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The Apicomplexa

The Apicomplexa have in common with dinoflagellates and ciliates (e.g., Tetrahy-mena, Paramecium) a subpellicular network of flattened vesicles ("alveoli") that are the ultrastructural justification for their inclusion in a common protozoan clade and the inspiration for the grouping name, Alveolata. This structural taxonomy is supported by phylogenetic analysis of small subunit rRNA, as well as combined protein sequence phylogenies. The evolutionary origin of the Apicomplexa within the Alveolata is unknown, but it is thought that they share ancestry with dinofla-gellate parasites of marine polychaetes. All apicomplexans are parasitic and possess a unique apical secretory complex and specialized secretory organelles, termed rhoptries and micronemes, mediating locomotion, tissue disruption, and invasion of target host cells. Evolutionary landmarks in the radiation of the Apicomplexa remain speculative but resulted in specialization to parasites of invertebrate, vertebrate, and insect hosts, as well as complex life cycles involving multiple hosts, including insect transmission vectors. The Apicomplexa can be generally classified into five groups: eugregarines, haemogregarines, haemosporines

(e.g., Plasmodium, Haemoproteus), eimeriorines (the coccidia, e.g., Toxoplasma, Sar-cocystis, Eimeria), and piroplasms (e.g., Babesia, Theileria).

As complete apicomplexan genome sequences are acquired and incorporated into comparative analyses, a better understanding will be acquired of the evolutionary adaptations and functional relationships of the apicomplexans, such as metabolic pathways associated with parasitic niches, the molecular basis of different host cell invasion mechanisms and tissue tropisms (e.g., hepatocytes, erythrocytes, and insect tissues in Plasmodium versus lymphocytes and tick tissues in Theileria), and an understanding of differentially present cellular structures such as flagellated gametes (present in Plasmodium but not in Cryptosporidium), parasi-tophorous vacuole (absent in Theileria), and extracellular cyst stages (present in Cryptosporidium and Toxoplasma but not in Plasmodium). The genome sequences for Plasmodium sp. and Cryptosporidium sp. are currently complete and it is anticipated that the next year or two will see the completion of genome sequences for Toxoplasma gondii, an opportunistic pathogen causing cerebral toxoplasmosis in immunocompromised individuals, and the cattle pathogen Theileria annulata, as well as largely complete DNA sequencing data for the economically important chicken pathogen Eimeria sp. In addition, the pending availability of complete genome sequence information for the alveolate Ciliophora Tetrahymena will hopefully shed light on evolutionary adaptations having origins in the distinction between free-living and parasitic lifestyles. Following are thumbnail sketches of genome projects for the important apicomplexan human pathogens Plasmodium, Cryptosporidium, and Toxoplasma.

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