Fitness Costs Of Non-Recessive Resistance To Cry1ac And CRY2AB Of Helicoverpa Armigera (H(?)bner)

The cotton bollworm (Helicoverpa armigera) is one of the most serious pests, and it has developed serious resistance to most insecticides. The extensive planting of Bt cotton in China has effectively controlled the cotton bollworm. However, there has been an upward trend of the resistance gene frequency in H. armigera due to continuous selection pressure from the Bt cotton in recent years. The evolution of resistance in field populations of H. armigera to Bt cotton is a major threat to the continued efficacy of Bt cotton. A number of factors may affect insect Bt resistance evolution, and the relative fitness of resistance genes is one of the most important factors. Refuge strategy is one of the main measures used to delay Bt resistance. The capacity of refuges to delay resistance is affected by rare initial frequency of resistance alleles, susceptible insects mating with resistant insects freely and recessive resistance gene. Two H. armigera strains XJ-rl5and An2Ab developed high levels of non-recessive resistance to CrylAc and Cry2Ab respectively. The non-recessive resistance in XJ-rl5is caused by a deletion mutation in the intracellular region of cadherin. The current study investigated the fitness costs of non-recessive resistance to Cryl Ac and Cry2Ab in XJ-r15and An2Ab strains respectively, impacts of the mutation (r15allele) in the intracellular region of cadherin on sperm competition, and effects of transgenic and conventional cotton leaves on the growth and development of An2Ab with resistance to Cry2Ab.1. Relative fitness of the XJ-r15strain of H. armigera with non-recessive resistance to CrylAcThe strain XJ-r15had about140-fold resistance to CrylAc activated toxin compared with susceptible SCD strain. Life tables of XJ-r15, SCD, SCD♀×XJ-r15♂XJ-r15♀×SCD♂strains/crosses on artificial diet in the laboratory were constructed. The results showed that the larval development time in XJ-r15was0.9d significantly longer than in SCD. The net replacement rate (R0) was calculated to determine the relative fitness for the above strains/crosses. The relative fitness of XJ-r15was0.91and that of the two F1 progenies was the same (0.94). It shows that the resistant XJ-rl5strain had a non-recessive fitness cost on artificial diet.2. Effects of intracellular domain deletion mutation of cadherin on sperm competition in H. armigeraResistance to Cry1Ac in XJ-r15was due to the intracellular domain deletion mutation of cadherin. Among the four mating combinations, XJ-r15males had sperm competition disadvantage as compared to SCD males. When H. armigera females mated effectively with two males, only one male sired the progenies according to PCR detection of the genotypes in the progenies. It shows that the intracellular domain deletion mutation of cadherin in the XJ-r15strain had a fitness cost on sperm competition.3. Relative fitness of the An2Ab strain of H. armigera with non-recessive resistance to Cry2AbThe An2Ab strain had636-fold resistance to Cry2Ab toxin compared with susceptible An strain. Life tables of An, An2Ab, and F1progenies (An♀×An2Ab♂; An♂×An2Ab♀) on artificial diet in the laboratory were constructed. The results showed that the larval development time and pupa development time of the resistance strain were1.4d and1.6d significantly longer than in the susceptible strain respectively. The pupal weights and pupation rate of the resistance strain were27.3mg and10%significantly lower than in the susceptible strain respectively. The realized fecundity per female of the resistance strain was56.3eggs significantly fewer than in the susceptible strain. The net replacement rate (R0) was calculated to determine the relative fitness for the above strains. The relative fitness of An2Ab was0.78, F1progenies was1.13compared with the susceptible strain An. It indicates that the resistant strain An2Ab had a recessive fitness cost on artificial diet.4. Effects of Bt-transgenic and conventional cotton leaves on the growth and development of the Cry2Ab-resistant An2Ab strainThe effects of two Bt cotton varieties (expressing Cry1Ac and CrylAc+Cry2Ab, respectively) and one conventional cotton (Simian3) on the growth and development of the An2Ab strain, the susceptible An and their Fi progeny were compared with cotton leaf bioassays in the laboratory. The resistant strain and F1progenies showed fitness costs when fed on conventional cotton (Simian3) leaves. The larval survival rates, pupation rates, pupal weight and realized fecundity of the resistant strain and F1progenies were significantly lower than those of the susceptible strain fed on conventional cotton (Simian3). However, the resistant strain and F1progenies fed on trainsgenic cotton, exhibited higher survival rate than the susceptible strain, although their median lethal time was longer. The survival rates of the4th instar larvae of the resistant strain and that of the F1progenies fed on conventional cotton (Simian3) were significantly lower than that of the susceptible strain after9days of feeding. However, the survival rates of the3th instar larvae of the resistant strain and the F1progenies were significantly higher than those of the suscepitable strain when fed on the two Bt cotton varieties. The intrinsic rate of increase (rm) of the resistant strain, F1progenies and the susceptible strain were0.114,0.124and0.155on conventional cotton (Simian3) respectively. The resistant strain and F1progenies had0.052and0.049intrinsic rates of increase (rm) on the transgenic Cryl Ac cotton.The intrinsic rates of increase for the resistant strain and the F1progenies on the transgenic Cry1Ac+Cry2Ab cotton were0.037and0.030respectively. These results show that both the resistant strain and F1progenies can survive to pupation on two transgenic cottons. It is also observed that the fitness costs in the Cry2Ab-resistant strain was non-reccessive on the conventional cotton.