Inheritance of Resistance to Bt Toxin Cry1Ac in a Field-Derived Strain of Pink Bollworm (Lepidoptera: Gelechiidae)

Abstract

Laboratory selection with Cry1Ac, the Bacillus thuringiensis (Bt) toxin in transgenic cotton, initially produced 300-fold resistance in a field-derived strain of pink bollworm, Pectinophora gossypiella (Saunders), a major cotton pest. After additional selection increased resistance to 3,100-fold, we tested the offspring of various crosses to determine the mode of inheritance of resistance to Cry1Ac. The progeny of reciprocal F1 crosses (resistant male x susceptible female and vice versa) responded alike in bioassays, indicating autosomal inheritance. Consistent with earlier findings, resistance was recessive at a high concentration of Cry1Ac. However, the dominance of resistance increased as the concentration of Cry1Ac decreased. Analysis of survival and growth of progeny from backcrosses (F1 x resistant strain) suggest that resistance was controlled primarily by one or a few major loci. The progression of resistance from 300- to 3,100-fold rules out the simplest model with one locus and two alleles. Overall the patterns observed can be explained by either a single resistance gene with three or more alleles or by more than one resistance gene. The pink bollworm resistance to Cry1Ac described here fits "mode 1" resistance, the most common type of resistance to Cry1A toxins in Lepidoptera.

Bacillus thuringiensis Vip3Aa Toxin Resistance in Heliothis virescens (Lepidoptera: Noctuidae)

“…Dominance of resistance in other selected populations against Cry toxins has revealed both recessive and incompletely dominant resistance that can vary depending on the concentration of the toxin used. However, the general pattern frequently found shows that the degree of dominance decreases with increasing toxin concentration (34,44,45), the opposite trend to that found in the present study.…”

“…The degree of dominance (h) was estimated using the single-concentration method, based on Hartl's definition of dominance and on survival at any single concentration (44).…”

“…The survival rate was estimated as 100 Ϫ % mortality. Values of h range from 0 (completely recessive) to 1 (completely dominant) (44). The backcross data were used as a direct test of a monogenic model of resistance (59).…”

Inheritance of Resistance to the Cry1Ab Bacillus thuringiensis Toxin in Ostrinia nubilalis (Lepidoptera: Crambidae)

“…Subsequent molecular work with the NO-QA strain mapped genetic control of its Cry1A resistance to a single chromosomal location (Heckel et al 1999). With pink bollworm resistance to Cry1Ac in the AZP-R strain from Arizona, the best Þt occurred with one-or two-locus models (Tabashnik et al 2002), and molecular work identiÞed a cadherin locus tightly linked with high levels of resistance. Diamondback moth resistance to Cry1C in the Cry1C-Sel strain from South Carolina Þt best to a one-locus model (Zhao et al 2000), and subsequent molecular mapping with a related strain (SC1) revealed contributions from two linkage groups (Baxter et al 2005).…”

“…This approach follows the one described by Tabashnik et al (1992Tabashnik et al ( , 2002, except that backcrosses were done with the susceptible parental strains. The slopes of concentrationÐresponse lines for the parental strains were calculated as the mean of the slopes experimentally determined for each parental strain.…”