title: Endosymbiosis in statu nascendi: close phylogenetic relationship between obligately endosymbiotic and obligately free-living Polynucleobacter strains (Betaproteobacteria).
abstract: Bacterial strains affiliated to the phylogenetically shallow subcluster C (PnecC) of the Polynucleobacter cluster, which is characterized by a minimal 16S rRNA gene sequence similarity of approximately 98.5%, have been reported to occur as obligate endosymbionts of ciliates (Euplotes spp.), as well as to occur as free-living cells in the pelagic zone of freshwater habitats. We investigated if these two groups of closely related bacteria represent strains fundamentally differing in lifestyle, or if they simply represent different stages of a facultative endosymbiotic lifestyle. The phylogenetic analysis of 16S rRNA gene and 16S-23S ITS sequences of five endosymbiont strains from two different Euplotes species and 40 pure culture strains demonstrated host-species-specific clustering of the endosymbiont sequences within the PnecC subcluster. The sequences of the endosymbionts showed characteristics indicating an obligate endosymbiotic lifestyle. Cultivation experiments revealed fundamental differences in physiological adaptations, and determination of the genome sizes indicated a slight size reduction in endosymbiotic strains. We conclude that the two groups of PnecC bacteria represent obligately free-living and obligately endosymbiotic strains, respectively, and do not represent different stages of the same complex life cycle. These closely related strains occupy completely separated ecological niches. To our best knowledge, this is the closest phylogenetic relationship between obligate endosymbionts and obligately free-living bacteria ever revealed.
authors.affiliation: Department of Biology-Protistology and Zoology Unit, University of Pisa, Via A. Volta 4/6, I-56126 Pisa, Italy.
authors.forename: Claudia
authors.initials: C
authors.lastname: Vannini
authors.forename: Matthias
authors.initials: M
authors.lastname: Pöckl
authors.forename: Giulio
authors.initials: G
authors.lastname: Petroni
authors.forename: Qinglong L
authors.initials: QL
authors.lastname: Wu
authors.forename: Elke
authors.initials: E
authors.lastname: Lang
authors.forename: Erko
authors.initials: E
authors.lastname: Stackebrandt
authors.forename: Martina
authors.initials: M
authors.lastname: Schrallhammer
authors.forename: Paul M
authors.initials: PM
authors.lastname: Richardson
authors.forename: Martin W
authors.initials: MW
authors.lastname: Hahn
mesh_terms: D000818:Animals; D001482:Base Composition; D042521:Burkholderiaceae / Q000145:classification* / Q000235:genetics / Q000502:physiology; D021903:DNA, Ribosomal Spacer / Q000737:chemistry / Q000145:classification; D016812:Euplotes / Q000382:microbiology; D005618:Fresh Water / Q000382:microbiology; D016680:Genome, Bacterial; D017404:In Situ Hybridization, Fluorescence; D008969:Molecular Sequence Data; D010802:Phylogeny; D012336:RNA, Ribosomal, 16S / Q000737:chemistry / Q000145:classification; D017422:Sequence Analysis, DNA; D013559:Symbiosis
chemical_list: D021903:DNA, Ribosomal Spacer; D012336:RNA, Ribosomal, 16S