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

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.

growth ratesThis file contains data on growth rates (intrinsic rate of
increase, µ) of Skeletonema marinoi strains isolated from the Baltic Sea
and the Swedish west coast. The average values are based on measured
growth rates of 10 monoclonal strains grown in f/4+Si for 5-7 days in +10
C in a light intensity of 40 µmol s-1 m-2. Strains were acclimatized to
respective salinity during one week prior to the growth
experiment.Microsatellite genotypes (Skeletonema marinoi)This file
contains allelic information on 354 individuals (8 loci) of Skeletonema
marinoi isolated from the Baltic Sea and NE North
Sea.genotypes.xlsxsampling locationsThis file contains the exact
coordinates of the ten sampling stations in this study.directional
relative migrationThis file contains the raw data for directional relative
migration between the ten Skeletonema marinoi populations. More
information in the ReadMe file.oceanographic connectivityThis file
contains matrices of oceanographic connectivity between the ten sampling
stations. More information in the associated ReadMe file.