title: Fitness Impact of Obligate Intranuclear Bacterial Symbionts Depends on Host Growth Phase.
abstract: According to text book definition, parasites reduce the fitness of their hosts whereas mutualists provide benefits. But biotic and abiotic factors influence symbiotic interactions, thus under certain circumstances parasites can provide benefits and mutualists can harm their host. Here we addressed the question which intrinsic biotic factors shape a symbiosis and are crucial for the outcome of the interaction between the obligate intranuclear bacteriumHolospora caryophila(Alphaproteobacteria; Rickettsiales) and its unicellular eukaryotic hostParamecium biaurelia(Alveolata; Ciliophora). The virulence ofH. caryophila, i.e., the negative fitness effect on host division and cell number, was determined by growth assays of severalP. biaureliastrains. The performances of genetically identical lines either infected withH. caryophilaor symbiont-free were compared. Following factors were considered as potentially influencing the outcome of the interaction: (1) host strain, (2) parasite strain, and (3) growth phases of the host. All three factors revealed a strong effect on the symbiosis. In presence ofH. caryophila, theParameciumdensity in the stationary growth phase decreased. Conversely, a positive effect of the bacteria during the exponential phase was observed for several host × parasite combinations resulting in an increased growth rate of infectedP. biaurelia. Furthermore, the fitness impact of the tested endosymbionts on differentP. biaurelialines was not only dependent on one of the two involved strains but distinct for the specific combination. Depending on the current host growth phase, the presence ofH. caryophilacan be harmful or advantageous forP. biaurelia. Thus, under the tested experimental conditions, the symbionts can switch from the provision of benefits to the exploitation of host resources within the same host population and a time-span of less than 6 days.
authors.affiliation: Microbiology, Institute of Biology II, Albert-Ludwigs Universität FreiburgFreiburg, Germany; Zoology-Anthropology Unit, Biology Department, Università di PisaPisa, Italy.
authors.forename: Chiara
authors.initials: C
authors.lastname: Bella
authors.affiliation: Microbiology, Institute of Biology II, Albert-Ludwigs Universität FreiburgFreiburg, Germany; Institute of Hydrobiology, Technische Universität DresdenDresden, Germany.
authors.forename: Lars
authors.initials: L
authors.lastname: Koehler
authors.affiliation: Microbiology, Institute of Biology II, Albert-Ludwigs Universität FreiburgFreiburg, Germany; Institute of Hydrobiology, Technische Universität DresdenDresden, Germany.
authors.forename: Katrin
authors.initials: K
authors.lastname: Grosser
authors.affiliation: Institute of Hydrobiology, Technische Universität Dresden Dresden, Germany.
authors.forename: Thomas U
authors.initials: TU
authors.lastname: Berendonk
authors.affiliation: Zoology-Anthropology Unit, Biology Department, Università di Pisa Pisa, Italy.
authors.forename: Giulio
authors.initials: G
authors.lastname: Petroni
authors.affiliation: Microbiology, Institute of Biology II, Albert-Ludwigs Universität FreiburgFreiburg, Germany; Institute of Hydrobiology, Technische Universität DresdenDresden, Germany.
authors.forename: Martina
authors.initials: M
authors.lastname: Schrallhammer
keywords: Holospora caryophila; Paramecium biaurelia; beneficial effect; context-dependent mutualism; endosymbiont; fitness reduction; parasite; symbiosis