Origin and evolution of plastid-like organella in Apicomplexa

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In order to estimate origin and evolution of the apicoplast, a plastid-like organelle that is usually found in most of apicomplexan parasites, and to gain more insights into the phylogenetic relationships concerning the phylum Apicomplexa, we cloned and sequenced genes encoding ribosomal RNAs, elongation factors, isoleucyl-and valyl-tRNA synthetases from diverse taxonomic lineages. These include Apicomplexa (Plasmodium, Toxoplasma, Babesia, Cryptosporidiuni), stramenopiles (Blastocystis, Proterornonas), Euglenozoa (Thypanosoma), Amoebozoa (Entamoeba, Mastigamoeba), and Microsporidia (Glugea, Encephalitozoon). Using these data in combination with related sequences in the public databases, detailed phylogenetic analyses were carried out on each of the individual genes by the maximum likelihood method of molecular phylogeny. The estimated log-likelihoods derived from the analyses of individual genes were summed up and combined phylogeny was performed in a framework of the maximum likelihood approach. Based on these analyses the following phylogenetic findings have been obtained up to now: (1)The genus Cryptosporidiurn is the earliest offshoot of Apicomplexa; (2) Plasmodium coatneyi, the phylogenetic position of which had been unknown in the genus Piasmodiuni (malaria 'parasites), is close to P vivax and other simian parasites;(3)The closest relative of the phylum Apicomplexa is dianofiagellates, and the genus Ciliophora shares a sister group' position to the common ancestor of Apicomplexa and dianoflagellates; (4)Monophyly of the higher-order lineage, Alveolata, including Apicomplexa, dianofiagellates, and Ciliophora is supported by several genes; (5)Alveolata is close to the other higher-order lineage, stramenopiles; (6)The common ancestor of Alveolata and strarnenopiles is close to Euglenozoa or to Viiridiplantae in the tree of Eukaryota.