title: Unique phylogenetic position of Diplomonadida based on the complete small subunit ribosomal RNA sequence of Giardia ardeae, G. muris, G. duodenalis and Hexamita sp.
abstract: Complete small-subunit rRNA (SSU-rRNA) coding region sequences were determined for two species of the intestinal parasite Giardia: G. ardeae and G. muris, both belonging to the order Diplomonadida, and a free-living member of this order, Hexamita sp. These sequences were compared to published SSU-rDNA sequences from a third member of the genus Giardia, G. duodenalis (often called G. intestinalis or G. lamblia) and various representative organisms from other taxa. Of the three Giardia sequences analyzed, the SSU-rRNA from G. muris is the smallest (1432 bases as compared to 1435 and 1453 for G. ardeae and G. duodenalis, respectively) and has the lowest G+C content (58.9%). The Hexamita SSU-rRNA is the largest in this group, containing 1550 bases. Because the sizes of the SSU-rRNA are prokaryotic rather than typically eukaryotic, the secondary structures of the SSU-rRNAs were constructed. These structures show a number of typically eukaryotic signature sequences. Sequence alignments based on constraints imposed by secondary structure were used for construction of a phylogenetic tree for these four taxa. The results show that of the four diplomonads represented, the Giardia species form a distinct group. The other diplomonad Hexamita and the microsporidium Vairimorpha necatrix appear to be distinct from Giardia.
authors.affiliation: Department of Biology, Cleveland State University, Ohio 44115.
authors.forename: H
authors.initials: H
authors.lastname: van Keulen
authors.forename: R R
authors.initials: RR
authors.lastname: Gutell
authors.forename: M A
authors.initials: MA
authors.lastname: Gates
authors.forename: S R
authors.initials: SR
authors.lastname: Campbell
authors.forename: S L
authors.initials: SL
authors.lastname: Erlandsen
authors.forename: E L
authors.initials: EL
authors.lastname: Jarroll
authors.forename: J
authors.initials: J
authors.lastname: Kulda
authors.forename: E A
authors.initials: EA
authors.lastname: Meyer
mesh_terms: D000818:Animals; D001483:Base Sequence; D004275:DNA, Ribosomal / Q000145:classification / Q000235:genetics*; D016828:Diplomonadida / Q000145:classification* / Q000235:genetics; D005872:Giardia / Q000145:classification / Q000235:genetics; D008969:Molecular Sequence Data; D009690:Nucleic Acid Conformation; D010802:Phylogeny; D012337:RNA, Ribosomal, 18S / Q000145:classification / Q000235:genetics*; D017422:Sequence Analysis, DNA; D012689:Sequence Homology, Nucleic Acid
chemical_list: D004275:DNA, Ribosomal; D012337:RNA, Ribosomal, 18S