| The environmentally friendly insecticide crystal proteins produced by Bacillus thuringiensis are of great scientific interest because of their potency and specificity to a wide range of insect pests. As the genes coding for Cry proteins of Bt are transferred to the genomes of plants, the effect of Bt on pest management become more and more important. However, greatly increased use of Bt, weather delivered by conventional sprays or by genetically engineered crops, raises the likelihood of insect resistance to Bt. The laboratory selection experiments have demonstrated that many pests can develop resistance to Bt toxins. So far, documented cases of resistance to Bt in open-field populations of pests are limited to one insect species, the diamondback moth, Plutella xylostella. Managing the evolution of resistance to Bt toxins requires extensive knowledge about the mechanisms, and genetics of resistance genes. In present study, resistance status of Bt formulation and Bt toxins in P. xylostella from southeast China was monitored, a laboratory resistant strain (PSBT) was selected and its inheritance and resistance mechanisms were investigated.1. Resistance status of Bt resistance in P. xylostella from southeast ChinaFour field populations of diamondback moth were collected respectively in Guangdong, Fujian, Zhejiang and Jiangsu provinces during 2003 autumn and 2004 spring. The resistance levels to Cry1Aa, Cry1Ab, Cry1Ac, Cry2Aa and Bt formulation (Btk) in these four populations were bioassayed. The results showed that Guangdong population had high levels of resistance to Cry1Ab, Cry1Ac toxins and middle level of resistance to Btk. Fujian population had middle levels of resistance to Cry1Ab, Cry1Ac and Btk. However, Zhejiang and Jiangsu populations only had low levels of resistance to both Bt toxins and Btk.2. Selection for resistance to activated Cry1Ac and cross-resistance in P. xylostellaA field population of P. xylostella, collected from Pingshan town, GuangdongProvince, in 2002 was divided into two sub-strains. One was selected with activated Cry1Ac toxin for 20 generations and designated as PSBT strain. Another was reared in laboratory for 20 generations without exposure to any insecticides and designated as PS strain. The PSBT strain developed 39.3-fold resistance to Cry1Ac compared with the PS strain. Compared with the PHI-S susceptible strain, the PSBT strain had 1178-fold resistance to Cry1Ac.In the process of selection with activated Cry1Ac toxin, the development of cross-resistance to Cry1Aa, Cry1Ab, Cry2Aa and Btk was monitered every four generations. With the increase of resistance to Cry1Ac in strain of PSBT, the level of cross-resistance to Cry1Aa, Cry1Ab was 13.4-fold and 17.8-fold, respectively. However, there was only 2.8-fold increase of resistance to Btk and no significant change of resistance to Cry2Aa. These results indicated that there was serious cross-resistance to Cry1Aa, Cry1Ab in the strain of PSBT, low level cross-resistance to Btk and no cross-resistance to Cry2Aa.3. Inheritance mode of Cry1Ac resistance in the PSBT strain of P. xylostellaPSBT strain and PHI-S strain were used for reciprocal crosses and backcrosses. The LD-P lines of PSBT, PHI-S, F1 and F1' progeny to Cry1Ac were constructed. The LC50 of F1 and F1' was 0.052 and 0.046 μg·ml-1 respectively which was not significantly different, which suggested that Cry1Ac resistance was inherited as an autosomal character. The value of dominance level of F1 and F1' was -0.27 and -0.31 respectively and that showed the resistance was incompletely recessive. In the LD-P curve of mortality due to Cry1Ac of the backcross progeny, there was a distinct plateau corresponding to 50% mortality. This plateau suggests that Cry1Ac resistance may be controlled by one gene. The x2 test of the goodness of fit between the observation and predictions under the assumption of monofactorial inheritance further supported the single-gene hypothesis.4. Binding analyses of Cry1Ac using BBMV from resistant and susceptible strains of P. xylostellaCry1 Ac-specific bindi...
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