High genetic diversity and fine-scale spatial structure in the marine flagellate Oxyrrhis marina (Dinophyceae) uncovered by microsatellite loci

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 (H(e)) 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 F(ST) 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.

Figure 1. The distribution and genetic structure of Oxyrrhis marina sampled across UK coastal waters.

A: The distribution of O. marina samples collected from UK coastal waters. Samples were grouped into 6 regions for pairwise comparisons; n = number of isolates per region. B: The probabilities of membership of individual O. marina isolates to hypothetical clusters in a two cluster simulation. Each bar represents an individual isolate and the proportion of the bar that is black or white represents the proportion of assignment to cluster 1 or 2 respectively. C: (i) Mean L(K) (±95% CI) over 5 independent runs for each K value; (ii) ÄK, the second order rate of change of Ln P(D) with respect to K; the modal value of this distribution corresponds to the true value of K or the uppermost level of genetic structure (see Methods and for further details).

Figure 2. Spatial-autocorrelation-based isolation by distance plot indicating average (all loci) kinship (Fij with 95% confidence intervals, 40) as a function of distance (km) for the 2 model clusters identified in Figure 1 .

Filled and open circles corresponded to model clusters 1 and 2, respectively (following Figure 1).