A single sym plasmid type predominates across diverse chromosomal lineages of Cupriavidus nodule symbionts

Cupriavidus nodule symbionts from Mimosa host legumes indigenous to five locations around the Caribbean region were analyzed by sequencing portions of five chromosomal housekeeping loci and five sym plasmid loci in 80 isolates. Nodule symbionts did not form a single clade separated from non-symbiotic reference strains of Cupriavidus and Ralstonia, implying that either convergent losses or independent gains of the trait of legume symbiosis have taken place. Chromosomal genes exhibited significantly higher nucleotide polymorphism and haplotype diversity than sym plasmid loci. A single derived sym plasmid haplotype (A1) was found to predominate in four of the populations, and was shared by multiple housekeeping gene clades. This suggests that one sym plasmid variant has recently spread geographically and has been acquired by diverse chromosomal lineages within the region. Inoculation of two Mimosa host species indicated that strains carrying the predominant A1 haplotype ranked either first or second among the five major sym plasmid haplotype groups with respect to plant growth enhancement. Symbiotic outcomes also varied greatly among chromosomally diverse strains that all shared the A1 haplotype. Thus, chromosomal as well as sym plasmid variants likely contribute to differential interactions with Mimosa host species.