Intragenomic homogeneity on Liberibacter 16S rDNA confirms phylogeny and explains ecological strategy

Three of the five currently recognized “Candidatus Liberibacter” spp., “Ca. L. asiaticus” (Las), “Ca. L. americanus” (Lam) and “Ca. L. solanacearum”, and the newly erected genus Liberibacter species, L. crescens (Lcr), have had their genomes sequenced. In all four cases there are three homogeneous copies of the 16S rRNA gene, one present as the reverse complement of the other two. 16S intragenic homogeneity is common within the α-Proteobacteria. The presence of three 16S rRNA copies indicates an advantage for a rapid response of population increase to favourable growth conditions. The metabolic cost of carrying multiple copies is avoided during periods of low cellular activity as this situation occurs at low temperatures, for example overwintering in deciduous plants or in a dormant insect host. A large insertion in the 16S rDNA sequence of three species compared to the other three species indicates a dichotomy in the Liberibacter genus and provides a phylogenetic signal of closeness to the proximal node within the Rhizobiaceae. In spite of similar symptoms in Citrus crops associated with Lam and Las infections, these species belong on either side of this dichotomy, thus confirming Lam as phylogenetically closer to the proximal node than Las.