EVOLUTIONARY RELATEDNESS OF SOME PRIMATE MODELS OF PLASMODIUM

Primate-and, specifically, monkey-malaria infections are commonly used for understanding the pathology of and immune response to the human d isease because they are thought to resemble most closely the host-para site relationship found in humans. Plasmodium cynomolgi is used extens ively as a model for the human parasite, P. vivax, and P. knowlesi is used primarily as a model for the development of erythrocytic-stage va ccines. Both of these simian parasites can naturally infect man, resul ting in mildly symptomatic episodes of the disease. The phylogenetic r elationship between these two simian parasites and previously characte rized Plasmodium species, including P. vivax, was examined by comparis on of the asexually expressed small-subunit ribosomal RNA genes. Our a nalysis confirmed that P. vivax is most closely related to P. cynomolg i and that it remains an appropriate model of the human pathogen. Furt hermore, with P. knowlesi and P. fragile, these two species form a gro up of closely related species, distant from other Plasmodium species. What is considered to be the most ancient of the human malaria pathoge ns, P. malariae, was also included in the analysis and does not group at all with other simian or human parasites.