| The Large-scale plantings of Bt cotton have helped to control the damage of cotton bollworm effectively in China, and reduce the use of pesticides, make it more economical and more ecofriendly. Bt cotton can express Bt toxin continuously during its whole growth season, so it is inevitable that the cotton bollworm (Helicoverpa armigera) as the main target insect of Bt cotton, can evolve resistance to Bt cotton, which will directly affect the long-term use of Bt cotton. Understanding the molecular mechanism of Bt toxin action and the fittness cost relate to Bt resistance is important to the development of adaptive resistance monitoring techniques, and make it clear to forecast the Cry 1 Ac-resistance evolutionary options.The resistant strain SCD-rl, which is homozygous for a truncated allele of HaCad (r1), has 422-fold resistance to Cry 1 Ac activated toxin. Here we selected SCD-rl strain for 10 generations with Cry 1 Ac protoxin, then resistance levels to activated Cry 1 Ac toxin and protoxin were compared, the protease enzyme activities were determined, and activation and degradation of Cry 1 Ac by midgut juices before and after the selection were characterized. To evaluate whether cadherin-based resistance is associated with fitness costs reducing male paternity in H. armigera, we examined the effects of a major cadherin resistance allele (r1) on sperm competition within and between male ejaculates. The results here will be of great important for elucidating the interaction between Cry 1 Ac toxin and cadherin receptor, and understanding the potential evolution of HaCad-based Bt resistance.1.Different resistance levels to CrylAc activated toxin and protoxin in SCD-rl strainThe resistant strain SCD-rl, which is homozygous for a truncated allele of HaCad (r1), has 422-fold resistance to CrylAc activated toxin and 22-fold resistance to CrylAc protoxin. There is 19-fold different resistant level between these two different toxins. In order to investigate the factor that may result in this difference, we selected SCD-r1 strain with CrylAc protoxin for 10 generations. The selected strain obtained 69-fold to CrylAc protoxin and 622-fold to Cry1Ac activated toxin, the different resistance level to these two toxins decreased to 9-fold. To identify contributions of midgut proteases to the different resistance levels to CrylAc protoxin and activated toxin, the protease enzyme activities, in vitro activation and degradation of CrylAc by midgut juices were compared among two resistant strains (SCD-rl and SCD-rl-selected strain) and a susceptible strain SCD. The results showed:protease activities against trypsin and chymotrypsin substrates in SCD-rl were less 2 fold compared with that from SCD-rl-selected strain and SCD, and the whole protease enzyme activities were similar among the three strains, so the proteinase activities were not correlated with resistance levels to these two toxins. The speed of degradation of CrylAc by midgut juices become slower as the resistance level to CrylAc protoxin increased, but prolong the time of incubation, the digested products of this three strain were same, no laggard degradation was appeard. The results indicated that the different resistance levels to activated CrylAc toxin and protoxin did not correlate with the midgut proteases. Considering that the action mechnisms of Cry 1A activated toxin and protoxin are different, we speculated that the main reason causing different resistance levels to CrylAc toxin and protoxin in SCD-r1 is the different interactions between these two types of the toxin and their midgut receptors.2. Effects of a deletion mutation of cadherin on male paternity and sperm competition in H. armigeraCadherins are transmembrane proteins that mediate cell-cell adhesion and tissue morphogenesis, suggesting that fitness costs associated with cadherin mutations may be present in many aspects of life history. It has been reported that cadherin mutations can reduce insect fitness of Bt resistance strains. To evaluate whether cadherin-based resistance is associated with fitness costs reducing male paternity in H. armigera, we examined the effects of a major cadherin resistance allele on sperm competition within and between male ejaculates:(1) Paternity competition between two males; (2) Sperm competition between R and S sperm. The results showed that:the fertilization advantage of RR males with SS females, and the fertilization advantage of SS males with RR females, was similar irrespective of the order in which the male genotypes mated. It seemed that females discriminate between males on the basis of cadherin genotype, females preferentially used sperm from males of the opposite genotype. When RS male mated with homozygous female RR or SS, the proportion of offspring genotypes was not always 1:1. Heterozygous males mated to resistant females, however, resulted in a disproportionate number of heterozygous offspring. All the above results show that cadherin-based resistance to CrylAc has significant impacts on paternity in H. armigera, Rather, effects of cadherin-based resistance on paternity depended on interactions between male and female genotypes. |
PDF Full Text Request