ABSTRACT
Two strains of pink bollworm, Pectinophora gossypiella (Saunders), each derived in 1997 from a different field population, were selected for resistance to Bacillus thuringiensis (Bt) toxin Cry1Ac in the laboratory. One strain (MOV97-R) originated from Mohave Valley in western Arizona; the other strain (SAF97-R) was from Safford in eastern Arizona. Relative to a susceptible laboratory strain, Cry1Ac resistance ratios were 1700 for MOV97-R and 520 for SAF97-R. For the two resistant strains, larval survival did not differ between non-Bt cotton and transgenic cotton producing CrylAc. In contrast, larval survival on Bt cotton was 0% for the two unselected parent strains from which the resistant strains were derived. Previously identified resistance (r) alleles of a cadherin gene (BtR) occurred in both resistant strains: r1 and r3 in MOV97-R, and r1 and r2 in SAF97-R. The frequency of individuals carrying two r alleles (rr) was 1.0 in the two resistant strains and 0.02 in each of the two unselected parent strains. Furthermore, in two hybrid strains with a mixture of susceptible (s) and r alleles at the BtR locus, all survivors on Bt cotton had two r alleles. The results show that resistance to Cry1Ac-producing Bt cotton is associated with recessive r alleles at the BtR locus in the strains of pink bollworm tested here. In conjunction with previous results from two other Bt-resistant strains of pink bollworm (APHIS-98R and AZP-R), results reported here identify the cadherin locus as the leading candidate for molecular monitoring of pink bollworm resistance to Bt cotton.
Subject(s)
Bacterial Proteins/genetics , Bacterial Toxins/genetics , Cadherins/genetics , Endotoxins/genetics , Gossypium/genetics , Insecticide Resistance/genetics , Moths/genetics , Plants, Genetically Modified , Animals , Bacillus thuringiensis Toxins , Genotype , Hemolysin ProteinsABSTRACT
Classical and molecular genetic analyses show that two independently derived resistant strains of pink bollworm, Pectinophora gossypiella (Saunders), share a genetic locus at which three mutant alleles confer resistance to Bacillus thuringiensis (Bt) toxin Cry1Ac. One laboratory-selected resistant strain (AZP-R) was derived from individuals collected in 1997 from 10 Arizona cotton fields, whereas the other (APHIS-98R) was derived from a long-term susceptible laboratory strain. Both strains were previously reported to show traits of "mode 1" resistance, the most common type of lepidopteran resistance to Cry1A toxins. Inheritance of resistance to a diagnostic concentration of Cry1Ac (10 microg per gram of diet) was recessive in both strains. In interstrain complementation tests for allelism, F1 progeny from crosses between the two strains were resistant to the diagnostic concentration of Cry1Ac. These results indicate that a major resistance locus is shared by the two strains. Analysis of DNA from the pink bollworm cadherin gene (BtR) using allele-specific polymerase chain reaction (PCR) tests showed that the previously identified resistance alleles (r1, r2, and r3) occurred in both strains, but their frequencies differed between strains. In conjunction with previous findings, the results reported here suggest that PCR-based detection of the three known cadherin resistance alleles might be useful for monitoring resistance to Cry1Ac-producing Bt cotton in field populations of pink bollworm.