The plastid in Apicomplexa: what use is it?

An extrachromosomal genome of between 27 and 35 kb has been described in several apicomplexan parasites including Plasmodium falciparum and Toxoplasma gondii. Examination of sequence data proved the genomes to be a remnant plastid genome, from which all genes encoding photosynthetic functions had been lost. Localisation studies had shown that the genome was located within a multi-walled organelle, anterior to the nucleus. This organelle had been previously described in ultrastructural studies of several genera of apicomplexa, but no function had been attributed to it. This invited review describes the evolution of knowledge on the apicomplexan plastid, then discusses current research findings on the likely role of the plastid in the Apicomplexa. How the plastid may be used to effect better drug treatments for apicomplexan diseases, and its potential as a marker for investigating phylogenetic relationships among the Apicomplexa, are discussed.