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Legume-rhizobium symbiotic promiscuity does not determine plant invasiveness.

Legume-rhizobium symbiotic promiscuity does not determine plant invasiveness.
Jan-Hendrik KEET1, Cang Hui 2,3, Allan G. Ellis1, Johannes J. Le Roux1,2
1Department of Botany and Zoology, Stellenbosch University, Matieland 7602, South Africa
2Centre for Invasion Biology, Department of Mathematical Sciences , Stellenbosch University, Matieland 7602, South Africa
3Mathematical and Physical Biosciences, African Institute for Mathematical Sciences, Cape Town 7945, South Africa

The fixing of atmospheric nitrogen is thought to play an important role in the invasion success of legumes. Interactions between legumes and nitrogen fixing bacteria (rhizobia) span a continuum of specialisation and therefore, promiscuous legumes should have higher chances of forming effective symbioses in novel ranges. Using eight widespread and eleven localised acacias, we aimed to determine host promiscuity and its link to invasion success by assessing the diversity and structure of rhizobia communities associated with these acacias in South Africa. We hypothesised widespread acacias to be more generalist in their symbiotic requirements, thus associating with a higher rhizobial diversity and differ in community composition, compared to localised acacias. Also, we predicted symbiotic effectiveness to have a strong host plant phylogenetic signal. Using next generation sequencing data for the nodulation gene, nodC, we compared the identity, richness, diversity and compositional similarity of rhizobia associated with these acacias. Using the latest acacia phylogeny and published data, we also explored symbiotic response between various cross-inoculated acacias as a function of host phylogenetic relatedness. Overall, widespread and localised acacias did not differ significantly in rhizobia richness and diversity. However, some diversity metrics were dependent on geography, status and their interaction. Widespread and localised acacias associated with compositionally different suites of rhizobia. Symbiotic responses of cross-inoculated acacias declined as host phylogenetic distance increased. These results suggest that differences in invasion success for these trees may be driven by symbiotic effectiveness rather than rhizobia diversity. 

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