| In this paper, we studied the organophosphorus resistance mechanism in cotton bollworm Helicoverpa armigera (Hubner), one of the serious cotton pests in the world. Our study especially put stress on the biochemical and molecular analysis of acetylcholinesterase (AChE, EC 3.1.1.7), which is the inhibition target enzyme of organophosphate and carbamate insecticides.Monocrotophos resistance was selected in laboratory with the field resistant cotton bollworm collected from Liaocheng County (LC-R), Shandong Province. After ten-generation selections with monocrotophos during sixteen generations, the resistance increased 6.67-fold and was as high as 211.88-fold when compared with a susceptible strain collected from Xinjiang (XJ-S). But the resistance of the selected strain to fenvalerate, cypermethrin and methomyl decreased by 8.13-, 7.44- and 13.36-fold, respectively, during the selection. Methylparathion and phoxim resistance was basically unchanged as compared with that of FQ. These indicated there was no significant cross-resistance between monocrotophos and the other 5 insecticides tested. Toxicity of 8 insecticides to resistant and susceptible H. armigera showed that the LC-R strain has developed high resistance to monocrotophos (RR=211.9) and low resistance or high insecticide-proof ability to the other 7 insecticides.By constructing life table of resistant strain and susceptible strain, the effects of monocrotophos resistance on fitness in the cotton bollworm were evaluated in terms of developmental and reproductive characteristics. The results indicated that the LC-R strain possessed developmental disadvantages including prolonged larva and pupa duration, lighter pupae weight, lower pupation and emergence rate, and reproductive disadvantages including lower copulation rate, fecundity and hatchability when compared with the XJ-S strain. Relative fitness was determined by population number tendency index (I). The LC-R strain was calculated to have a fitness value of 0.14relative to the XJ-S strain, which could be contributed to the slow development of organophosphate resistance in cotton bollworm in fields.Acetylcholinesterase (AChE) in the resistant and susceptible H. armigera was also studied. The results showed that AChE in resistant strain was much less sensitive to monocrotophos. The I50 value for resistant AChE was 3.18-fold higher than that for susceptible. Partial purification of AChE to remove esterase and some other proteins increased the sensitivity dramatically. The I50 reduced by 6.69-fold and 3.49-fold for resistant and susceptible strain, respectively. This indicated that there might be some factors protecting AChE, which was more powerful in resistant strain. Even so, there was still 1.66-fold difference in the I50 value of purified AChEs from resistant and susceptible strains. Kinetic parameters of AChE also showed significant difference between these two strains. Esterase activity was 5-fold higher in resistant larvae than in the susceptible strain. So, it is concluded that the decreased sensitivity of AChE together with some other factors in the resistant strain contributes to monocrotophos resistance.With the degenerate primers we have amplified a 281bp cDNA fragment of AChE gene in cotton bollworm by reverse transcription-polymerase chain reaction (RT-PCR) method using total RNA extracted from 4th larva as the template. The cDNA fragment was inserted into pGEM-T vector and then cloned. The deduced amino acid sequence of AChE was consisted of 94 residues. The sequence analysis indicated that the deduced amino acid sequence of the cDNA fragment shared high identity with AChE gene from other published insects and animals. The acquired sequence had 84%, 79%, 74%, 70%, 70%, 72%, 68%, 61%, 55% and 57% of amino acid residues identical to those of Leptinotarsa decemlineata (L.d.), Nephotettix cincticeps (N.c.), Anopheles stephensi (A.s.), Aedes aegypti (A.a.), Lucilia cuprina (L.c.\ Drosophila melanogaster (D.m.), Musca domestica (M.d.), Meloidogyne incognita (M. i.), Torpedo californica...
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