ULTRASTRUCTURAL EFFECTS OF BACILLUS-THURINGIENSIS VAR SAN-DIEGO ON MIDGUT CELLS OF THE COTTONWOOD LEAF BEETLE

Sequential observations of the ultrastructural effects of Bacillus thuringiensis var. san diego were made on midgut epithelial cells of the cottonwood leaf beetle, Chrysomela scripta F. Larvae imbibed a droplet of B. thuringiensis var. san diego containing δ-endotoxin and live spores. Sections of midgut were prepared for transmission electron microscopy at 15-min intervals during the first hour and then hourly for a duration of 3 hr. No signs of cellular disruption were observed until 2 hr after treatment. At this time midgut epithelial cells from treated larvae appeared more elongate and swollen than cells observed from control larvae, with the apical region of the cell bulging into the gut lumen. The cells contained large cytoplasmic spaces, expansion of the basal labyrinth, disruption of rough endoplasmic reticulum, and apical displacement of nuclei. No adverse effects were observed on microvilli, membranes, or organelles. After 3 hr cellular damage had increased, with cells ruptured at the apical surface and leakage of cytoplasmic materials into the gut lumen. B. thuringiensis var. san diego proliferated in the midgut lumen, producing additional δ-endotoxin crystals during sporulation. Although cellular swelling caused by B. thuringiensis var. san diego δ-endotoxin is similar to that reported for B. thuringiensis var. kurstaki δ-endotoxin in lepidopterans, some differences were observed. These differences included lack of membrane lesions and microvillar damage and comparatively slow response. © 1992.