Genotypic diversity and differentiation among populations of two benthic freshwater diatoms as revealed by microsatellites

Abstract

Given their large population sizes and presumed high dispersal capacity, protists are expected to exhibit homogeneous population structure over large spatial scales. On the other hand, the fragmented and short-lived nature of the lentic freshwater habitats that many protists inhabit promotes strong population differentiation. We used microsatellites in two benthic freshwater diatoms, Eunotia bilunaris 'robust' and Sellaphora capitata, sampled from within a pond and connected ponds, through isolated ponds from the same region to western Europe to determine the spatial scale at which differentiation appears. Because periods of low genotypic diversity contribute to population differentiation, we also assessed genotypic diversity. While genotypic diversity was very high to maximal in most samples of both species, some had a markedly lower diversity, with up to half (Eunotia) and over 90% (Sellaphora) of the strains having the same multilocus genotype. Population differentiation showed an isolation-by-distance pattern with very low standardized FST values between samples from the same or connected ponds but high values between isolated ponds, even when situated in the same region. Partial rbcL sequences in Eunotia were consistent with this pattern as isolated ponds in the same region could differ widely in haplotype composition. Populations identified by Structure corresponded to the source ponds, confirming that 'pond' is the main factor structuring these populations. We conclude that freshwater benthic diatom populations are highly fragmented on a regional scale, reflecting either less dispersal than is often assumed or reduced establishment success of immigrants, so that dispersal does not translate into gene flow.

Clonal expansion behind a marine diatom bloom

“…Intra-population gene flow, exerted by frequent sexual events in a relatively secluded environment, may explain this long-term genetic homogenization in S. marinoi (Godhe et al, 2014). Among diatom populations there is thus evidence for both short-term differentiation (Saravanan and Godhe, 2010;Vanormelingen et al, 2015;Godhe et al, 2016), as also reported in the present study and likely driven by clonality, and long-term homogenization (Härnström et al, 2011), driven by sexual recombination.…”

“…Nonetheless, low diversity was observed in some cases: the dominance of asexual reproduction and clonal evolution were considered at the base of the relatively low genotypic richness (0.667) reported for the coccolithophore Emiliania huxleyi during a bloom in the North Sea (Krueger-Hadfield et al, 2014). Extremely low genotypic richness (down to~0) was also reported for a benthic freshwater diatom collected in small pond (Vanormelingen et al, 2015), although the hypothesis of clonal expansion could not be investigated due to the limited number of samples.…”

One species for one island? Unexpected diversity and weak connectivity in a widely distributed tropical hydrozoan

“…Extremely high genetic differentiation between populations of a single species is unusual, although similar levels have been documented over large geographic scales in sponges (Chaves-Fonnegra et al, 2015), coastal sharks (Ashe et al, 2015) and marine mammals (Fruet et al, 2014), but also at distances o100 km in terrestrial animals (Sethuraman et al, 2013) or freshwater diatoms (Vanormelingen et al, 2015). Our results indicate that the populations of L. brevirostris α present clear geographic boundaries and are consistent with two of the characteristics of a metapopulation model (Grimm et al, 2003): (1) local populations have their own dynamics and (2) they are connected by limited dispersal.…”

Patterns of population structure and complex haplotype sharing among field isolates of the green alga Chlamydomonas reinhardtii

“…Exploring the extent of population structure and its causes can be used to test between the ubiquity and moderate endemicity models, as the former predicts a lack of divergence in allopatry or isolation by distance, and little evidence for recent speciation events, in contrast to what is observed in many plants and animals. Evidence for genetically structured populations has recently been reported across a variety of taxa and habitats, including examples from ciliates (Zufall, Dimond, & Doerder, 2013), amoebae (Douglas, Kronforst, Queller, & Strassmann, 2011;Heger, Mitchell, & Leander, 2013), diatoms (Casteleyn et al, 2010;Sjöqvist, Godhe, Jonsson, Sundqvist, & Kremp, 2015;Vanormelingen et al, 2015;Whittaker & Rynearson, 2017), dinoflagellates (Lowe, Martin, Montagnes, & Watts, 2012;Rengefors, Logares, & Laybourn-Parry, 2012), raphidophytes (Lebret, Tesson, Kritzberg, Tomas, & Rengefors, 2015) and fungi (Carriconde et al, 2008;Ellison et al, 2011). While many of these studies showed clear evidence for geographic structure (supporting the moderate endemicity model), the majority were limited in resolution due to the small number of marker loci used.…”