title: Amoeba stages in the deepest branching heteroloboseans, including Pharyngomonas: evolutionary and systematic implications.
abstract: The taxon Heterolobosea (Excavata) is a major group of protists well known for its diversity of life stages. Most are amoebae capable of transforming into flagellates (amoeboflagellates), while others are known solely as flagellates or solely as amoebae. The deepest-branching heterolobosean taxon confirmed previously, Pharyngomonas, was generally assumed to be a pure flagellate, suggesting that the amoeba form arose later in the evolution of Heterolobosea sensu lato. Here we report that multiple isolates of Pharyngomonas are actually amoeboflagellates that also have cyst stages, with only amoebae transforming into cysts. The amoeba form of Pharyngomonas showed heterolobosean characteristics (e. g. eruptive movement), but also possessed unusual morphological features like slow-flowing crenulated hyaline crescents with conical subpseudopodia, finger-like projections and branching posterior extensions. Furthermore, phylogenetic analyses of 18S ribosomal RNA gene sequences that included two undescribed species of amoebae showed that Pharyngomonas is not the only deep-branching heterolobosean to possess an amoeba stage. These results suggest that possession of an amoeba stage was ancestral for Heterolobosea, unifying this taxon as a group of species with amoeba stages in their lifecycle or derived from organisms with such stages.
authors.affiliation: Department of Biochemistry and Molecular Biology, Dalhousie University, Halifax, Nova Scotia, Canada.
authors.forename: Tommy
authors.initials: T
authors.lastname: Harding
authors.forename: Matthew W
authors.initials: MW
authors.lastname: Brown
authors.forename: Andrey
authors.initials: A
authors.lastname: Plotnikov
authors.forename: Elena
authors.initials: E
authors.lastname: Selivanova
authors.forename: Jong Soo
authors.initials: JS
authors.lastname: Park
authors.forename: John H
authors.initials: JH
authors.lastname: Gunderson
authors.forename: Manuela
authors.initials: M
authors.lastname: Baumgartner
authors.forename: Jeffrey D
authors.initials: JD
authors.lastname: Silberman
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Roger
authors.forename: Alastair G B
authors.initials: AG
authors.lastname: Simpson
mesh_terms: D016000:Cluster Analysis; D016054:DNA, Protozoan / Q000737:chemistry / Q000235:genetics; D004275:DNA, Ribosomal / Q000737:chemistry / Q000235:genetics; D056890:Eukaryota / Q000145:classification* / Q000166:cytology* / Q000235:genetics / Q000502:physiology; D005407:Flagella / Q000502:physiology; D020459:Genes, rRNA; D008124:Locomotion; D008853:Microscopy; D008969:Molecular Sequence Data; D010802:Phylogeny; D016053:RNA, Protozoan / Q000235:genetics; D012337:RNA, Ribosomal, 18S / Q000235:genetics; D017422:Sequence Analysis, DNA; D033761:Spores, Protozoan / Q000166:cytology
chemical_list: D016054:DNA, Protozoan; D004275:DNA, Ribosomal; D016053:RNA, Protozoan; D012337:RNA, Ribosomal, 18S