Horizontal transfer of potential mobile units in phytoplasmas

Abstract

Phytoplasmas are uncultivated phy- topathogenic bacteria that cause diseases in a wide range of economi- cally important plants. Through secre- tion of effector proteins, they are able to manipulate their plant hosts to facilitate their multiplication and dispersal by insect vectors. The genome sequences of several phytoplasmas have been charac- terized to date and a group of putative composite transposons called potential mobile units (PMUs) are found in these highly reduced genomes. Recently, our team reported the genome sequence and comparative analysis of a peanut witches’ broom (PnWB) phytoplasma, the first representative of the phytoplasma 16SrII group. Comparisons between the spe- cies phylogeny and the phylogenies of the PMU genes revealed that the PnWB PMU is likely to have been transferred from the 16SrI group. This indicates that PMUs are not only the DNA unit for transposition within a genome, but also for horizontal transfer among divergent phytoplasma lineages. Given the asso- ciation of PMUs with effector genes, the mobility of PMUs across genomes has important implications for phytoplasma ecology and evolution.