Description:
<jats:p>Sequences of small-subunit rRNA genes have been obtained for four new isolates of <jats:italic>Entamoeba</jats:italic>. Phylogenetic analyses give new insights into the evolution of these organisms. A novel <jats:italic>Entamoeba</jats:italic> from pigs in Vietnam that produces uninucleate cysts proved to be unrelated to other uninucleated cyst-producing species. Revival of the name <jats:italic>Entamoeba suis</jats:italic> for this organism is proposed. Instead of being related to <jats:italic>Entamoeba polecki</jats:italic>, it shares a recent common ancestor with the non-encysting <jats:italic>Entamoeba gingivalis</jats:italic> in a lineage that is basal to the tetranucleate cyst-producing clade. This suggests that species producing cysts with four nuclei are descended from an ancestor that produced cysts with a single nucleus. An <jats:italic>Entamoeba</jats:italic> from a horse was isolated in culture. No cysts were observed in the original stool sample but the sequence is placed unequivocally within the clade of tetranucleate cyst-producing species with no other sequences being specifically related. Revival of the name <jats:italic>Entamoeba equi</jats:italic> for this organism is proposed. The <jats:italic>Entamoeba ecuadoriensis</jats:italic> sequence was found to be the most closely related to <jats:italic>Entamoeba histolytica</jats:italic> and <jats:italic>Entamoeba dispar</jats:italic>, as predicted, despite the organism having been an environmental isolate originally assigned to <jats:italic>Entamoeba moshkovskii</jats:italic>. Finally, a partial <jats:italic>E. polecki</jats:italic> gene sequence from a pig proved to be virtually identical to that of <jats:italic>Entamoeba struthionis</jats:italic> from an ostrich, suggesting that the latter name is a synonym.</jats:p>