Researchers studied how a fungal plant pathogen called Fusarium oxysporum rapidly adapts to new environments by analyzing genetic changes during repeated passages through tomato plants and laboratory media. They discovered that jumping genes (transposons) were responsible for most mutations driving adaptation, and surprisingly found that genes located in specialized ‘accessory’ regions of the fungus’s genome controlled important functions like growth and virulence. This research reveals how fungal pathogens can evolve quickly to become better competitors or invaders.