Molecular Variation and Phylogeny of Thymidylate Kinase Genes of Candidatus Phytoplasma ziziphi from Different Resistant and Susceptible Jujube Cultivars in China

The thymidylate kinase ( ) gene is indispensable for the proliferation and survival of phytoplasma. To reveal the molecular variation and phylogeny of the genes of phytoplasma ziziphi, in this study, the genes of 50 phytoplasma strains infecting different resistant and susceptible jujube cultivars from different regions in China were amplified and analyzed. Two sequence types, and , were identified using clone-based sequencing. The JWB phytoplasma strains were classified into three types, type-X, type-Y, and type-XY, based on the sequencing chromatograms of the genes. The type-X and type-Y strains contained only and genes, respectively. The type-XY strain contained both and genes. The type-X, type-Y, and type-XY strains comprised 42%, 12%, and 46% of all the strains, respectively. The type-X and type-XY strains were identified in both susceptible and resistant jujube cultivars, while type-Y strain was only identified in susceptible cultivars. Phylogenetic analysis indicated that the genes of the phytoplasmas were divided into two categories: phylo-S and phylo-M. The phylo-S gene was single-copied in the genome, with an evolutionary pattern similar to the 16S rRNA gene; the phylo-M gene was multi-copied, related to PMU-mediated within-genome transposition and between-genome transfer. Furthermore, the phylogenetic tree suggested that the genes shuttled between the genomes of the Paulownia witches' broom phytoplasma and JWB phytoplasma. These findings provide insights into the evolutionary and adaptive mechanisms of phytoplasmas.