Genomic similarities among 16SrI phytoplasmas and implications on species boundaries

Phytoplasmas are wall less phytopathgenic bacteria that have great impacts on agriculture worldwide. The current taxonomy for these bacteria is mostly based on their 16S rRNA gene sequences, which may lack sufficient resolution or accuracy. With the increased availability of genome sequences for the 16SrI group within this ‘Candidatus ’ genus, this work quantified their genetic divergence and investigated possible species boundaries. The results indicated that strains belonging to different 16SrI subgroups were sufficiently diverged to be possibly considered as distinct ‘Candidatus Phytoplasma’ species. However, intragenomic variations could cause conflicting ribosomal subgroup assignments. Thus, genome-based identification of marker genes for multilocus sequence analysis (MLSA) is critical for improving phytoplasma taxonomy and diagnostics.