Congruence between fine-scale genetic breaks and dispersal potential in an estuarine seaweed across multiple transition zones


 Genetic structure in biogeographical transition zones can be shaped by several factors including limited dispersal across barriers, admixture following secondary contact, differential selection, and mating incompatibility. A striking example is found in Northwest France and Northwest Spain, where the estuarine seaweed Fucus ceranoides L. exhibits sharp, regional genetic clustering. This pattern has been related to historical population fragmentation and divergence into distinct glacial refugia, followed by post-glacial expansion and secondary contact. The contemporary persistence of sharp ancient genetic breaks between nearby estuaries has been attributed to prior colonization effects (density barriers) but the effect of oceanographic barriers has not been tested. Here, through a combination of mesoscale sampling (15 consecutive populations) and population genetic data (mtIGS) in NW France, we define regional genetic disjunctions similar to those described in NW Iberia. Most importantly, using high resolution dispersal simulations for Brittany and Iberian populations, we provide evidence for a central role of contemporary hydrodynamics in maintaining genetic breaks across these two major biogeographic transition zones. Our findings further show the importance of a comprehensive understanding of oceanographic regimes in hydrodynamically complex coastal regions to explain the maintenance of sharp genetic breaks along continuously populated coastlines.

barriers that were key in shaping phylogeographic structure of many plant and animal species.

51
Of these, the Last Glacial Maximum is probably the most significant and recent historical event

93
In this study, we use phylogeographic analyses and Lagrangian Particle Simulations 132 Table SI1). The second dataset (Data set 2) consisted of sequences previously analysed in

257
Previous studies have shown that mtIGS differentiation and contemporary F.  into distinct refugia (estuarine refugia within regional refugia), followed by expansion and 266 secondary contact of vicariant phylogroups (Neiva et al., 2012b).

267
In addition to southern European refugia, the ice-free paleo-shores of northwest

171
Both regions of simulation were gridded to a common spatial resolution of 0.01° (approx. 1km).

229
The assignment of oceanographic regions performed by the leading eigenvector algorithm  Fig. 1d) was 10-fold higher than 236 between regions, with the exception of those between R1 and R2 (Fig. 1d), which were of the 237 same order of magnitude. Following a similar pattern, the probabilities of connectivity in 238 northwest Iberia were 10 to 100-fold higher within regions than between regions (diagonal of 239 Fig. 1e), with a marked increase as the distance between groups increased.  into distinct refugia (estuarine refugia within regional refugia), followed by expansion and 267 secondary contact of vicariant phylogroups (Neiva et al., 2012b).