A photosynthetic alveolate closely related to apicomplexan parasites

Many parasitic Apicomplexa, such as Plasmodium falciparum , contain an unpigmented chloroplast remnant termed the apicoplast, which is a target for malaria treatment. However, no close relative of apicomplexans with a functional photosynthetic plastid has yet been described. Here we describe a newly cultured organism that has ultrastructural features typical for alveolates, is phylogenetically related to apicomplexans, and contains a photosynthetic plastid. The plastid is surrounded by four membranes, is pigmented by chlorophyll a , and uses the codon UGA to encode tryptophan in the psbA gene. This genetic feature has been found only in coccidian apicoplasts and various mitochondria. The UGA-Trp codon and phylogenies of plastid and nuclear ribosomal RNA genes indicate that the organism is the closest known photosynthetic relative to apicomplexan parasites and that its plastid shares an origin with the apicoplasts. The discovery of this organism provides a powerful model with which to study the evolution of parasitism in Apicomplexa. An organelle with history The apicomplexans are protozoan parasites of animals, including the pathogens causing malaria, toxoplasmosis and other human diseases. Most apicomplexans contain an unpigmented chloroplast remnant — the apicoplast — that is essential for the parasite's survival. It is thought that photosynthetic genes were lost during organelle evolution, but no descendant of a 'photosynthetic apicomplexan' with a photosynthetic plastid was known. Now an organism isolated from a stony coral in Sydney Harbour comes close to fitting that bill. The alga lives in association with corals, but can be cultivated as a free-living organism. It is phylogenetically related to Apicomplexa and its chloroplast features a genetic novelty — the presence of photosynthetic genes in combination with the use of the UGA codon to encode tryptophan — the latter being characteristic of apicoplasts. The first apicomplexan, Chromera velia , with a functional photosynthetic plastid has been isolated and characterized, providing insight into early apicomplexan evolution.