Abstract
BACKGROUND: The fungus Metarhizium brunneum has evolved a remarkable ability to switch between different lifestyles. It develops as a saprophyte, an endophyte establishing mutualistic relationships with plants, or a parasite, enabling its use for the control of insect pests such as the aphid Myzus persicae. We tested our hypothesis that switches between lifestyles must be accompanied by fundamental transcriptional reprogramming, reflecting adaptations to different environmental settings. RESULTS: We combined high throughput RNA sequencing of M. brunneum in vitro and at different stages of pathogenesis to validate the modulation of genes in the fungus and its host during the course of infection. In agreement with our hypothesis, we observed transcriptional reprogramming in M. brunneum following conidial attachment, germination on the cuticle, and early-stage growth within the host. This involved the upregulation of genes encoding degrading enzymes and gene clusters involved in synthesis of secondary metabolites that act as virulence factors. The transcriptional response of the aphid host included the upregulation of genes potentially involved in antifungal activity, but antifungal peptides were not induced. We also observed the induction of a host flightin gene, which may be involved in wing formation and flight muscle development. CONCLUSIONS: The switch from saprophytic to parasitic development in M. brunneum is accompanied by fundamental transcriptional reprogramming during the course of the infection. The aphid host responds to fungal infection with its own transcriptional reprogramming, reflecting its inability to express antifungal peptides but featuring the induction of genes involved in winged morphs that may enable offspring to avoid the contaminated environment.