Evolutionary implications of plant host interactions with a generalist pathogen

Eco-evolutionary feedback arising from host–pathogen interactions shapes the number and frequency of resistance (R) genotypes and their allelic polymorphism. A subset of R genes exhibits unusually strong signatures of balancing selection, sometimes even existing as a trans-specific polymorphism. Here, we explore the role of alternative hosts on R gene evolution through a simple model of two closely related host species that share a single generalist pathogen. We ask (i) how shared interactions determine the R gene repertoire and polymorphism in each host and (ii) under which circumstances (trans-specific) polymorphism is maintained. Our results indicate that interactions with a generalist pathogen are more likely to sustain polymorphism at a shared R gene compared with the maintenance of polymorphism at R genes private to each host. The former can translate into trans-specific resistance gene polymorphism. Further, increasing the relative proportion of a single host species favours fixation of private resistance in the less common host, as the pathogen segregates for escaping recognition by the common host and often retains the effector recognized by the less common host. Our model thus sheds light on how R gene dynamics are shaped by interactions with a shared generalist pathogen and the pathogen’s response to alternative hosts.