A NOVEL INSECTICIDAL TOXIN FROM PHOTORHABDUS-LUMINESCENS, TOXIN COMPLEX A (TCA), AND ITS HISTOPATHOLOGICAL EFFECTS ON THE MIDGUT OF MANDUCA-SEXTA

Photarhabdus luminescens is a bacterium which is mutualistic with ento mophagous nematodes and which secretes high-molecular-weight toxin com plexes following its release into the insect hemocoel upon nematode in vasion. Thus, unlike other protein toxins from Bacillus thuringiensis (delta-endotoxins and Vip's), P. luminescens toxin (Pht) normally acts from within the insect hemocoel. Unexpectedly, therefore, the toxin c omplex has both oral and injectable activities against a wide range of insects, We have recently fractionated the protein toxin and shown it to consist of several native complexes, the most abundant of which we have termed Toxin complex a (Tca), This complex is highly active agai nst the lepidopteran Manduca sexta, Zn view of the difference in the n ormal mode of delivery of P, luminescens toxin and the apparent commun ality in the histopathological effects of other gut-active toxins from B. thuringiensis, as well as cholesterol oxidase, we were interested ire investigating the effects of purified Tea protein on larvae of M. sexta. Here we report that the histopathology of the M, sexta midgut i s similar to that for other novel midgut-active toxins. Following oral ingestion of Tea by ill. sexta, we observed are acceleration in the b lebbing of the midgut epithelium into the gut lumen and eventual lysis of the epithelium, The midgut shows a similar histopathology followin g injection of Tea into the insect hemocoel, These results not only sh ow that Tea is a highly active oral insecticide but also confirm the s imilar histopathologies of a range of very different gut-active toxins , despite presumed differences In modes off action and/or delivery, Th e implications for the mode of action of Tea are discussed.