Comparison of host susceptibility to native and exotic pathogens provides evidence for pathogen imposed selection in forest trees

The extent to which spatial structuring of host resistance in wild plant populations reflects direct pathogen imposed selection is a subject of debate. To examine this issue, genetic susceptibility to an exotic and co-evolved native fungal pathogen were compared using two Australian host tree species. Damage to common host germplasm of Corymbia citriodora subsp. variegata (CCV) and Eucalyptus globulus, caused by recently introduced (Austropuccinia psidii) and native (Quambalaria pitereka and Teratosphaeria sp.) pathogens was evaluated in common-garden experiments. There was significant additive genetic variation within host species for susceptibility to both the exotic and native pathogens. However, susceptibility to A. psidii was not genetically correlated with susceptibility to either native pathogen, providing support for pathogen specific rather than general mechanisms of resistance. Population differentiation (QST ) for susceptibility to the native pathogens was greater than neutral expectations (molecular FST ), arguing for divergent selection. Coupled with lower native, but not exotic, pathogen susceptibility in host populations from areas climatically more prone to fungal proliferation, these findings suggest that pathogen imposed selection has directly contributed to a geographic mosaic of host resistance to native pathogens. This article is protected by copyright. All rights reserved