Shared binding sites in lepidoptera for Bacillus thuringiensis Cry1Ja and Cry1A toxins

Bacillus thuringiensis toxins act by binding to specific target sites in th e insect midgut epithelial membrane. The best-known mechanism of resistance to B. thuringiensis toxins is reduced binding to target sites. Because alt eration of a binding site shared by several toxins may cause resistance to all of them, knowledge of which toxins share binding sites is useful for pr edicting cross-resistance. Conversely, cross-resistance among toxins sugges ts that the toxins share a binding site. At least two strains of diamondbac k moth (Plutella xylostella) with resistance to Cry1A toxins and reduced bi nding of Cry1A toxins have strong cross-resistance to Cry1Ja. Thus, we hypo thesized that Cry1Ja shares binding sites with Cry1A toxins. We tested this hypothesis in six moth and butterfly species, each from a different family : Cacyreas marshalli (Lycaenidae), Lobesia botrana (Tortricidae), Manduca s exta (Sphingidae), Pectinophora gossypiella (Gelechiidae), P. xylostella (P lutellidae), and Spodoptera exigua (Noctuidae). Although the extent of comp etition varied among species, experiments with biotinylated Cry1Ja and radi olabeled Cry1Ac showed that Cry1Ja and Cry1Ac competed for binding sites in all six species. A recent report also indicates shared binding sites for C ry1Ja and Cry1A toxins in Heliothis virescens (Noctuidae). Thus, shared bin ding sites for Cry1Ja and Cry1A occur in all lepidopteran species tested so far.