Identification of a Linkage Group with a Major Effect on Resistance to Bacillus thuringiensis CrylAc Endotoxin in the Tobacco Budworm (Lepidoptera: Noctuidae)

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Abstract
Resistance to Bacillus thuringiensis (Berliner) CrylAc toxin was subjected to genetic analysis in the YHD2 strain of Heliothis virescens (F.). Resistance in this strain allows survival at high toxin concentrations that kill all susceptible individuals, and permits rapid growth at lower sublethal concentrations that inhibit growth of susceptibles. To quantify the number and relative effects of loci contributing to rapid growth on low concentrations of toxin, a genetic linkage analysis was done with markers on 10 of the 31 chromosomes of H. virescens in a backcross design. Linkage group 9 (marker locus MPl) contributed as much as 80% of the total resistance to CrylAc growth inhibition in YHD2. Recombination between the resistance locus or loci and the marker locus used to identify linkage group 9 occurred only when the informative hybrid parent was the father, which was expected because crossing-over does not occur in H. virescens females. Linkage group 11 (marker locus GDA) made a smaller contribution to resistance that was only detectable when the effect of linkage group 9 was removed. In addition to the effects of these 2 linkage groups, slight but significant differences between families suggested that additional unlinked loci had minor effects on resistance. Measurements of the resistance levels conferred by a small number of genes with the largest effects may be useful in predicting the selection response of H. virescens in the field following the release of transgenic cotton expressing B. thuringiensis toxins.

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