Invasional meltdown via horizontal gene transfer of a European symbiosis island variant in North American nodule symbionts of Cytisus scoparius

New data from sites in Spain, Sicily and North America establish that the spread of the European legume Cytisus scoparius (Scotch broom) across North America involved the concurrent invasion of a European mobile genetic element (symbiosis island [SI]) into North American Bradyrhizobium nodule bacteria. At four SI loci, bacteria from nine C. scoparius populations across the continent were all identical to haplotypes present in European Bradyrhizobium strains. At seven non-symbiotic (housekeeping) loci, these American C. scoparius bacteria grouped into eight diverse lineages related to, or in some cases identical to, symbionts from several native North American legumes. Inoculation experiments showed that American Bradyrhizobium strains with the European SI variant conferred superior growth to C. scoparius plants compared to strains with SI variants found indigenously in North America. Cytisus scoparius invasion in North America thus enabled proliferation of a European SI variant that spread to multiple indigenous American Bradyrhizobium lineages via horizontal gene transfer. Spread of this SI variant, in turn, likely facilitated C. scoparius invasion of North American habitats.