Nuclear-encoded, plastid-targeted genes suggest a single common origin forapicomplexan and dinoflagellate plastids

The phylum Apicomplexa encompasses a large number of intracellular protozoa n parasites, including the causative agents of malaria (Plasmodium), toxopl asmosis (Toxoplasma), and many other human and animal diseases. Apicomplexa have recently been found to contain a relic, nonphotosynthetic plastid tha t has attracted considerable interest as a possible target for therapeutics . This plastid is known to have been acquired by secondary endosymbiosis, b ut when this occurred and from which type of alga it was acquired remain un certain. Based on the molecular phylogeny of glyceraldehyde-3-phosphate deh ydrogenase (GAPDH) genes, we provide evidence that the apicomplexan plastid is homologous to plastids found in dinoflagellates-close relatives of apic omplexa that contain secondary plastids of red algal origin. Surprisingly, apicomplexan and dinoflagellate plastid-targeted GAPDH sequences were also found to be closely related to the plastid-targeted GAPDH genes of heteroko nts and cryptomonads, two other groups that contain secondary plastids of r ed algal origin. These results address several outstanding issues: (1) apic omplexan and dinoflagellate plastids appear to be the result of a single en dosymbiotic event which occurred relatively early in eukaryotic evolution, also giving rise to the plastids of heterokonts and perhaps cryptomonads; ( 2) apicomplexan plastids are derived from a red algal ancestor; and (3) the ancestral state of apicomplexan parasites was photosynthetic.