SSU rRNA gene variation resolves population heterogeneity and ecophysiological differentiation within a morphospecies (Stramenopiles, Chrysophyceae)
The objective of this study was to analyze the extent of ecophysiological and molecular homogeneity, or heterogeneity, of distinct protist populations and to test for correlations between molecular distance and ecophysiological adaptation in a widespread and ubiquitous protistan taxon. We selected heterotrophic nanoflagellates of the Spumella morphotype as a model organism, because these flagellates are widespread and among the dominant bacterivores in many microbial communities. We investigated the molecular microdiversity and the ecophysiological tolerances for a total of 13 strains originating from two freshwater samples. We further investigated the affiliation of different ecotypes with distinct small subunit ribosomal ribonucleic acid (SSU rRNA) genotypes, or molecular operational taxonomic units. These regional population studies were further compared to the molecular and ecophysiological diversity of 28 strains originating from remote sampling sites. None of the investigated populations are homogenous, but are rather heterogeneously composed of different ecotypes and genotypes, possibly corresponding to cryptic species. This population heterogeneity may partly explain the deviations between studies on single strains and populations in both laboratory and field studies. The molecular distance between the strains was correlated with the salinity and temperature adaptation of the respective strains, contradicting the assumption that SSU rRNA variation reflects accumulated neutral mutations. Independent of whether this correlation reflects adaptation above the (biological) species level or variation between asexually reproducing lineages, we demonstrate the unsuitability of the current classification system (species concept) for the investigated organisms, at least with respect to ecological and ecophysiological investigations. The ecology of protistan species
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