Phylogenetic Relationship among the Malaria Parasites Based on Small Subunit rRNA Gene Sequences: Monophyletic Nature of the Human Malaria Parasite,Plasmodium falciparum1

Abstract

We analyzed the small subunit ribosomal RNA (SSUrRNA) gene sequences from 13 malaria species parasitic to humans, chimpanzees/gorillas, Old World monkeys, rodents, birds, and lizards in order to reconstruct the phylogenetic relationships among thePlasmodiumspecies. The SSUrRNA genes ofPlasmodium vivaxandP. ovalewere sequenced by the dideoxy method in our laboratory; other sequences were retrieved from GenBank. These sequences were aligned with the SSUrRNA gene sequence of outgroup species,ParameciumandToxoplasma. After gaps and ambiguous regions were deleted, the aligned sequences were used for phylogenetic analysis by maximum likelihood and distance methods. The tree defines two major clades, the first with the bird and reptile parasites, the second with the rest of the species. The two bird parasites,P. gallinaceumandP. lophurae,do not closely cluster with human, chimpanzee/gorilla, Old World monkey, or rodent parasites, but cluster with the lizard parasites.P. vivaxclusters with three Old World monkey parasites,P. cynomolgi, P. fragile,andP. knowlesiin decreasing order of closeness.P. ovale,while in a separate clade, is more closely related toP. vivaxthan toP. malarieorP. falciparum. P. malariaeandP. bergheido not closely cluster with any of the other clades or with each other. Statistical analysis proves that the placement ofP. falciparumin the bird malaria clade is less likely than in the mammalian malaria clade. Our analysis reveals that: (1) human malaria parasites have an evolutionary independent origin; (2)P. falciparumis most closely related toP. reichenowiand did not arise from lateral transfer of a bird parasite, as was previously suggested; and (3) the lizard malaria parasites are true members of the genusPlasmodium.