A NONPERMISSIVE ENTOMOPHTHORALEAN FUNGAL INFECTION INCREASES ACTIVATION OF INSECT PROPHENOLOXIDASE

Entomophaga maimaiga and Entomophaga aulicae are entomopathogenic fung i that show species-specific infection in Lepidoptera. These fungi gro w as protoplasts in the hemolymph of permissive insect hosts. E. maima iga infects gypsy moth larvae, Lymantria dispar, and E. aulicae infect s hemlock looper, Lambdina fiscellaria. Cross-infections do not occur and are referred to as a nonpermissive response. We circumvented cutic ular barriers and injected E. aulicae protoplasts into L. dispar and i nvestigated 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 nonperm issive host, (ii) fungal toxins specific to host insect, (iii) hemocyt e 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 act ivity, were observed for L. dispar challenged with E. aulicae when com pared to an E. maimaiga challenge. Three isoforms of phenoloxidase (pI 5.0-5.5) and at least six isoforms of trypsin activity (four basic tr ypsins pI 8-10 and two acidic trypsins pI 4-6) with preferences for sm all amino acid residues were activated in L. dispar after challenge. I n vitro prophenoloxidase activation experiments showed that treatment of L. dispar hemolymph with E. aulicae protoplast plasma membranes con sistently resulted in higher prophenoloxidase activation than E. maima iga. We suggest that differences in protoplastic components of Entomop haga species, such as the surface glycoproteins, are implicated in act ivation of zymogenic trypsins in the insect which in turn activate the prophenoloxidase cascade as a nonpermissive response. (C) 1998 Academ ic Press.