Seed dispersal along sheep herding routes influences genetic connectivity at the landscape scale

Background/Question/Methods

For plants, seed dispersal is the process that determines population demographic connectivity, whereas both seed and pollen flow contribute to population genetic connectivity. The contribution of seeds to population genetic connectivity is typically expected to be negligible, as pollen may travel larger distances and in larger numbers. However, this may not be the case for species dispersed by animal vectors. This study used as model system a network of calcareous grasslands in Bavaria, Germany, to investigate whether landscape-scale patterns of population genetic connectivity in Dianthus carthusianorum are influenced by seed dispersal by large-flock sheep herding. We hypothesized that (i) landscape-scale patterns of genetic structure are related to sheep herding in three-non overlapping rotational systems; (ii) spatial patterns of population genetic structure in ungrazed patches are related to isolation by geographic distance (IBD) resulting from limited dispersal; whereas (iii) for populations connected by grazing, we expect weak or no IBD effect, but instead population genetic structure should be associated with a distance-dependent effect of connectivity along sheep herding routes. Based on the amplification of eleven nuclear microsatellite loci, we performed PCA analysis and Bayesian clustering methods to assess population genetic structure. Peason correlations were performed to test for the effects of IBD and sheep herding connectivity against population genetic distances. 

Results/Conclusions

We found clear evidence of spatial genetic structure at the landscape-scale differentiating populations at the west and the east of the study area differentiating populations of separate sheep herding systems. We found a significant and strong effect of IBD in ungrazed patches (r = 0.64, P= 0.001), whereas IBD for grazed patches within herding systems had no significant effect, but sheep herding was significantly related to population genetic connectivity (r = 0.41, P = 0.001). This positive association remained significant after controlling for the effect of IBD on sheep herding connectivity. Our study showed the potential of seed-mediated gene flow to determine landscape-scale patterns of population genetic connectivity in calcareous grassland plants.