Phytoplasmas have diverged from gram-positive eubacteria, and belong to the Genus Phytoplasma within the Class Mollicutes (Fig. 1).

Fig. 1. Phytoplasmas are firmicutes. A. Phylogenetic relationships of several bacterial clades containing bacterial pathogens. B. The 5 phylogenetic groups within the Class Mollicutes. Plant pathogenic/symbiotic bacteria are indicated in green. GL, gene loss; WL, loss of cell wall.

Mycoplasmas dramatically differ phenotypically from other bacteria by their minute size (0.3 - 0.5 µm) (Fig. 2) and lack of cell wall. The lack of cell wall was used to separate mycoplasmas from other bacteria in a class named Mollicutes. Due to degenerative or reductive evolution, accompanied by significant losses of genomic sequences, the genomes of mollicutes have shrunk and are relatively small compared to other bacteria, ranging from 580 kb. to 2,200 kb.

Fig. 2. Comparison of sizes of some eubacteria. Phytoplasmas have the same size as mycoplasmas.