Evolutionary implications of plant host interactions with a generalist pathogen

It is widely believed that eco-evolutionary feedbacks arising from host-pathogen interactions shape the number and frequency of resistance (R) genotypes and their allelic polymorphism. A subset of R genes exhibit 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 repertoires 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 to the maintenance of polymorphism at R genes private to each host. The former can translate into trans-specific resistance gene polymorphism. Further, we observe that increasing the relative proportion of a single host species favors fixation of private resistance in the less common host, which stays effective because the pathogen tracks the more common host. Our model thus sheds light on how R gene dynamics are shaped by interactions with a shared generalist pathogen and the pathogens response to alternative hosts.