PLASTIDS ARE WIDESPREAD AND ANCIENT IN PARASITES OF THE PHYLUM APICOMPLEXA

Current evidence supports the presence of a non-photosynthetic chlorop last-like organelle in several apicomplexan parasites, including Plasm odium falciparum and Toxoplasma gondii. This apicomplexan organelle, r eferred to here as the ''plastid'', may have been acquired through a p rimary or secondary endosynmbiosis of a photosynthetic organism. Alter natively, apicomplexan plastids may have been acquired through several independent endosymbiotic events, as appears to be the case for the a cquisition of chloroplasts by dinoflagellates. The likelihood of multi ple origins of an apicomplexan plastid is enhanced by the close evolut ionary relatedness of apicomplexan and dinoflagellate taxa. In this st udy, we have tested the hypothesis that apicomplexan plastids are deri ved from a single ancient ancestor. Two lines of evidence supporting t his hypothesis are presented. First, this study supports the widesprea d presence of plastid DNA in apicomplexan species. Second, the topolog ies of the phylogenetic trees derived from plastid and nuclear-encoded rRNA gene sequences suggest the co-evolution of the DNAs localised in these two compartments. Taken together, these data support a single a ncient lineage for the plastids of parasites in the phylum Apicomplexa . (C) 1998 Australian Society for Parasitology. Published by Elsevier Science Ltd. All rights reserved.