Variable patterns of local adaptation along an elevation gradient in a host-pathogen interaction

Michael Rechsteiner , Fletcher William Halliday, Michael Giolai, Andreas Wagner, Anna-Liisa Laine

The strength of biotic interactions is hypothesized to decrease with elevation, yet it remains unclear whether this decline results in weaker selection at higher elevations. Further, it is not known if varying ecological interactions along elevation gradients affect the patterns of local adaptation between hosts and pathogens. Here, we experimentally test whether the direction and strength of local adaptation vary between four populations of a host and its specialist fungal pathogen along an elevation gradient. This system represents a steep climatic gradient and is characterized by variation in infection prevalence along altitude, where infected plants are rarely observed in the highest elevation population. We measured infection success and severity across several hundred plant-pathogen pairs and tested for differences between sympatric and allopatric host-pathogen combinations. We found signs of both local adaptation and maladaptation in the severity of infections between populations. The high-elevation pathogen population was locally adapted, whereas the low-elevation pathogen population was locally maladapted. Our results suggest a relationship between varying pathogen prevalence and the pattern of local adaptation along an environmental gradient, highlighting that climate driven shifts in pathogen distributions may reshape coevolutionary dynamics between hosts and pathogens.