The evolution of complex metarhizium-insect-plant interactions

Metarhizium species interact with plants, insects, and microbes within a diffuse co-evolutionary framework that benefits soil health, biodiversity, and plant growth. The insect host ranges of these fungi vary greatly. Specialization to a narrow host range usually occurs in the tropics with its stable insect populations, and is characterized by the rapid evolution of existing protein sequences, sexual recombination, and small genomes. Host-generalists are associated with temperate regions and ephemeral insect populations. Their mutualistic plant-colonizing lifestyle increases survival when insects are rare, while facultative entomopathogenicity feeds both the fungi and plants when insects are common. Host-generalists have lost meiosis and associated genome defense mechanisms, enabling gene duplications to diversify functions related to plant colonization and host exploitation. Horizontal gene transfer events via transposons have also contributed to host range changes, while parasexuality combines beneficial mutations within individual clones of host-generalists. There is also a lot of genetic variation in insect populations and an increasing understanding that both pathogen virulence and insect immunity are linked with stress responses. Thus, susceptibility to host-generalists can vary due to non-specific resistance to multiple stressors, multipurpose physical and chemical barriers, and heterogeneity in physiological and behavioral factors, such as sleep.