that, relative to eubacteria and archaebacteria, the It has been proposed that certain extant anaerobic eukaryotic domain is very deep (Sogin, 1989). Indeed, protozoa are descended from organisms that diverged the number of kingdoms into which eukaryotes have early in eukaryotic evolution prior to the acquisition been subdivided has expanded from the Plantae and of mitochondria. Among these are the extracellular Animalia to include Fungi, Protozoa, Chromista, and parasites Giardia lamblia, Trichomonas vaginalis, Archezoa, according to Cavalier-Smith (1993).
and Entamoeba histolytica, and the obligately intracel-
The Archezoa represent the earliest branches in eulular microsporidia. Phylogenetic analysis of rRNA sekaryotic evolution, prior to the acquisition of mitochonquences from these amitochondrial organisms sugdria by endosymbiosis (Cavalier-Smith, 1987). The four gests that G. lamblia, T. vaginalis, and microsporidia phyla originally included in Archezoa, with representaare near the base of the eukaryotic tree, while E. histotive pathogenic protozoa, were Archamoeba (Entalytica clusters with mitochondria-containing species. moeba histolytica), Metamonada (Giardia lamblia), However, since eukaryotes likely evolved by symbiotic Microsporidia (Encephalitozoon hellem), and Parabasassociations, it is important to analyze other sealia (Trichomonas vaginalis). Subsequently, rRNA sequences which may have independent origins. Unlike quence analysis supported the early origins of these ribosomes, microtubules appear to be unique to euamitochondrial organisms, with the exception that E. karyotes. Complete gene sequences for the -tubulin histolytica diverged later, after several mitochondriasubunit of microtubules from T. vaginalis, E. histolytcontaining protozoa including Euglena gracilis (Sogin, ica, and the microsporidian Encephalitozoon hellem 1989). Consistent with this, two genes encoding prohave recently been determined. Phylogenetic relationteins normally localized in mitochondria have been ships among these, G. lamblia, and 20 additional -tu-
identified in this organism (Clark and Roger, 1995).
bulins were analyzed by distance matrix and parsimony methods, using ␣and ␥-tubulin outgroups. All However, based on the amino acid sequence of translaanalyses placed the E. histolytica sequence at the base tional elongation factor 1α (EF1α), E. histolytica apof the -tubulin evolutionary tree. Similar results were pears to branch earlier than E. gracilis (Bauldauf and obtained for E. histolytica ␣-tubulin using a less repre- Palmer, 1993;Hasegawa et al., 1993). Various other assentative set of sequences. In contrast, the E. hellem pects of its cell biology and metabolism argue for an sequence branched considerably higher, within the early evolutionary origin for E. histolytica (Meza, 1992; lineage containing animal and fungal -tubulins. Pos- Bakker-Grunwald and Wo ¨stmann, 1993). Recently, sible explanations are considered for these unex- Cavalier-Smith (1993) has revised his definition of pected differences between the -tubulin and rRNA Archezoa to include organisms primitively without trees.