Spatial Scales of Geographical Isolation by Distance and Barriers, and Heterogeneity in the Genetic Structure of a Seagrass

Abstract Gene flow is a crucial concept in the delineation of conservation units for natural populations of a species. Seagrasses are marine species targeted for conservation because their abundance has declined worldwide during the last century. However, knowledge of how to delineate seagrass conservation units based on their genetic structure is inadequate. In this study, the genomes of Zostera marina L. (eelgrass) populations in three semi-enclosed areas with different spatial scales in Japan were analyzed using single nucleotide polymorphisms within genome-wide loci, and their genetic structures were explored in the context of their isolation caused by geographical distance and barriers. Isolation by distance was apparent at spatial scales of 200–2000 km. A barrier apparently due to a narrow strait in an area with linear dimensions less than 200 km led to greater genetic isolation than did distance. On a more local scale, we found that heterogeneity among sites and intra-site variability rather than isolation by distance or barriers governed the genetic structure of eelgrass on spatial scales less than ~ 20 km. Our results imply that future seagrass conservation efforts should make more use than before of genomic tools so that greater attention can be focused on the complex structures formed on local scales. Identifying the ecological importance of genetic heterogeneity and the functional meaning of genetic differences on local scales will be among the next challenges to the delineation of seagrass conservation units.