Spatial patterns of genomic variation and temporal genomic offset in a common grassland plant and their relation to seed transfer zones

Introduction: of large amounts of seeds is essential for restoration of temperate grasslands and is often regulated by seed transfer zones. These zones are commonly derived from abiotic parameters only. In order to evaluate seed zones as a means for the protection of genetic diversity and to avoid potential detrimental effects such as maladaptation and homogenisation of seed transfer within zones, empirical data on spatial genetic structure is paramount. Here, we focussed on Galium album, a widespread perennial grassland species, which we thoroughly sampled on average once per 25 km x 25 km in all of Germany. Based on 9,403 SNP loci, we described the genetic structure using Bayesian clustering. We identified four spatially coherent genetic clusters that were rarely congruent with the current seed zones. Therefore, current zone-based seed transfer potentially distorts and homogenises some spatial differentiation. Yet, we found significant isolation-by-distance among seed zones, showing that they still reflect a substantial part of spatial genetic differentiation. Seed transfer practice is challenged by climate change that shifts adaptive requirements for populations. We conducted redundancy analysis (RDA)-based genotype-environment association (GEA) analysis, and assessed necessary genomic turnover to maintain current levels of adaptation (temporal genomic offset). The resulting patterns suggest that climate change might impose a risk for regional adaptation in parts of Southern and Central Germany. We found that targeted assisted migration across seed zone borders might in some cases mitigate the most adverse GEA disruptions in seed zones that do not harbour suitable donor material themselves.