Effector loss and gain drives pathogen host range at a fitness cost

The genetic constraints for microbial host adaptation to emerge for generalist and specialist behaviors remain unclear but is key for epidemiological preparedness. Here, we show that generalist cereal pathogen Xanthomonas translucens arose from a specialist ancestor via the loss of a single effector gene, xopAL1 . Deleting barley-specialist X. translucens xopAL1 recapitulated the host jump to wheat and demonstrates risk across each globally distributed genetic lineages. However, this niche expansion via XopAL1 loss incurs a significant pathogenic fitness cost on barley. Moreover, the specialist lineage gained additional effector genes, such as xopAJ , enhancing virulence on barley while restricting oat infection, thereby reinforcing niche specialization. We further identified and validated key host defense genes that restrict host infection, providing new, undefined targets for crop improvement. Our work provides a validated evolutionary framework of intergenera host jump, demonstrating that single loss and gain events create a dynamic trade-off between niche breadth and specialization.