Seascape analysis reveals regional gene flow patterns among populations of a marine planktonic diatom

Abstract

We investigated the gene flow of the common marine diatom, Skeletonema marinoi, in Scandinavian waters and tested the null hypothesis of panmixia. Sediment samples were collected from the Danish Straits, Kattegat and Skagerrak. Individual strains were established from germinated resting stages. A total of 350 individuals were genotyped by eight microsatellite markers. Conventional F-statistics showed significant differentiation between the samples. We therefore investigated whether the genetic structure could be explained using genetic models based on isolation by distance (IBD) or by oceanographic connectivity. Patterns of oceanographic circulation are seasonally dependent and therefore we estimated how well local oceanographic connectivity explains gene flow month by month. We found no significant relationship between genetic differentiation and geographical distance. Instead, the genetic structure of this dominant marine primary producer is best explained by local oceanographic connectivity promoting gene flow in a primarily south to north direction throughout the year. Oceanographic data were consistent with the significant FST values between several pairs of samples. Because even a small amount of genetic exchange prevents the accumulation of genetic differences in F-statistics, we hypothesize that local retention at each sample site, possibly as resting stages, is an important component in explaining the observed genetic structure.

Keywords: Bacillariophyceae; Skeletonema marinoi; microsatellites; oceanographic connectivity.

Figure 1.

(a) Southern Scandinavia. Strains of S. marinoi were established from sediment samples collected from inshore and offshore sites in the Skagerrak, Kattegat and Öresund. (b–m) Oceanographic trajectories for the seven sampling stations for each month of the year. The trajectories for each sampling station are colour coded according to the legend in (b). Connectivity is based on trajectories released from 15 grid cells per site. The total numbers of trajectories released at each site over the period 1995–2002 was 5880. (b) January, (c) February, (d) March, (e) April, (f) May, (g) June, (h) July, (i) August, (j) September, (k) October, (l) November, (m) December.