Physical and biochemical properties of entomopathogenic fungal spores
Entomopathogenic fungi (EPF) are poised to become major commercial insect control agents, and their spores are the key to any mycoinsecticide or biological control strategy. Pie purpose of this study was to characterize and compare the surface properties of spores produced by hyphomycete EPF and investigate how these attributes relate to spore association with a host insect Melanoplus sanguinipes. The salt-mediated aggregation and sedimentation (SAS) assay was developed to quantitatively estimate spore surface hydrophobicity. The surface properties of aerial conidia (AC) from 24 EPF isolates were studied and compared using: the SAS assay, electron microscopy, fluorescently-labelled lectins, and spore dimensions. Spores with rugose surfaces were hydrophobic whereas hydrophilic spores possessed smooth surfaces. Correlation analysis found no link between spore dimensions and either hydrophobicity or surface carbohydrates. However, there was a strong positive con-elation between spore hydrophobicity and surface carbohydrates. The three spore types of 'Beauveria bassiana' were shown to possess discrete surface hydrophobicities that were also strongly linked to surface carbohydrate profiles. Various chemical treatments had pronounced effects on spore surface properties, with sodium dodecyl sulfate (SDS) and formic acid reducing both lectin binding and surface hydrophobicity. When formic acid protein extracts were separated and analysed using SDS-polyacrylamide gel electrophoresis, only the hydrophobic spores were found to possess low molecular weight hydrophobin-like peptides that were unglycosylated and contained disulfide bonds. For the association of spores with the grasshopper body, surfaces possessing hairs and cavities accumulated and retained spores in higher densities. The infection of grasshoppers by spore ingestion was efficacious, and the palps and the mouthparts were found to be the primary infection sites, not the alimentary tract. Hydrophobicity was not found to be a major determinant of spore association with grasshoppers, suggesting that it is the complex and varied topography of this insect's cuticular surface that primarily promotes spore association and retention through entrapment. The findings of this work have produced some new insights into the surface properties of EPF spores that are significant to the study of spore-host interactions and in the selection, production and formulation of spores for future mycoinsecticides.
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