Beware of the impact of land use legacy on genetic connectivity: A case study of the long-lived perennial Primula veris

Land use change over the past century has drastically decreased the amount of semi-natural grasslands and jeopardises the functional connectivity, i.e., exchange of genes through pollen and seed dispersal, among populations of plants characteristic to these valuable habitats. Increasing evidence shows that landscape elements can influence gene flow between isolated plant populations. Still, the role of historical land use legacies in shaping the relationship between current landscape characteristics and gene flow in perennial grassland plants remains unclear. We studied the impact of landscape characteristics on the functional connectivity of 32 populations of the grassland plant Primula veris in two landscapes, both historically dominated by semi-natural grasslands. Nowadays, one landscape remains grassland-dominated, while forests and agricultural fields prevail in the other. We examined the genetic diversity and structure of these populations and assessed the role of landscape characteristics on gene flow using resistance- and corridor-based approaches. Two gene flow measures were calculated from >2300 SNP markers: pairwise genetic differentiation (FST) and pairwise mean assignment probability (MAP). The relative permeability of landscape elements for gene exchange depended on landscape context, likely mirroring opposite land use trajectories in these landscapes. Furthermore, the outcome was also affected by different gene flow indices, indicating their context-dependency. We conclude that applying connectivity measures based on findings from a landscape setting reflecting only one type of land use history should be avoided. Additionally, caution is needed when interpreting gene flow measures of long-lived plant species due to possible delays in their response to landscape change.