Abstract
Genetically engineered cotton and corn plants producing insecticidal Bacillus thuringiensis (Bt) toxins kill some key insect pests. Yet, evolution of resistance by pests threatens long-term insect control by these transgenic Bt crops. We compared the genetic basis of resistance to Bt toxin Cry1Ac in two independently derived, laboratory-selected strains of a major cotton pest, the pink bollworm (Pectinophora gossypiella [Saunders]). The Arizona pooled resistant strain (AZP-R) was started with pink bollworm from 10 field populations and selected with Cry1Ac in diet. The Bt4R resistant strain was started with a long-term susceptible laboratory strain and selected first with Bt cotton bolls and later with Cry1Ac in diet. Previous work showed that AZP-R had three recessive mutations (r1, r2, and r3) in the pink bollworm cadherin gene (PgCad1) linked with resistance to Cry1Ac and Bt cotton producing Cry1Ac. Here we report that inheritance of resistance to a diagnostic concentration of Cry1Ac was recessive in Bt4R. In interstrain complementation tests for allelism, F1 progeny from crosses between AZP-R and Bt4R were resistant to Cry1Ac, indicating a shared resistance locus in the two strains. Molecular analysis of the Bt4R cadherin gene identified a novel 15-bp deletion (r4) predicted to cause the loss of five amino acids upstream of the Cry1Ac-binding region of the cadherin protein. Four recessive mutations in PgCad1 are now implicated in resistance in five different strains, showing that mutations in cadherin are the primary mechanism of resistance to Cry1Ac in laboratory-selected strains of pink bollworm from Arizona.
Highlights
Insecticidal crystalline proteins from the common soil bacterium Bacillus thuringiensis (Bt) kill some insect pests, but cause little or no harm to most non-target organisms including people [1,2,3]
Survival was 0% for the F1 progeny of a cross between Arizona pooled resistant strain (AZP-R) and the susceptible strain (Fig. 1), which confirmed that resistance of AZP-R was completely recessive at the diagnostic concentration [33,38,39]
The results here show that recessive mutations in the cadherin gene are associated with resistance to Bt toxin Cry1Ac in two strains of pink bollworm, AZP-R and Bt4R, that differ in their origin and the way in which they were selected for resistance in the laboratory
Summary
Insecticidal crystalline proteins from the common soil bacterium Bacillus thuringiensis (Bt) kill some insect pests, but cause little or no harm to most non-target organisms including people [1,2,3]. Engineered crops producing Bt proteins for insect control were first cultivated commercially in 1996 [3] and grew on more than 58 million hectares worldwide in 2010 [4]. Such Bt crops can improve yields and reduce reliance on conventional insecticides, thereby providing economic, health, and environmental benefits [5,6,7,8]. Field-evolved resistance to Bt crops has been reported for some populations of several insect pests [9,10,11,12,13,14,15]. The most common mechanism involves changes in larval midgut target sites that reduce binding of Bt toxins [20,21,22,23,24,25]
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