Amoeba at attention: phylogenetic affinity of Sappinia pedata.

The genus Sappinia, a taxon of free-living amoebae with trophozoites that typically have two closely appressed nuclei, contains two named species, Sappinia pedata, the type species, and S. diploidea. The amoebae of both species are essentially identical according to the literature. The two species are distinguished by S. pedata having a standing amoeba stage, incorrectly interpreted as a cyst, and S. diploidea having sessile, bicellular cysts. Using four isolates of S. pedata collected from around the world, we present detailed light micrographic illustrations of all stages of its life cycle. We confirm that the standing amoeba lacks a cell wall. In two isolates of S. pedata, there are bicellular cysts indistinguishable from those of S. diploidea. Using sequence data from the nuclear small subunit ribosomal RNA gene, we conclude that S. pedata and the published neotype of S. diploidea are congeneric but not conspecific. The genus branches within Thecamoebidae. Sequencing of the actin gene confirms the inclusion of Sappinia in Thecamoebidae. Resolving the taxonomy of Sappinia is gaining importance because it has recently been attributed as an opportunistic human pathogen.