title: Negative frequency-dependent selection is intensified at higher population densities in protist populations.
abstract: Natural populations of free-living protists often exhibit high-levels of intraspecific diversity, yet this is puzzling as classic evolutionary theory predicts dominance by genotypes with high fitness, particularly in large populations where selection is efficient. Here, we test whether negative frequency-dependent selection (NFDS) plays a role in the maintenance of diversity in the marine flagellate Oxyrrhis marina using competition experiments between multiple pairs of strains. We observed strain-specific responses to frequency and density, but an overall signature of NFDS that was intensified at higher population densities. Because our strains were not selected a priori on the basis of particular traits expected to exhibit NFDS, these data represent a relatively unbiased estimate of the role for NFDS in maintaining diversity in protist populations. These findings could help to explain how bloom-forming plankton, which periodically achieve exceptionally high population densities, maintain substantial intraspecific diversity.
authors.affiliation: Department of Biology, University of York, Wentworth Way, York, Yorkshire YO10 5DD, UK ewan.minter@york.ac.uk.
authors.forename: Ewan J A
authors.initials: EJ
authors.lastname: Minter
authors.affiliation: Department of Ecology, University of Oulu, PO Box 3000, 90014 Oulu, Finland.
authors.forename: Phillip C
authors.initials: PC
authors.lastname: Watts
authors.affiliation: Centre for Ecology and Conservation, College of Life and Environmental Sciences, University of Exeter, Cornwall Campus, Falmouth TR10 9FE, UK.
authors.forename: Chris D
authors.initials: CD
authors.lastname: Lowe
authors.affiliation: Department of Biology, University of York, Wentworth Way, York, Yorkshire YO10 5DD, UK.
authors.forename: Michael A
authors.initials: MA
authors.lastname: Brockhurst
mesh_terms: D004141:Dinoflagellida / Q000235:genetics* / Q000502:physiology; D014644:Genetic Variation; D011156:Population Density; D012641:Selection, Genetic
keywords: density dependence; diversity; frequency dependence; plankton; selection