Coevolutionary outcomes between interacting species are predicted to vary across landscapes, as environmental conditions, gene flow, and the strength of selection vary among populations. Using a combination of molecular, experimental, and field approaches, we describe how broad-scale patterns of environmental heterogeneity, genetic divergence, and regional adaptation have the potential to influence coevolutionary processes in the Linum marginale-Melampsora lini plant-pathogen interaction. We show that two genetically and geographically divergent pathogen lineages dominate interactions with the host across Australia, and demonstrate a hybrid origin for one of the lineages. We further demonstrate that the geographic divergence of the two lineages of M. lini in Australia is related to variation among lineages in virulence, life-history characteristics, and response to environmental conditions. When correlated with data describing regional patterns of variation in host resistance diversity and mating system these observations highlight the potential for gene flow and geographic selection mosaics to generate and maintain coevolutionary diversification in long-standing host-pathogen interactions.
