A PLASTID OF PROBABLE GREEN ALGAL ORIGIN IN APICOMPLEXAN PARASITES

Protozoan parasites of the phylum Apicomplexa contain three genetic el ements: the nuclear and mitochondrial genomes characteristic of virtua lly all eukaryotic cells and a 35-kilobase circular extrachromosomal D NA. In situ hybridization techniques were used to localize the 35-kilo base DNA of Toxoplasma gondii to a discrete organelle surrounded by fo ur membranes. Phylogenetic analysis of the tufA gene encoded by the 35 -kilobase genomes of coccidians T. gondii and Eimeria tenella and the malaria parasite Plasmodium falciparum grouped this organellar genome with cyanobacteria and plastids, showing consistent clustering with gr een algal plastids. Taken together, these observations indicate that t he Apicomplexa acquired a plastid by secondary endosymbiosis, probably from a green alga.