Local adaptation and oceanographic connectivity patterns explain genetic differentiation of a marine diatom across the North Sea–Baltic Sea salinity gradient

Abstract

Drivers of population genetic structure are still poorly understood in marine micro-organisms. We exploited the North Sea–Baltic Sea transition for investigating the seascape genetics of a marine diatom, Skeletonema marinoi. Eight polymorphic microsatellite loci were analysed in 354 individuals from ten locations to analyse population structure of the species along a 1500-km-long salinity gradient ranging from 3 to 30 psu. To test for salinity adaptation, salinity reaction norms were determined for sets of strains originating from three different salinity regimes of the gradient. Modelled oceanographic connectivity was compared to directional relative migration by correlation analyses to examine oceanographic drivers. Population genetic analyses showed distinct genetic divergence of a low-salinity Baltic Sea population and a high-salinity North Sea population, coinciding with the most evident physical dispersal barrier in the area, the Danish Straits. Baltic Sea populations displayed reduced genetic diversity compared to North Sea populations. Growth optima of low salinity isolates were significantly lower than those of strains from higher native salinities, indicating local salinity adaptation. Although the North Sea–Baltic Sea transition was identified as a barrier to gene flow, migration between Baltic Sea and North Sea populations occurred. However, the presence of differentiated neutral markers on each side of the transition zone suggests that migrants are maladapted. It is concluded that local salinity adaptation, supported by oceanographic connectivity patterns creating an asymmetric migration pattern between the Baltic Sea and the North Sea, determines genetic differentiation patterns in the transition zone.

Mare Incognitum: A Glimpse into Future Plankton Diversity and Ecology Research

“…However, many planktonic species exhibit local adaptation (Peijnenburg and Goetze, 2013;Sjöqvist et al, 2015) or consist of several ecotypes with different environmental preferences, and phenotypic plasticity, dispersal, and evolutionary changes could mitigate climate change impacts as they could help species to adapt to changing conditions (O'Connor et al, 2012). One possibility to account for both local adaptation and phenotypic plasticity is to include a population-dependent component in mixed effect models (e.g., Valladares et al, 2014).…”

Genetic diversity affects ecological performance and stress response of marine diatom populations

“…It was not logistically feasible using our method to quantify monoclonal performance of all 20 strains. However, the high diversity level was included in order to reflect a more realistic situation, as earlier work has shown that S. marinoi populations naturally consist of hundreds of genotypes (Sjöqvist et al, 2015). All monoclonal cultures and diversity levels were incubated at a native salinity of 5 psu and at a salinity of 3 psu, the latter representing the lower end of the Baltic Sea salinity gradient and a potentially stressful condition.…”

“…All 20 experimental strains were established from a sediment sample collected in the Bothnian Sea (18.55E, 62.12N), where ambient mean salinities are 5 psu. Culture establishment and genetic characterization using microsatellites are described in detail in Sjöqvist et al (2015).…”

“…Baltic low-salinity-adapted populations of the species are genotypically diverse and display considerable phenotypic diversity (Sjöqvist et al, 2014(Sjöqvist et al, , 2015. Using microsatellite-based genotyping methods we were able to monitor and confirm true clonal diversity in experimental mixtures.…”

Diatoms diversify and turn over faster in freshwater than marine environments*

“…) or ecology (Whittaker and Rynearson ), sometimes even at small spatial scales (Rynearson and Armbrust ; Godhe and Härnström ; Sjöqvist et al. ). These findings suggest that population subdivision might be even stronger in freshwaters (Vanormelingen et al.…”