Insertion of an Esterase Gene into a Specific Locust Pathogen ( Metarhizium acridum) Enables It to Infect Caterpillars

Abstract

An enduring theme in pathogenic microbiology is poor understanding of the mechanisms of host specificity. Metarhizium is a cosmopolitan genus of invertebrate pathogens that contains generalist species with broad host ranges such as M. robertsii (formerly known as M. anisopliae var. anisopliae) as well as specialists such as the acridid-specific grasshopper pathogen M. acridum. During growth on caterpillar ( Manduca sexta) cuticle, M. robertsii up-regulates a gene ( Mest1) that is absent in M. acridum and most other fungi. Disrupting M. robertsii Mest1 reduced virulence and overexpression increased virulence to caterpillars ( Galleria mellonella and M. sexta), while virulence to grasshoppers ( Melanoplus femurrubrum) was unaffected. When Mest1 was transferred to M. acridum under control of its native M. robertsii promoter, the transformants killed and colonized caterpillars in a similar fashion to M. robertsii. MEST1 localized exclusively to lipid droplets in M. robertsii conidia and infection structures was up-regulated during nutrient deprivation and had esterase activity against lipids with short chain fatty acids. The mobilization of stored lipids was delayed in the Mest1 disruptant mutant. Overall, our results suggest that expression of Mest1 allows rapid hydrolysis of stored lipids, and promotes germination and infection structure formation by M. robertsii during nutrient deprivation and invasion, while Mest1 expression in M. acridum broadens its host range by bypassing the regulatory signals found on natural hosts that trigger the mobilization of endogenous nutrient reserves. This study suggests that speciation in an insect pathogen could potentially be driven by host shifts resulting from changes in a single gene.

Author Summary

Host selectivity and host switching have been widely documented in diverse pathogens, but in most cases the underlying mechanisms are poorly understood. Entomopathogenic fungi in the genus Metarhizium are being used as environmentally friendly alternatives to chemical insecticides in agricultural and human disease-vector control programs, and as model systems for studying the interactions between invertebrate hosts and pathogenic fungi. In this paper we describe molecular mechanisms controlling the host selectivity of M. robertsii strain Mr2575, a generalist able to infect hundreds of insect species, and M. acridum strain Ma324, a specialist pathogen of grasshoppers and locusts. The esterase gene ( Mest1) from Mr2575 is required for virulence against caterpillars but not grasshoppers. Ma324 lacks Mest1, but insertion of Mest1 into Ma324 broadens its host range to include caterpillars. Our results suggest that expression of Mest1 allows rapid mobilization of endogenous lipid reserves and promotes germination and infection structure formation. This study suggests that speciation in insect pathogens can be driven by host shifts due to the gain or loss of a pathogen gene that confers wide host specificity.