Comparison of plastid 16S rRNA (rrn16) genes from Helicosporidium spp.: evidence supporting the reclassification of Helicosporidia as green algae (Chlorophyta)

Abstract

The Helicosporidia are invertebrate pathogens that have recently been identified as 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 rRNA (rrn16) genes were amplified successfully 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 neighbour-joining methods were used to infer phylogenetic trees from an 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 rRNA genes confirmed that the Helicosporidia are non-photosynthetic green algae, close relatives of the genus Prototheca (Chlorophyta, Trebouxiophyceae). Such phylogenetic affinities suggest that Helicosporidium spp. are likely to possess Prototheca-like organelles and organelle genomes.

The endosymbiotic origin, diversification and fate of plastids

“…In another previously enigmatic lineage of parasites, the Helicosproidia, a similar story has unfolded: while these were previously believed to be related to parasites such as Apicomplexa, molecular phylogenetic analysis showed they are in fact green algae (Tartar et al 2002). This led to the suggestion that they contain a cryptic plastid, the presence of which was confirmed by identifying the plastid genome (Tartar et al 2003;de Koning & Keeling 2006), and several nuclear genes for plastid-targeted proteins (de Koning & Keeling 2004). Despite these advances, the organelle itself has yet to be identified.…”

Phylogeny and Molecular Evolution of the Green Algae

“…The non-photosynthetic, parasitic Prototheca and Helicosporidium are also included in the Chlorellales based on nuclear and chloroplast gene data (Huss & Sogin, 1990;Tartar et al, 2003;Ueno et al, 2003;de Koning et al, 2005;Ueno et al, 2005). Although plastids are not apparent in Helicosporidium cells, molecular evidence indicates that it maintains a functional plastid genome (Tartar et al, 2003;de Koning & Keeling, 2004;Tartar & Boucias, 2004;de Koning & Keeling, 2006). Diversity and phylogenetic relationships within the Chlorellales have been well studied Krienitz et al, 2003;Henley et al, 2004;Zhang et al, 2008;Luo et al, 2010;Pazoutova et al, 2010;Pröschold et al, 2010).…”

Nucleus-Encoded Genes for Plastid-Targeted Proteins in Helicosporidium : Functional Diversity of a Cryptic Plastid in a Parasitic Alga

“…Recently, however, the first molecular data from this group were characterized and, surprisingly, showed Helicosporidium to be a trebouxiophyte green alga that is closely related to another enigmatic parasite, Prototheca (38). Recent descriptions of a plastid-like small subunit rRNA (39) and a fragment containing the elongation factor Tu and ribosomal protein genes (37) in Helicosporidium confirmed that, like its green algal relations, this nonphotosynthetic parasite possesses a plastid genome. The metabolic role of the Helicosporidium plastid is of considerable comparative interest because it is a primary plastid of green algal ancestry, while the cryptic plastid of Plasmodium is a secondary plastid derived from the red algal lineage.…”