Genetics and molecular biology of fungus-insect interactions

Fungal pathogens of insects play important roles in regulating insect populations in nature, most of which are ascomycetes. The species like Metarhizium spp. and Beauveria bassiana has been developed as environmentally friendly biocontrol agents against different insect pests. Comparative and phylogenomic analyses revealed that fungal entomopathogenicity is polyphyletic and more closely related to phytopathogens instead of mammalian pathogens. Relative non-insect pathogens, similar expansions of insect cuticle degrading enzymes reflect a convergent evolution unique for fungus-insect interactions. Genomics analysis of different Metarhizium species with varied host range indicated that the specialist species with a narrow host range diverged first and then the transitional species with intermediate host ranges evolved and followed by the generalists in association with protein family expansions. Similar to plant pathogens, an array of effector-like proteins are also encoded by animal pathogens despite that the gene-for-gene relationship is still suspected in fungus-animal interactions. Our molecular biology studies indicated that the insect fungi like M. robertsii evolved with a lineage-specific collagen-like protein to camouflage cell wall components for evading insect-host immunities. Divergent LysM proteins encoded in insect pathogens such as B. bassiana can bind chitin to suppress chitin-induced immune responses in insects and or protect fungal cell wall against chitinases. Besides the polypeptide effectors, we also found that secondary metabolites biosynthesized by insect pathogens could also be deployed to inhibit host innate immune responses and thereby facilitate fungal propagation within insect body cavity. Our studies advanced the understanding of the evolution and effector mechanisms of fungus-host interactions.