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Phylogenetic positions of phytoplasmas associated with dieback, yellow crinkle and mosaic diseases of papaya, and their proposed inclusion in ‘ Candidatus Phytoplasma australiense’ and a new taxon, ‘Candidatus Phytoplasma australasia’

¹Plant Sciences Group, Primary Industries Research Centre, School of Biological and Environmental Sciences, Central Queensland University, Rockhampton Qld 4702, Australia
²Centre for Bacterial Diversity and Identification, Department of Microbiology, University of Queensland, Brisbane Qld 4072, Australia

Author for correspondence: Daniel T. White. Tel: +61 7 4930 6324. Fax: +61 7 4930 6536. e-mail: whited{at}topaz.cqu.edu.au

ABSTRACT

DNA extracted from three papaya (Carica papaya L.) plants, individually affected by dieback, yellow crinkle or mosaic diseases, was subjected to PCR using phytoplasma-specific primers to amplify the 16S rRNA gene plus 16S–23S rRNA intergenic spacer region. Near-complete DNA sequences obtained for the three PCR amplimers were subjected to phylogenetic analyses and direct sequence comparison with other phytoplasma 16S rDNA and 16S-23S spacer region DNA sequences. The papaya yellow crinkle (PpYC) and papaya mosaic (PpM) sequences were identical to each other, but distinctly different from the papaya dieback (PpDB) sequence, showing 90.3% identity in the 16S rDNA and 87.8% identity in the 16S–23S spacer region DNA sequences. A phylogenetic tree based on 16S rDNA sequences was calculated, in which PpYC and PpM are most closely related to the tomato big bud phytoplasma (TBB; 99.7% 16S rDNA sequence identity) from Australia, within subclade iii. This subclade consists of strains only reported occurring in the Southern Asian region and Australia, which indicates an Asian/Australasian origin. PpDB is most closely related to the Phormium yellow leaf phytoplasma from New Zealand (PYL; 99.9% identity) and the Australian grapevine yellows phytoplasma (AGY; 99.7% identity). These three phytoplasma strains form a distinct clade within subclade xii, which also includes the European strains STOL and VK as another distinct clade. The origin of the closely related but geographically separated AGY-like strains and STOL-like strains of subclade xii is unclear. It is proposed that phytoplasma strains PpDB, PYL and AGY be included in the previously described taxon ‘Candidatus Phytoplasma australiense’, and that PpYC PpM and TBB be assigned to a new taxon, ‘Candidatus Phytoplasma australasia’.

Key Words: phytoplasmas • phylogeny • papaya • 16S rDNA • classification

The EMBL accession numbers for the sequences reported in this paper are Y10095 (PpDB), Y10097 (PpYC) and Y10096 (PpM).

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