Host-to-Pathogen Gene Transfer Facilitated Infection of Insects by a Pathogenic Fungus

Abstract

Metarhizium robertsii is a plant root colonizing fungus that is also an insect pathogen. Its entomopathogenicity is a characteristic that was acquired during evolution from a plant endophyte ancestor. This transition provides a novel perspective on how new functional mechanisms important for host switching and virulence have evolved. From a random T-DNA insertion library, we obtained a pathogenicity defective mutant that resulted from the disruption of a sterol carrier gene ( Mr-npc2a). Phylogenetic analysis revealed that Metarhizium acquired Mr-npc2a from an insect by horizontal gene transfer (HGT). Mr-NPC2a binds to cholesterol, an animal sterol, rather than the fungal sterol ergosterol, indicating it retains the specificity of insect NPC2 proteins. Mr-NPC2a is an intracellular protein and is exclusively expressed in the hemolymph of living insects. The disruption of Mr-npc2a reduced the amount of sterol in cell membranes of the yeast-like hyphal bodies that facilitate dispersal in the host body. These were consequently more susceptible to insect immune responses than the wild type. Transgenic expression of Mr-NPC2a increased the virulence of Beauveria bassiana, an endophytic insect-pathogenic fungus that lacks a Mr-NPC2a homolog.

Author Summary

The ability of infectious agents to evolve different host ranges contributes to the emergence of new diseases, and this host switching could also account for the wide variety of fungal associations with animals, plants and other fungi. There must be mechanisms for such host shifts, but these remain largely unknown. In this study, we phylogenetically predict that the endophytic fungus Metarhizium robertsii acquired a sterol carrier gene from insects through horizontal gene transfer (HGT). This sterol carrier is involved in maintaining cell membrane sterols, and thus membrane integrity, when M. robertsii proliferates in the haemocoel of living insects. Therefore, the acquisition of genetic material from a host has contributed to the development of fungal entomopathogenicity. In order to simulate this evolutionary event, the sterol carrier gene was transformed into an endophytic insect-pathogenic fungus ( Beauveria bassiana) that lacks an endogenous Mr-NPC2a homolog. The virulence of B. bassiana was increased by expression of Mr-NPC2a.