Control of Resistant Pink Bollworm ( Pectinophora gossypiella ) by Transgenic Cotton That Produces Bacillus thuringiensis Toxin Cry2Ab

Abstract

Crops genetically engineered to produce Bacillus thuringiensis toxins for insect control can reduce use of conventional insecticides, but insect resistance could limit the success of this technology. The first generation of transgenic cotton with B. thuringiensis produces a single toxin, Cry1Ac, that is highly effective against susceptible larvae of pink bollworm (Pectinophora gossypiella), a major cotton pest. To counter potential problems with resistance, second-generation transgenic cotton that produces B. thuringiensis toxin Cry2Ab alone or in combination with Cry1Ac has been developed. In greenhouse bioassays, a pink bollworm strain selected in the laboratory for resistance to Cry1Ac survived equally well on transgenic cotton with Cry1Ac and on cotton without Cry1Ac. In contrast, Cry1Ac-resistant pink bollworm had little or no survival on secondgeneration transgenic cotton with Cry2Ab alone or with Cry1Ac plus Cry2Ab. Artificial diet bioassays showed that resistance to Cry1Ac did not confer strong cross-resistance to Cry2Aa. Strains with >90% larval survival on diet with 10 g of Cry1Ac per ml showed 0% survival on diet with 3.2 or 10 g of Cry2Aa per ml. However, the average survival of larvae fed a diet with 1 g of Cry2Aa per ml was higher for Cry1Ac-resistant strains (2 to 10%) than for susceptible strains (0%). If plants with Cry1Ac plus Cry2Ab are deployed while genes that confer resistance to each of these toxins are rare, and if the inheritance of resistance to both toxins is recessive, the efficacy of transgenic cotton might be greatly extended.

Asymmetrical cross-resistance betweenBacillus thuringiensistoxins Cry1Ac and Cry2Ab in pink bollworm

“…1). Confirming previous results (26,27), the AZP-R strain selected with Cry1Ac had little or no crossresistance to Cry2Ab or Cry2Aa (Tables 1 and 2). Relative to the susceptible strain APHIS-S, the maximum resistance ratios for BX-R1 were 240 for Cry2Ab and 420 for Cry1Ac.…”

“…Whereas previous laboratory selection of pink bollworm with Cry1Ac produced little or no cross-resistance to Cry2A toxins (26,27), selection with Cry2Ab here yielded up to 420-fold crossresistance to Cry1Ac ( Table 1). As far as we know, asymmetrical cross-resistance between Cry1 and Cry2 toxins has not been reported before.…”

“…Diet Bioassays and Selection. We measured susceptibility to Cry2Ab, Cry1Ac, and Cry2Aa protoxins by using bioassays in which the toxins were incorporated in diet at 6 -8 concentrations, neonates were put individually on diet, and survival was recorded after 21 days at 29 Ϯ 2°C (22,27,28). We also measured susceptibility to combinations of Cry1Ac (10 or 100 g toxin/mL diet) and Cry2Ab in diet (10 g toxin/mL diet).…”

“…The powder was made by grinding freeze-dried leaves of corn (Mon84006) that produced Cry2Ab2. Cry2Aa2 provided by William Moar (Auburn University, Auburn, AL) was purified from transformed Eschericia coli that produced Cry2Aa protoxin inclusion bodies (27). We refer to Cry2Ab2 as Cry2Ab and Cry2Aa2 as Cry2Aa because the amino acid sequence is the same for Cry2Ab2 and Cry2Ab, as well as for Cry2Aa2 and Cry2Aa (http://www.lifesci.sussex.…”

“…AZP-R is a Cry1Ac-resistant strain that was started by pooling survivors of exposure to Cry1Ac in diet from 10 strains derived in 1997 from Arizona cotton fields (22). AZP-R had been selected repeatedly with 10 or 100 g Cry1Ac/mL diet, yielding Ͼ1,000-fold resistance to Cry1Ac, but little cross-resistance to Cry2Aa or Cry2Ab (27,28). BX-H is a hybrid strain that included a mixture of Cry1Ac-resistant and susceptible individuals (34).…”