Multilocus Genetic Characterization of Phytoplasmas

Classification of phytoplasmas into 16S ribosomal groups and subgroups and ‘Candidatus Phytoplasma’ species designation have been primarily based on the conserved 16S rRNA gene. However, distinctions among closely related ‘Ca. Phytoplasma’ species and strains based on 16S rRNA gene alone have limitations imposed by the high degree of rRNA nucleotide sequence conservation across diverse phytoplasma lineages and by the presence in a phytoplasma genome of two, sometimes sequence heterogeneous, copies of this gene. Thus, in recent years, moderately conserved genes have been used as additional genetic markers with the aim to enhance the resolving power in delineating distinct phytoplasma strains among members of some 16S ribosomal subgroups. The present chapter is divided in two parts: the first part describes the non-ribosomal single-copy genes less conserved (housekeeping genes) such as ribosomal protein (rp), secY, secA, rpoB, tuf, and groEL genes, which have been extensively used for differentiation across the majority of phytoplasmas; the second part describes the differentiation of phytoplasmas in the diverse ribosomal groups using multiple genes including housekeeping genes and variable genes encoding surface proteins.