Human-mediated admixture shapes high genetic diversity and the invasion dynamics of Lupinus nootkatensis in Iceland.

The invasion of the Nootka lupin (Lupinus nootkatensis) in Iceland presents a fascinating scenario: an introduced species on an isolated island that retains exceptionally high genetic diversity. Using inter-simple sequence repeat (ISSR) markers, we analysed the population structure of this widespread coloniser to understand the mechanisms behind its success. Contrary to expectations of a founder effect, our results reveal substantial polymorphism (100% at the dataset level) and high gene diversity, indicating that high propagule pressure from repeated, large-scale introductions has effectively countered genetic bottlenecks. Spatial analyses (PCoA, AMOVA) demonstrated a highly admixed structure where genetic clustering explains twice as much variance as geographical regions (12.3% vs. 5.9%). Importantly, a Mantel test confirmed a remarkably weak isolation-by-distance signal (explaining around 6% of the variance), aligning directly with the AMOVA results. This structural concordance shows that human-mediated transport-particularly from Reykjavík, the demographic hub-has completely overridden natural spatial gradients. We conclude that historical land reclamation efforts have acted as massive hyper-vectors, mixing diverse lineages to form a highly resilient, interconnected metapopulation. Therefore, effective management should shift from broad-scale eradication to targeted biosecurity measures that limit further human-facilitated mixing of these distinct genetic lineages.