The diapause inducement condition, cold hardiness, and flight ability in Cry1Ac-resistant (BtR) and Cry1Ac-susceptible (96S) strains of Helicoverpa armigera (Hübner) were compared in the laboratory. The BtR strain was derived from the 96S strain and shows 1375-fold resistance to the Cry1Ac toxin after having been selected for 52 generations. Compared with the 96S strain, the Bt-resistant strain was more likely to go into diapause under a short-photoperiod environment. At 11L : 13D, 12L : 12D and 13L : 11D photoperiods, the percentages of BtR insects entering diapause were 72.7%, 82.9% and 68.7%, respectively, which were significantly higher than those in the 96S strain (58.6%, 67.4% and 46.3%, respectively) under the same conditions. The supercooling points (SCP) and freezing points (FP) were not significantly different between the BtR and 96S strains. The LT50 (50% lethal time) and LT90 (90% lethal time) of BtR pupae were also not significantly different from those of the 96S strain at -15°C. The moths from both strains had similar flight ability when their larvae were fed with nontoxic control diet. However, the total flight distance of these BtR moths was 56.2 km whose larvae fed on normal diet, which was more than twice as much as for those feeding on Bt diet (26.2 km). Flight duration for these BtR moths was longer after feeding on normal diet (11.6 h) than after feeding on Bt diet (7.3 h).
A cDNA encoding aminopeptidase N3 was cloned by degenerated PCR and RACE techniques. The full-length of APN3harm is 3486bp. Open reading frame is 3042bp in length, encoding 1014 amino acid residues. Its predicted molecular weight and isoelectric point are 117.04 kDa and 5.14, respectively. This deduced amino acid sequence shares some common structural features with aminopeptidase N from Lepidoptera, including the consensus zinc-binding motif HEXXHX18E and the GAMEN motif common to gluzincin aminopeptidases. The full-length of the APN3harm gene from three susceptible and three resistant strains were cloned and sequenced. Comparison analysis revealed fourteen amino acid differences in the APN3harm gene from resistant and susceptible strains and six mutations of amino acids exist in all resistant strains. It is possible that these mutations are related to the resistance of Helicoverpa armigera to Cry1Ac toxin. The results of semi-quantitative RT-PCR showed that the resistance of H. armigera to Cry1Ac is unrelated to the amount of APN3harm mRNA in midgut tissue. In susceptible strains, APN3harm is highly expressed in mid-gut, foregut and hindgut but not in other tissues. To determine if the APN3harm is the receptor of Cry1Ac, recombinant APN3harm protein was successfully expressed in E. coli. A ligand binding experiment showed purified product could bind Cry1Ac toxin. So it is proposed that APN3harm is a putative receptor of Cry1Ac in H. armigera. The sequence of APN3harm was deposited in GenBank with the accession number AY052651.