Abstract
Free-living marine protists are often assumed to be broadly distributed and genetically homogeneous on large spatial scales. However, an increasing application of highly polymorphic genetic markers (e.g., microsatellites) has provided evidence for high genetic diversity and population structuring on small spatial scales in many free-living protists. Here we characterise a panel of new microsatellite markers for the common marine flagellate Oxyrrhis marina. Nine microsatellite loci were used to assess genotypic diversity at two spatial scales by genotyping 200 isolates of O. marina from 6 broad geographic regions around Great Britain and Ireland; in one region, a single 2 km shore line was sampled intensively to assess fine-scale genetic diversity. Microsatellite loci resolved between 1–6 and 7–23 distinct alleles per region in the least and most variable loci respectively, with corresponding variation in expected heterozygosities (He) of 0.00–0.30 and 0.81–0.93. Across the dataset, genotypic diversity was high with 183 genotypes detected from 200 isolates. Bayesian analysis of population structure supported two model populations. One population was distributed across all sampled regions; the other was confined to the intensively sampled shore, and thus two distinct populations co-occurred at this site. Whilst model-based analysis inferred a single UK-wide population, pairwise regional FST values indicated weak to moderate population sub-division (0.01–0.12), but no clear correlation between spatial and genetic distance was evident. Data presented in this study highlight extensive genetic diversity for O. marina; however, it remains a substantial challenge to uncover the mechanisms that drive genetic diversity in free-living microorganisms.
Highlights
Free-living marine protists commonly have broad distributions [1,2,3] and potentially large population sizes [3]
Here, we characterise a panel of polymorphic microsatellite loci that can be used to detect subtle patterns of spatial genetic structure and assess the extent of genetic diversity in the common marine protist Oxyrrhis marina
Our motivation was to acquire greater resolution than that afforded by the distribution of DNA sequence variation, where analyses to date have not identified spatial genetic structure in north-east Atlantic coastal waters
Summary
Free-living marine protists commonly have broad distributions [1,2,3] and potentially large population sizes [3]. In combination with a general assertion that marine environments offer few physical barriers to dispersal this has often lead to the assumption that protist populations may be genetically homogeneous over large spatial distances [4] This idea is largely consistent with studies based on the spatial distribution of sequence variation, typically at ribosomal gene loci, which often provide evidence for widely distributed phylotypes [e.g. 5,6]. The increasing application of highly polymorphic genetic markers (such as microsatellites) is a recognition that defining population structure and population level genetic variation is critical to understanding the demographic processes that drive free-living protists distributions Such high resolution studies are revealing extensive genetic diversity and significant population structuring within free-living protist species occurring on small spatial scales on the order of tens to hundreds of kilometres [9,10,11]
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