Phylogenetic analysis identifies the invertebrate pathogen Helicosporidium sp. as a green alga (Chlorophyta).

Abstract

Historically, the invertebrate pathogens of the genus Helicosporidium were considered to be either protozoa or fungi, but the taxonomic position of this group has not been considered since 1931. Recently, a Helicosporidium sp., isolated from the blackfly Simulium jonesi Stone & Snoddy (Diptera : Simuliidae), has been amplified in the heterologous host Helicoverpa zea. Genomic DNA has been extracted from gradient-purified cysts. The 18S, 28S and 5.8S regions of the Helicosporidium rDNA, as well as partial sequences of the actin and β-tubulin genes, were amplified by PCR and sequenced.Comparative analysis of these nucleotide sequences was performed using neighbour-joining and maximum-parsimony methods. All inferred phylogenetic trees placed Helicosporidium sp. among the green algae (Chlorophyta), and this association was supported by bootstrap and parsimony jackknife values. Phylogenetic analysis focused on the green algae depicted Helicosporidium sp. as a close relative of Prototheca wickerhamii and Prototheca zopfii (Chlorophyta, Trebouxiophyceae), two achlorophylous, pathogenic green algae. On the basis of this phylogenetic analysis, Helicosporidium sp. is clearly neither a protist nor a fungus, but appears to be the first described algal invertebrate pathogen. These conclusions lead us to propose the transfer of the genus Helicosporidium to Chlorophyta, Trebouxiophyceae.

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

“…1), the two Helicosporidium isolates are depicted as being monophyletic, sister taxa to Prototheca zopfii, and this relationship is supported by maximal bootstrap values. This is consistent with a previously published nuclear 18S rDNA phylogeny (Tartar et al, 2002), which associated the black fly Helicosporidium and Prototheca zopfii.…”

“…The relationships among green algal taxa depicted in Fig. 1 are consistent with affiliations suggested previously by other phylogenetic studies (Bhattacharya & Medlin, 1998;Huss et al, 1999;Nedelcu, 2001;Tartar et al, 2002). First, both classes (Trebouxiophyceae and Chlorophyceae) appear monophyletic.…”

“…The positions of Helicosporidium spp. are identical in phylogenies based on nuclear 18S rRNA genes (Tartar et al, 2002). Similar to the situation observed in the 18S rDNA phylogeny, the branch leading to the Helicosporidium+Prototheca zopfii clade is the longest of the tree, suggesting that this association could be an artifact due to long-branch attraction.…”

“…This plastid has been proposed to derive from an endosymbiotic interaction with a red alga (secondary symbiosis). However, the algal nature of Helicosporidium has already been suggested by morphological observations (Boucias et al, 2001) and strongly supported by phylogenetic analyses inferred from several nuclear genes (Tartar et al, 2002). Therefore, helicosporidial cells are likely to possess a plastid similar to other non-photosynthetic Chlorophyta, derived from a primary endosymbiosis.…”

“…This pellicle eventually splits open and releases either two or four daughter cells. This association between Helicosporidium and Prototheca was surprising, but was later confirmed by molecular sequence comparisons (Tartar et al, 2002). Phylogenetic analyses of several Helicosporidium sp.…”