Van Oevelen, Sandra

Three genera in the Rubiaceae ( Pavetta , Psychotria and Sericanthe ) harbour bacterial endosymbionts within leaf nodules or galls. The present paper identifies the bacterial endophytes in three leaf-nodulating Pavetta species. In order to reveal their identity and assess their phylogenetic position, 16S rRNA, recA and gyrB genes were sequenced from an extensive sampling of Burkholderia strains. This multigene approach results in a robust phylogeny, which places the bacterial endosymbionts of Pavetta at two distinct positions within the genus Burkholderia (class Betaproteobacteria ), suggesting that leaf-nodulating endosymbionts within Pavetta have different origins. The endophytes of nodulated Psychotria species were recognized as the closest relatives to the Pavetta endosymbionts. Our results suggest that the endosymbionts of Pavetta represent novel species, which can be classified as ‘ Candidatus Burkholderia hispidae’, ‘ Candidatus Burkholderia rigidae’ and ‘ Candidatus Burkholderia schumannianae’.

Phylogenetic 16S rRNA gene analysis was used to assign the bacterial leaf-nodulating endosymbionts of two tropical African Psychotria species to the genus Burkholderia. The microsymbionts of the different Psychotria hosts were recognized as distinct and novel species of Burkholderia on the basis of relatively low intersequence similarities and sufficiently large evolutionary distances when compared with each other and their closest validly named neighbours. The obligate endosymbiotic nature of the bacteria prevented their in vitro cultivation and the deposition of type strains to culture collections. Therefore, the provisional status Candidatus is assigned to the bacterial partners of Psychotria calva and Psychotria nigropunctata, with the proposal of the names ‘Candidatus Burkholderia calva’ and ‘Candidatus Burkholderia nigropunctata’, respectively.