The genetic consequences of population marginality: a case study in maritime pine

Aim Marginal tree populations, either those located at the edges of the species’ range or in suboptimal environments, are often a valuable genetic resource for biological conservation. However, there is a lack of knowledge about the genetic consequences of population’s marginality, estimated across entire species’ ranges. Our study addresses this gap by providing information about several genetic indicators and their variability in marginal and core populations identified using quantitative marginality indices. Location Southwestern Europe and North Africa. Methods Using 10,185 SNPs across 82 populations of maritime pine ( Pinus pinaster Ait.), a widespread and economically important conifer characterised by a fragmented range, we modelled the relationship of seven genetic indicators potentially related to population evolutionary resilience, namely genetic diversity (based on all SNPs and two kind of outlier loci), inbreeding, genetic differentiation, recessive genetic load and genomic offset, with population geographical, demo-historical and ecological marginality (as estimated by nine quantitative indices). Models were constructed for both regional (introducing gene pool as random factor) and range-wide spatial scales. Results We showed a trend towards decreasing overall genetic diversity (albeit not necessarily for genetic diversity estimates based on outlier loci) and increasing differentiation with geographic marginality, supporting the centre-periphery hypothesis (CPH). Moreover, we found no correlation between population inbreeding and marginality, while geographically marginal populations had a lower recessive genetic load (only models without the gene pool effect) and ecologically marginal populations had a higher genomic offset, suggesting higher maladaptation to future climate. Main conclusions Overall, our results indicate that marginal populations could be at higher risk of maladaptation to climate change than core populations, despite reduced levels of genetic load, a risk that is exacerbated by typically small effective population sizes and increasing human impact.