Chromera velia as a model to study evolution of parasitism

Apicomplexan parasites cause harmful diseases of animals including humans. These parasites includes also the most devastating ones, such as Plasmodium, a causative agent of malaria and Toxoplasma gondii causing numerous latent infections of humans with proposed impact to their behavior. It was shown that apicomplexans contain secondary non-photosynthetic plastid in their heterotrophic cells, suggesting their origin in a phototrophic alga. 

The team leader Miroslav Oborník initiated research in frame of MODBIOLIN on the new group of symbiotic algae, which was recently isolated from stony corals in Australia and named chromerids. Two chromerid species have been described so far, Chromera velia and Vitrella brassicaformis. "We have participated on isolation of the alga and extensive studies on its life cycle, morphology, ultrastructure, and plastid, mitochondrial and nuclear genomes; we have also investigated the synthesis of tetrapyrroles in C. velia. We have shown that in C. velia only asexual life was found, consisting of vegetative cells, autosporangia with autospores and zoosporangia containing motile bi-flagellated zoospores."

Plastid genome displaying linear topology is highly derived and it encodes large and divergent genes. Two of the genes were split into two genes, transcripts and proteins which are, however, assembled into functional photosystems. Mitochondrial genome is placed on heterogeneous linear molecules and codes only for two protein coding genes (cox1 and cox 3) and truncated rRNA fragments. Consequently, the respiratory complexes I and III are missing from the respiratory chain of C. velia.

Investigation of the nuclear genome showed very high number of gene losses on the pathway from a phototrophic alga to the obligatory parasitic Apicomplexa (over 3800 genes) and very few acquisitions (approximately 80) suggesting that phototrophic ancestor already contained in its genome most of the genes used later for parasitism. The tetrapyrrole pathway is non-canonical and resemble that in Apicomplexa by use of aminolevulinate synthase in the mitochondrion for aminolevulinic acid synthesis.

Studies on C. velia provide essential information to understand how phototrophic alga evolved into an obligate parasite.

Author: Prof. Ing. Miroslav Oborník, PhD.

We acknowledge the use of research infrastructure that has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement n° 316304.

This issue is processed eg. in:

Oborník M., Modrý D., Lukeš M., Černotíková-Stříbrná E., Cihlář J., Tesařová M., Kotabová E., Vancová M., Prášil O., Lukeš J. (2012) Morphology, Ultrastructure and Life Cycle of Vitrella brassicaformis n. sp., n. gen., a Novel Chromerid from the Great Barrier Reef. Protist 163, 306-323

Oborník M. & Lukeš J. (2013) Cell biology of chromerids, the autotrophic relatives to apicomplexan parasites. Int. Rev. Cell. Mol. Biol. 306, 333-369.

Janouškovec J., Sobotka R., Lai D.-H., Flegontov P., Koník P., Komenda J., Ali S., Prášil O., Pain A., Oborník M., Lukeš J., Keeling P.J. (2013) Split photosystem protein, linear-mapping topology and growth of structural complexity in the plastid genome of Chromera velia Molecular Biology and Evolution 30, 2447-2462.

Oborník M. & Lukeš J. (2015) The organellar genomes of Chromera and Vitrella, the phototrophic  relatives of apicomplexan parasites. Annu. Rev. Microbiol. (in press).