SMALL SUBUNIT RIBOSOMAL RNA-INFLATA AND THE QUEST FOR THE 1ST BRANCH IN THE EUKARYOTIC TREE( OF HEXAMITA)

A phylogenetic analysis of the small subunit ribosomal RNA (16S-like r RNA) coding region from Hexamita inflata demonstrates that parasitism alone cannot explain early diverging eukaryotic lineages. Parasitic an d free-living diplomonads, as well as trichomonads and microsporidia, diverge at the base of the eukaryotic tree. The relative branching ord er of diplomonads, trichomonads and microsporidia is influenced by out lying prokaryotic taxa with different G + C compositions in their rRNA coding regions. The high G + C prokaryotes position Giardia lamblia a t the base of the eukaryotic tree but split diplomonads into a paraphy letic group. When the outlying groups are restricted to rRNAs with nom inal G + C compositions, diplomonads form a monophyletic group that di verged after the microsporidia and trichomonads. This unstable branchi ng pattern correlates with unusual nucleotide compositions in the rRNA s of G. lamblia (75% G + C) and Vairimorpha necatrix (35% G + C). In c ontrast, the 51% G + C composition of the H. inflata rRNA is typical o f other eukaryotic rRNAs. Its divergence after trichomonads is strongl y supported by bootstrap replicates in distance analyses that do not i nclude G. lamblia. Because of a low G + C composition in its rRNA codi ng region, the phylogenetic placement of V. necatrix is uncertain and the identity of the deepest branching eukaryotic lineage is ambiguous.