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.
Highlights
Many free-living protists exhibit high levels of intraspecific diversity [1,2,3,4] despite their large population sizes, which offer the potential for natural selection to operate efficiently and fix the fittest genotype(s)
Understanding the mechanisms that maintain intraspecific variation in natural populations is an important challenge in evolutionary ecology, as this variation underpins numerous fundamental processes [20,21], including adaptation to environmental change (e.g. [22])
We observed an overall signature of negative frequencydependent selection (NFDS) between pairs of competing strains of O. marina, which was intensified at higher population densities
Summary
Many free-living protists exhibit high levels of intraspecific diversity [1,2,3,4] despite their large population sizes, which offer the potential for natural selection to operate efficiently and fix the fittest genotype(s). Selection coefficients [16] for multiple pairs of strains at a range of starting frequencies and at several population densities that were representative of natural populations [17]. Selection experiments to test for frequency and density dependence were performed by co-culturing six pairs of strains to estimate instantaneous selection coefficients [16]. A global model (i.e. including all pairwise assays) of selection coefficients was analysed using a mixed effects model, with random slopes and s as the dependent variable, density and frequency as fixed effects, and strain pair as a random effect using the R package ‘lme4’ [19]. Owing to the non-independent and reciprocal nature of selection coefficients (where for a given pair of strains the value of s for the target strain is equal to the negative value of s for its competitor) the strain with the positive mean s across treatments was designated the target strain. All statistical analyses were conducted in R v. 3.1.0 (R Core Development Team, 2014) and all data are presented as mean +1 s.e
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