FORUM Shared Genetic Basis of Resistance to Bt Toxin Cry1Ac in Independent Strains of Pink Bollworm BRUCE E. TABASHNIK, 1 YONG-BIAO LIU, 2 DEVIKA C. UNNITHAN, YVES CARRIE ` RE, TIMOTHY J. DENNEHY, AND SHAI MORIN 3 Department of Entomology, University of Arizona, Tucson, AZ 85721 J. Econ. Entomol. 97(3): 721Ð726 (2004) ABSTRACT 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 Þelds, 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 g per gram of diet) was recessive in both strains. In interstrain complementation tests for allelism, F 1 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-speciÞc polymerase chain reaction (PCR) tests showed that the previously identiÞed resistance alleles (r1, r2, and r3) occurred in both strains, but their frequencies differed between strains. In conjunction with previous Þndings, 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 Þeld populations of pink bollworm. KEY WORDS genetics, genetically modiÞed crops, resistance, Bacillus thuringiensis, Pectinophora gossypiella TRANSGENIC CORN AND COTTON producing Bacillus thu- ringiensis (Bt) toxins that kill larvae of some key lep- idopteran pests grow on millions of hectares annually (Schnepf et al. 1998, James 2003). Such Bt crops can reduce reliance on conventional insecticides, thereby providing economic, health, and environmental ben- eÞts (Shelton et al. 2002). Evolution of resistance to Bt toxins by insect pests, however, could cut short such beneÞts. Although Þeld-evolved resistance to Bt crops has not been reported yet (Carrie ` re et al. 2003, Tabashnik et al. 2003), many pests have been selected for resistance to Bt toxins in the laboratory (Tabashnik 1994, Ferre ´ and Van Rie 2002). Also, resistance to Bt sprays has evolved in greenhouse populations of cab- bage looper, Trichoplusia ni (Hu ¨ bner) (Janmaat and Myers 2003), and in Þeld populations of diamondback moth, Plutella xylostella (L.) (Tabashnik 1994, Ferre ´ and Van Rie 2002). Understanding the genetic basis of resistance to Bt is essential for developing and implementing strate- gies to delay and monitor resistance (Gould 1998). One crucial issue is the extent of variation in the genetic basis of resistance to Bt among and within pest species. If such variation is limited, as in some cases of resistance to conventional insecticides (ffrench- Constant et al. 2000, Daborn et al. 2002), results with a few representative cases may have broad applica- bility. Conversely, if the genetic basis of resistance varies greatly among and within pest species, detailed genetic analyses might be required for many popula- tions of each pest. In lepidopteran pests, the most common type of resistance to Bt toxins is called “mode 1.” Mode 1 resistance entails high levels of resistance to at least one Cry1A toxin, recessive inheritance, reduced bind- ing to midgut membranes of at least one Cry1A toxin, and little or no cross-resistance to Cry1C (Tabashnik et al. 1998). Mode 1 resistance has been reported in at least one strain of diamondback moth; tobacco bud- worm, Heliothis virescens (F.); Indianmeal moth, Plo- dia interpunctella (Hu ¨ bner); and pink bollworm, Pecti- nophora gossypiella (Saunders) (Tabashnik et al. 1998, Gonza ´ lez-Cabrera et al. 2003). 1 E-mail: brucet@ag.arizona.edu. 2 Current address: Agricultural Research Station, USDAÐARS, 1636 E. Alisal St., Salinas, CA 93905. 3 Department of Entomology, Faculty of Agricultural, Food and Environmental Quality Sciences, The Hebrew University of Jerusa- lem, P.O. Box 12, Rehovot 76100, Jerusalem, Israel. 0022-0493/04/0721Ð0726$04.00/0  2004 Entomological Society of America