1. School of Molecular and Microbial Biosciences, University of Sydney, Darlington, New South Wales 2006, Australia
2. Roy J. Carver Center for Comparative Genomics, Department of Biological Sciences, University of Iowa, Iowa City, Iowa 52242-1324, USA
3. Biology Centre of the Academy of Sciences of the Czech Republic, Institute of Parasitology, and University of South Bohemia, Faculty of Science, Braniovská 31, 37005 eské Budjovice, Czech Republic
4. Scottish Association for Marine Science, Dunstaffnage Marine Laboratory, Oban, Argyll PA37 1QA, UK
5. Australian Antarctic Division, Kingston, Tasmania 7050, Australia
6. Central Science Laboratory, University of Tasmania, Hobart, Tasmania 7001, Australia
7. School of Aquaculture, University of Tasmania, Launceston, Tasmania 7250, Australia
8. School of Marine and Tropical Biology, James Cook University, Townsville, Queensland 4811, Australia
9. Faculty of Veterinary Science, University of Sydney, Camperdown, New South Wales 2006, Australia
10. Centre for Marine Studies, University of Queensland, St Lucia, Queensland 4072, Australia
11. These authors contributed equally to this work.

Correspondence to: Dee A. Carter¹ Correspondence and requests for materials should be addressed to D.A.C. (Email: d.carter@mmb.usyd.edu.au).

Abstract

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.

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