Linking seascape with landscape genetics: Oceanic currents favour colonization across the Galápagos Islands by a coastal plant

Abstract

[Aim]: Coastal plants are terrestrial organisms for which ocean surface currents often act as long‐distance dispersal vectors (thalassochorous species) favouring broad distributions and connecting distant populations. However, few studies have statistically assessed the role of currents in modulating gene flow and species distributions of terrestrial organisms. Here we evaluate the hypothesis that some thalassochorous plants exhibit population connectivity, presumably due to effective seed dispersal driven by sea currents.

Location Galápagos Islands (Ecuador).

Taxon Salt bush (Cryptocarpus pyriformis Kunth), a Galápagos native and locally widespread coastal angiosperm.

Methods Using 1806 SNPs obtained by ddRADseq, we evaluated the genetic structure and differentiation of the Galápagos salt bush. To assess the role of sea currents in modulating inter‐population gene flow, four explicit hypotheses were tested using reciprocal causal modelling and spatial eigenvector analysis: (a) isolation by sea resistance, considering that only sea dispersal is possible; (b) isolation by sea and inland resistance, considering that inland dispersal is also possible; (c) isolation by barrier, considering the sea as an obstacle to seed dispersal; and (d) isolation by geographical distance.

Results Low differentiation and little genetic structure were detected among populations of C. pyriformis. Pairwise genetic distances between populations from different islands were significantly correlated with cost distances calculated from sea‐current direction and speed. Nonetheless, inland dispersal also accounted for some gene flow within each island.

Main conclusion Extensive and frequent seed dispersal by sea has apparently favoured strong inter‐island genetic connectivity within Galápagos. A combination of methods developed for terrestrial and marine domains (landscape and seascape genetics) aids in understanding how landscape features modulate gene flow of coastal plant species, as these terrestrial organisms are highly dependent on the sea for seed dispersal.

[Location]: Galápagos Islands (Ecuador).

[Taxon]: Salt bush (Cryptocarpus pyriformis Kunth), a Galápagos native and locally widespread coastal angiosperm.

[Methods]: Using 1806 SNPs obtained by ddRADseq, we evaluated the genetic structure and differentiation of the Galápagos salt bush. To assess the role of sea currents in modulating inter‐population gene flow, four explicit hypotheses were tested using reciprocal causal modelling and spatial eigenvector analysis: (a) isolation by sea resistance, considering that only sea dispersal is possible; (b) isolation by sea and inland resistance, considering that inland dispersal is also possible; (c) isolation by barrier, considering the sea as an obstacle to seed dispersal; and (d) isolation by geographical distance.

[Results]: Low differentiation and little genetic structure were detected among populations of C. pyriformis. Pairwise genetic distances between populations from different islands were significantly correlated with cost distances calculated from sea‐current direction and speed. Nonetheless, inland dispersal also accounted for some gene flow within each island.

Main conclusion Extensive and frequent seed dispersal by sea has apparently favoured strong inter‐island genetic connectivity within Galápagos. A combination of methods developed for terrestrial and marine domains (landscape and seascape genetics) aids in understanding how landscape features modulate gene flow of coastal plant species, as these terrestrial organisms are highly dependent on the sea for seed dispersal.