A nonpermissive entomophthoralean fungal infection increases activation of insect prophenoloxidase

Entomophaga maimaiga and Entomophaga aulicae are entomopathogenic fungi that show species-specific infection in Lepidoptera. These fungi grow as protoplasts in the hemolymph of permissive insect hosts. E. maimaiga infects gypsy moth larvae, Lymantria dispar, and E. aulicae infects hemlock looper, Lambdina fiscellaria. Cross-infections do not occur and are referred to as a nonpermissive response. We circumvented cuticular barriers and injected E. aulicae protoplasts into L. dispar and investigated factors which could potentially curtail entomophthoralean fungal infection in a nonpermissive host insect. There was no evidence of (i) restricted fungal growth in cell-free hemolymph of the nonpermissive host, (ii) fungal toxins specific to host insect, (iii) hemocyte encapsulation of fungal protoplasts in a nonpermissive or permissive infection, or (iv) fungal-specific induction of plasma proteins in L. dispar. However, higher levels of phenoloxidase activity for up to 96 h postchallenge, as well as a prophenoloxidase-activating trypsin activity, were observed for L. dispar challenged with E. aulicae when compared to an E. maimaiga challenge. Three isoforms of phenoloxidase (pI 5.0-5.5) and at least six isoforms of trypsin activity (four basic trypsins pI 8-10 and two acidic trypsins pI 4-6) with preferences for small amino acid residues were activated in L. dispar after challenge. In vitro prophenoloxidase activation experiments showed that treatment of L. dispar hemolymph with E. aulicae protoplast plasma membranes consistently resulted in higher prophenoloxidase activation than E. maimaiga. We suggest that differences in protoplastic components of Entomophaga species, such as the surface glycoproteins, are implicated in activation of zymogenic trypsins in the insect which in turn activate the prophenoloxidase cascade as a nonpermissive response. (C) 1998 Academic Press.