Abstract
Mutualistic interactions can strongly influence species invasions, as the inability to form successful mutualisms in an exotic range could hamper a host's invasion success. This barrier to invasion may be overcome if an invader either forms novel mutualistic associations or finds and associates with familiar mutualists in the exotic range. Here, we ask (1) does the community of rhizobial mutualists associated with invasive legumes in their exotic range overlap with that of local native legumes and (2) can any differences be explained by fundamental incompatibilities with particular rhizobial genotypes? To address these questions, we first characterized the rhizobial communities naturally associating with three invasive and six native legumes growing in the San Francisco Bay Area. We then conducted a greenhouse experiment to test whether the invasive legume could nodulate with any of a broad array of rhizobia found in their exotic range. There was little overlap between the Bradyrhizobium communities associated with wild‐grown invasive and native legumes, yet the invasive legumes could nodulate with a broad range of rhizobial strains under greenhouse conditions. These observations suggest that under field conditions in their exotic range, these invasive legumes are not currently associating with the mutualists of local native legumes, despite their potential to form such associations. However, the promiscuity with which these invading legumes can form mutualistic associations could be an important factor early in the invasion process if mutualist scarcity limits range expansion. Overall, the observation that invasive legumes have a community of rhizobia distinct from that of native legumes, despite their ability to associate with many rhizobial strains, challenges existing assumptions about how invading species obtain their mutualists. These results can therefore inform current and future efforts to prevent and remove invasive species.
Highlights
We examined the rhizobia associated with three invasive legumes (Genista monspessulana, Spartium junceum, and Ulex europaeus) and six native legumes (Acmispon glaber, A. heermannii, A. micranthus, A. strigosus, Lupinus arboreus, and L. bicolor) in the San Francisco (SF) Bay Area, California, USA (Fig. S1)
A small percentage of Bradyrhizobium genotypes associated with both native and invasive legumes under field conditions, which suggests that the surveyed invaders are not currently forming novel associations with local mutualists in their exotic range
Our results parallel growing evidence at sites worldwide that invasive legumes utilize rhizobial communities that differ from those of native legumes (Chen et al, 2005; Lafay & Burdon, 2006; Rodriguez-Echeverria, 2010; Weir et al, 2004), the opposite trend was observed in an Australian Mimosa invasion (Parker, Wurtz, & Paynter, 2006)
Summary
We examined the rhizobia associated with three invasive legumes (Genista monspessulana, Spartium junceum, and Ulex europaeus) and six native legumes (Acmispon glaber, A. heermannii, A. micranthus, A. strigosus, Lupinus arboreus, and L. bicolor) in the San Francisco (SF) Bay Area, California, USA (Fig. S1). Phylogenetic diversity of genotypes associated with each legume host under field conditions was calculated as the mean pairwise molecular distance using the Jukes-Cantor metric between all pairs of genotypes associated with that legume species (note, mean pairwise molecular distance for the concatenated ITS and nifD loci of A. glaber and A. micranthus and the nifD locus of A. glaber were set to 0 for this analysis, as all rhizobia isolated from these species were identified as the same genotype; qualitatively similar results were obtained in separate analyses that excluded these species). Patterns identified when examining the ITS and nifD loci separately generally supported the observation that, in nature, the Bradyrhizobium communities associated with native and invasive legume hosts did not significantly differ in richness or phylogenetic diversity. Vicia sp. and native A. wrangelianus), only rarely nodulated test host plants (Figure 5)
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