Submitted to: International Organization for Mycoplasmology
Publication Type: Abstract Only
Publication Acceptance Date: 5/3/2004
Publication Date: 7/11/2004
Citation: Zhao, Y., Davis, R.E., Lee, I. 2004. Phylogenetic position of phytoplasmas as inferred from multiple sets of concatenated indispensable proteins. International Organization for Mycoplasmology. p. 127-128.
Technical Abstract: Phytoplasmas are cell wall-less, nonculturable bacteria that cause diseases in more than 300 vegetable, ornamental, and perennial plant species representing over one hundred families. These phytopathogenic agents are restricted to sieve cells of phloem tissue and are transmitted from diseased to healthy plants by insect vectors, mainly leafhoppers and psyllids. Phytoplasmas are classified as Mollicutes and have been thought to be derived from ancestral low G+C Gram-positive bacteria, possibly some ancient members of the Bacillus/Clostridium group through retrogressive evolution and genome reduction. Since phytoplasmas parasitize both plant hosts and insect vectors, knowledge of phytoplasma evolution is important to understand their parasitic life style and pathogenicity. The availability of the first complete and several partial phytoplasma genome sequences enabled us to investigate the evolutionary relationship between phytoplasmas and other micro-organisms, especially gram-positive bacteria, using a comparative genomics approach. In this study, more than 50 proteins that are ubiquitous to all bacteria were selected to build a consensus tree. These selected proteins are evolutionarily conserved and are involved in fundamental life processes such as DNA replication/repair, RNA transcription, protein synthesis, protein translocation, and carbohydrate metabolism. These proteins were concatenated into five superproteins according to their functional categories and phylogenetic trees were reconstructed based on the five superproteins using both Maximum likelihood and parsimony methods. A total of 40 bacteria species including two phytoplasmas, eight other mollicutes, and twenty-one low G+C and nine high G+C gram-positive bacteria, of which the complete or partial genome sequences are available in the public domain, were used for the analysis. Our results showed that the primary topologies of the five trees are congruent in that phytoplasmas are clustered together with other Mollicutes, forming a monophyletic group. In all five trees, the Bacillaceae, along with Lactobacillales, is paraphyletic to mollicutes, rather than being ancestral. Thus, it appears that an unknown common ancestor gave rise to two sister clades, Bacillus/Mollicutes and Clostridia. The results of this study support the concept that phytoplasmas and other Mollicutes may have evolved earlier than previously thought. Our results also revealed possible events of horizontal gene acquisition in early stages of mollicute evolution.