbiosis island (SI) portion of the Bradyrhizobium chromosome. Bayesian phylogenetic analysis showed that North American C. scoparius symbionts had highly heterogeneous ancestry. Some were grouped into three distinct clades of European C. scoparius symbionts. One isolate had both housekeeping and SI genes belonging to a Bradyrhizobium clade from native legumes in western North America. Two other clades had mosaic ancestry: sequences for nifD as well as two other SI genes (nifH, nodC) were highly similar or identical to a C. scoparius strain from Spain, while their housekeeping loci belonged to American Bradyrhizobium clades. Thus, it appears that bacteria ancestrally associated with other North American legumes have evolved to utilize C. scoparius, by acquiring SI-region genes from European C. scoparius symbionts. Inoculation assays indicated that North American isolates were as competent as European strains in promoting plant growth, consistent with the findings on symbiont ancestry.

Specificity in Legume-Rhizobia Symbioses

“…genistearum [175]. In contrast, rhizobia sampled from invasive Cytisus scoparius sampled in six states in the United States, differed with respect to housekeeping and symbiosis gene sequences [174]. Specifically, one group of isolates had both housekeeping and symbiosis gene sequences similar to a Bradyrhizobium clade from native legumes in Western North America, but two clades had nifD , nifH and nodC sequences highly similar or identical to a Cytisus scoparius strain isolated in Spain, while their housekeeping genes were similar to American Bradyrhizobium clades.…”

Cointroductions of Australian acacias and their rhizobial mutualists in the Southern Hemisphere

“…On the other hand, many legumes associate with identical rhizobia in both their native and their non-native ranges (e.g. Birnbaum, Bissett, Thrall, & Leishman, 2016;Horn, Parker, Malek, Rodríguez-Echeverría, & Parker, 2014;McGinn et al, 2016;Ndlovu, Richardson, Wilson, & Le Roux, 2013), supporting cointroduction.…”

Co‐introduction vs ecological fitting as pathways to the establishment of effective mutualisms during biological invasions

“…The presence of co‐introduced alien rhizobia also opens up the possibility for invasive legumes to recruit native bacteria (which are potentially better adapted to local abiotic conditions) following HGT of symbiotic gene islands from non‐native to native bacteria (e.g. Horn et al ., ; Ling et al ., ). The effectiveness of N‐fixation in native legumes may be further compromised if HGT leads to less effective symbiotic interactions (e.g.…”