Mutualistic interactions can be important drivers of species invasions. In particular, the mutualism between legumes and nitrogen-fixing soil bacteria (i.e., rhizobia) may be influential in plant invasion success. Acacias, which are legumes native to Australia, have been introduced around the world, with certain species becoming invasive. We examined the acacia-rhizobia symbiosis to determine whether this mutualism plays a role in invasiveness of introduced acacias. Specifically, we assessed whether acacias varying in invasiveness were co-introduced abroad with their native rhizobial symbionts. We predicted that invasive acacias would be found associating with rhizobia from their native range, whereas non-invasive acacias would not. To determine whether acacias were co-introduced abroad with their native rhizobia, we selected four Acacia species occurring in California (two invasive and two non-invasive), collected root nodules from each species in both their native and introduced ranges, and identified rhizobial strains associating with each species, sequencing the 16S rRNA, nifD, and nodC genes. We conducted a maximum likelihood analysis for each gene to determine whether there was geographic distinction in the identity of rhizobial symbionts associating with acacias across continents and invasive status.
Results/Conclusions
Our results indicate that all Acacia species introduced to California, regardless of invasive status, associate with rhizobia of Australian origin. This suggests that concurrent acacia-rhizobia introductions have occurred for all species tested. For all genes (16S, nifD, and nodC) we found overlap in the genetic structure of rhizobia collected from acacias in Australia and California, regardless of acacia invasive status. Similar to previous studies, acacias were most commonly found in both ranges associating with Bradyrhizobium spp. Our findings clarify the role that the co-introduction of rhizobial mutualists plays in the invasion of acacias abroad, indicating that acacia invasive status in California is not delineated by the availability of native rhizobial symbionts. Our research aids in ruling out an important mechanism of acacia invasion, and sheds light on a key aspect of legume biology, clarifying the driving forces of acacia invasion abroad.