II. 'Candidatus Phytoplasma' species*.

In the second system, phytoplasmas are classified into 'Candidatus Phytoplasma' species, based on the nucleotide sequence of the 16S rRNA gene. In this system, the nucleotide sequences of 16S rRNA genes from different phytoplasmas are aligned by a sequence alignment computer program, and the % identities among the sequences are calculated. If the 16S rRNA genes of two phytoplasmas share 97.5% or less nucleotide sequence identity, they are considered to represent two separate 'Candidatus Phytoplasma' species.

The Table below illustrates relationships between the two systems of phytoplasma classification. Each 16S rDNA RFLP group represents at least one distinct 'Candidatus Phytoplasma' species. The Table lists only phytoplasmas that have been formally named as 'Candidatus Phytoplasma' species, phytoplasmas for which 'Candidatus Phytoplasma' names have been suggested but not formally published in the literature, and phytoplasmas recognized as potential new 'Candidatus Phytoplasma' species but for which a 'Candidatus' name has not yet been suggested.

In a separate but related system, phytoplasmas are classified into groups and subgroups, based on the "fingerprint" (RFLP** pattern) of a segment of the gene that encodes 16S rRNA. (LINK TO GROUP section)

--R.E. Davis

*Currently, there is no formal taxonomy of phytoplasmas. Phytoplasmas belong to class Mollicutes, which embraces bacteria that lack a cell wall. In class Mollicutes, formal recognition of species, and assignment of binomial Latin names to species, require description of properties of the species growing in pure culture. It is not possible to determine these properties for phytoplasmas, because they cannot be isolated in pure culture. The convention of 'Candidatus Phytoplasma' species has therefore been adopted as a means to refer to distinct phytoplasma lineages and putative species.

**RFLP, restriction fragment length polymorphism

Table 1. 16S rDNA RFLP group-subgroup classification and 'Candidatus Phytoplasma' species.

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¹Abbreviations are as follows: AY, aster yellows; WB, witches'-broom; Y, yellows; GY, grapevine yellows; Vir, virescence; WL, white leaf.

²Group and subgroup are determined on the basis of RFLP patterns of 1.2 kbp segments of 16S rDNA that are delimited by the annealing sites of PCR primers R16F2n and R16R2. 'Candidatus Phytoplasma' species are distinguished on the basis of 16S rDNA nucleotide sequence homology/identity.

³GenBank accession number of 16S rRNA gene sequence documenting description of the 'Candidatus Phytoplasma' species or strain.

⁴Potentially distinct 'Candidatus Phytoplasma' species names noted by the IRPCM Phytoplasma/Spiroplasma Working Team - Phytoplasma Taxonomy Working Group (2004. International Journal of Systematic and Evolutionary Microbiology 54:1243-1255.). These names have not been formally published.

⁵Recognized in Wei et al. 2007 (International Journal of Systematic and Evolutionary Microbiology 57:1855-1867.) as potential representatives of new 'Candidatus Phytoplasma' species.

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