Spatial variation in resistance and virulence in the host-pathogen system Salix triandra-Melampsora amygdalinae

1 Host-pathogen interactions in isolated populations may result in the adaptation of pathogens to local hosts. However, results of earlier studies of local adaptation in plant-pathogen systems have been contradictory and it has been suggested that specific, species characteristics, for example distribution, dispersal and the degree of pathogen dependence on the host, are important for the outcome of host-pathogen interactions. In addition, the scale of the study may influence whether or not local adaptation is found. 2 We argue that local adaptation of the pathogen to the host can be expected in a system where: (i) the pathogen is host-specific with a short generation time compared with the host; (ii) populations are isolated, allowing localized evolution to occur; and (iii) the study is performed on a geographical scale exceeding the maximum dispersal range of the interacting species. 3 To test these predictions we examined the within- and among-population variation in resistance and virulence of the plant-pathogen system Salix triandra -Melampsora amygdalinae. The pathogen occurs throughout the whole distribution range of the host, and the area of interest consists of highly isolated, small natural populations. 4 Resistance and virulence differed both within and between populations and all clones showed a unique resistance pattern. We conclude that M. amygdalinae is locally adapted to the host S. triandra as all pathogen populations have a higher probability of infecting sympatric than allopatric hosts. Furthermore, high resistance in the host population was accompanied by a high virulence in the pathogen population, suggesting that high resistance levels in a host population may select for more virulent pathogen populations, or vice versa. 5 The nature of host-pathogen interactions differs among systems, and the dynamics of the interaction between S. triandra and M. amygdalinae is governed by characteristics that have resulted in the evolution of local adaptation. 6 Thus, when studying local adaptation of pathogens to their hosts it is important to consider the biology, as well as the scale of dispersal and spatial distribution of both host and pathogen.