Résumé
The extent and speed at which pathogens adapt to host resistance varies
considerably. This presents a challenge for predicting when—and
where—pathogen evolution may occur. While gene flow and spatially
heterogeneous environments are recognized to be critical for the
evolutionary potential of pathogen populations, we lack an understanding
of how the two jointly shape coevolutionary trajectories between hosts and
pathogens. The rust pathogen Melampsora lini infects two ecotypes of its
host plant Linum marginale that occur in close proximity yet in distinct
populations and habitats. In this study, we found that within-population
epidemics were different between the two habitats. We then tested for
pathogen local adaptation at host population and ecotype level in a
reciprocal inoculation study. Even after controlling for the effect of
spatial structure on infection outcome, we found strong evidence of
pathogen adaptation at the host ecotype level. Moreover, sequence analysis
of two pathogen infectivity loci revealed strong genetic differentiation
by host ecotype but not by distance. Hence, environmental variation can be
a key determinant of pathogen population genetic structure and
coevolutionary dynamics and can generate strong asymmetry in infection
risks through space.