DOI: 10.5061/dryad.273d3
title: Data from: Low genetic and phenotypic divergence in a contact zone between freshwater and marine sticklebacks: gene flow constrains adaptation

description: Background: Distinct hybrid zones and phenotypic and genomic divergence is
 often observed between marine and freshwater threespine sticklebacks
 (Gasterosteus aculeatus). Nevertheless, cases also exist where
 marine-freshwater divergence is diffuse despite seemingly similar
 environmental settings. In order to assess what characterizes these highly
 different outcomes, we focused on the latter kind of system in the Odder
 River, Denmark. Here, a previous study based on RAD (Restriction site
 Associated DNA) sequencing found non-significant genome-wide
 differentiation between marine and freshwater sticklebacks. In the present
 study, we analyzed samples on a finer geographical scale. We assessed if
 the system should be regarded as panmictic, or if fine-scale genetic
 structure and local selection was present but dominated by strong
 migration. We also asked if specific population components, that is the
 two sexes and different lateral plate morphs, contributed
 disproportionally more to dispersal. Results: We assessed variation at 96
 SNPs and the Eda gene that affects lateral plate number, conducted
 molecular sex identification, and analyzed morphological traits. Genetic
 differentiation estimated by FST was non-significant throughout the
 system. Nevertheless, spatial autocorrelation analysis suggested fine
 scale genetic structure with a genetic patch size of 770 m. There was no
 evidence for sex-biased dispersal, but full-plated individuals showed
 higher dispersal than low- and partial-plated individuals. The system was
 dominated by full-plated morphs characteristic of marine sticklebacks, but
 in the upstream part of the river body shape and frequency of low-plated
 morphs changed in the direction expected for freshwater sticklebacks. Five
 markers including Eda were under possible diversifying selection. However,
 only subtle clinal patterns were observed for traits and markers.
 Conclusions: We suggest that gene flow from marine sticklebacks overwhelms
 adaptation to freshwater conditions, but the short genetic patch size
 means that the effect of gene flow on the most upstream region must be
 indirect and occurs over generations. The occurrence of both weak unimodal
 and strong bimodal hybrid zones within the same species is striking. We
 suggest environmental and demographic factors that could determine these
 outcomes, but also highlight the possibility that long-term population
 history and the presence or absence of genomic incompatibilities could be
 a contributing factor.
description: Genepop file with all
 SNP dataGenepop file with all
 SNP data of the study along with an EDA-linked marker.Genepop_all_SNPs_and_EDA.txtGenalex file with geographic coordinates for spatial autocorrelation: all individualsGenalex file with the data for fig. 3.a.Genalex_spatial_autocorrelation_all.xlsxGenalex_spatial_autocorrelation_malesGenalex file with data for spatial autocorrelation analysis of males (Fig. 3.c)Genalex_spatial_autocorrelation_femalesGenalex file with data for spatial autocorrelation analysis of females (Fig. 3.c)Genalex_spatial_autocorrelation_full _platedGenalex file with data for spatial autocorrelation analysis of full-plated individuals (Fig. 3.b)Genalex_spatial_autocorrelation_low+partial _platedGenalex file with data for spatial autocorrelation analysis of low and partial-plated individuals (Fig. 3.b)body_shape_coordinatesBody shape coordinates for analysis using PAST 3.12 (Fig. 2).