title: Long-term selection experiment produces breakdown of horizontal transmissibility in parasite with mixed transmission mode.
abstract: Evolutionary transitions from parasitism toward beneficial or mutualistic associations may encompass a change from horizontal transmission to (strict) vertical transmission. Parasites with both vertical and horizontal transmission are amendable to study factors driving such transitions. In a long-term experiment, microcosm populations of the protozoan Paramecium caudatum and its bacterial parasite Holospora undulata were exposed to three growth treatments, manipulating vertical transmission opportunities over ca. 800 host generations. In inoculation tests, horizontal transmission propagules produced by parasites from a "high-growth" treatment, with elevated host division rates increasing levels of parasite vertical transmission, showed a near-complete loss of infectivity. A similar reduction was observed for parasites from a treatment alternating between high growth and low growth (i.e., low levels of population turn-over). Parasites from a low-growth treatment had the highest infectivity on all host genotypes tested. Our results complement previous findings of reduced investment in horizontal transmission and increased vertical transmissibility of high-growth parasites. We explain the loss of horizontal transmissibility by epidemiological feedbacks and resistance evolution, reducing the frequency of susceptible hosts in the population and thereby decreasing the selective advantage of horizontal transmission. This illustrates how environmental conditions may push parasites with a mixed transmission mode toward becoming vertically transmitted nonvirulent symbionts.
authors.affiliation: Institute of Hydrobiology, Technische Universität Dresden, Germany; Institut des Sciences de l'Evolution - UMR 5554, Université de Montpellier, Place Eugène Bataillon, 34095, Montpellier, Cedex 05, France. eike.dusi@tu-dresden.de.
authors.forename: Eike
authors.initials: E
authors.lastname: Dusi
authors.forename: Claire
authors.initials: C
authors.lastname: Gougat-Barbera
authors.forename: Thomas U
authors.initials: TU
authors.lastname: Berendonk
authors.forename: Oliver
authors.initials: O
authors.lastname: Kaltz
mesh_terms: D005075:Biological Evolution; D005838:Genotype; D042281:Holosporaceae / Q000472:pathogenicity*; D054884:Host-Pathogen Interactions / Q000235:genetics*; D048488:Paramecium caudatum / Q000235:genetics / Q000382:microbiology*; D010641:Phenotype; D012641:Selection, Genetic
keywords: Horizontal transmission; host density; infectivity; loss of function; resistance; vertical transmission