Submitted to: International Journal of Systematic and Evolutionary Microbiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/19/2002
Publication Date: 3/28/2003
Citation: Tartar, A., Boucias, D.G., Becnel, J.J., Adams, B.J. 2003. Comparison of plastid 16s rdna (rrn16) genes from helicosporidium spp.: evidence supporting the reclassification of helicosporidia as green algae (chlorophyta). International Journal of Systematic and Evolutionary Microbiology. 53:1719-1723. Interpretive Summary: Naturally occurring protozoan parasites of insects are under study to evaluate and develop these disease causing organisms as biological control agents. A new species of Helicosporidia has been found in black flies in Florida. Helicosporida are known from a number of arthropods, but fundamental knowledge on their life cycles, modes of transmission and taxonomic placement are incomplete. In this investigation we have conducted molecular analysis of this Helicosporidium sp. and determined that it may represent a unique group of organisms.
Technical Abstract: The Helicosporidia are invertebrate pathogens that have recently been identified as being non-photosynthetic green algae (Chlorophyta). In order to confirm the algal nature of the genus Helicosporidium, the presence of a retained chloroplast genome in Helicosporidia cells was investigated. Fragments homologous to plastid 16S rDNA (rrn16) genes were successfully amplified from cellular DNA extracted from two different Helicosporidium isolates. The fragment sequences are 1269 and 1266 bp long, are very AT-rich (60.7%), and are similar to homologous genes sequenced from non-photosynthetic green algae. Maximum-parsimony, maximum-likelihood and neighbor-joining methods were used to infer phylogenetic trees from a rrn16 sequence alignment. All trees depicted the Helicosporidia as sister taxa to the non-photosynthetic, pathogenic alga Prototheca zopfii. Moreover, the trees identified Helicosporidium spp. as members of a clade that included the heterotrophic species Prototheca spp. and the mesotrophic species Chlorella protothecoides. The clade is always strongly supported by bootstrap values, suggesting that all these organisms share a most recent common ancestor. Phylogenetic analyses inferred from plastid 16S rDNA genes confirmed that the Helicosporidia are non-photosynthetic green algae, close relatives to the genus Prototheca (Chlorophyta, Trebouxiophyceae). Such phylogenetic affinities suggest that Helicosporidium spp. are likely to possess Prototheca-like organelles and organelle genomes.