Generalist and specialist Metarhizium insect pathogens retain ancestral ability to colonize plant roots

Metarhizium is widely recognized as an insect pathogenic fungus but it can also form symbiotic associations with plant roots. Here we assessed root rhizoplane, as well as endophytic colonization in monocots and dicots by ten Metarhizium strains representing species with either narrow or broad insect host ranges, as well as a related endophytic and nematophagous fungus, Pochonia chlamydosporia. In addition, insect pathogenicity of these strains was evaluated against mealworm (Tenebrio molitor), wax moth (Galleria mellonella) larvae and grasshoppers (Melanoplus sanguinipes). We found that all Metarhizium strains and Pochonia colonized the rhizoplane and rhizosphere of roots with varying degrees on all plants assessed. All Metarhizium strains tested showed a preference for endophytic colonization of monocot plants (within 20 d) when compared to dicots. While generalists showed pathogenicity towards all tested insect hosts, the specialists showed pathogenicity only towards grasshoppers, whereas Pochonia was avirulent towards insects. We observed variation in ability to colonize the rhizosphere amongst Metarhizium species. However, regardless of whether the Metarhizium species was a generalist or specialist insect pathogen, all strains tested retain ancestral ability to associate with plant roots. (C) 2019 Elsevier Ltd and British Mycological Society. All rights reserved.