Inheritance And Fitness Costs Of Vip3Aa19 Resistance In Mythimna Separata

The "high-dose/refuge" strategy is expected to work most effectively when resistance is inherited as a functionally recessive trait so that heterozygous progeny can be killed by the high dose Bt proteins expressed in the Bt plants.In addition,the fitness of Bt-resistant insects on refuges is an important factor in determining the speed of resistance development.Therefore,understanding the genetic basis of Bt resistance and the fitness cost associated with resistance are essential in developing IRM strategies for the sustainable use of Bt crop technology.In the present study,a laboratory selected Mythimna separata strain was used to determine the mode of inheritance for Vip3 Aa19 resistance.Besides,the fitness costs associated with the resistance of Vip3Aa19-resistant(RR),-susceptible(SS),and two reciprocal F1 colonies were assayed on non-toxic diet.The results are as follows:1.The resistant strain exhibited>634.5-fold resistance to Vip3Aal9 after selected for 18 generations.The LC50 values of R♀S♂ was significantly higher than the LC50 values of(R♂S♀),with the LC50 values are 254.58 μg/g and 14.75p,g/g,respectively.These results suggest that maternal effects and sex linkage were present in the Vip3Aa19 resistance.2.The LC50 of F1 progenies from mass cross yielded values of D of-0.30 and 0.68 respectively,which are equivalent to h values of 0.35 and 0.84.This indicated that resistance showed incomplete dominance for resistant female progeny and incomplete recessive for resistant male progeny at the LC50 concentration.The effective dominance h of the resistance varied depending on the concentrations.For mass crosses between females of resistant strain and males of susceptible strain,h ranged from 0.19 to 0.93 at the concentrations of 5.0 to 625.0 μg/g,suggesting that the resistance was functionally nearly completely dominant to incompletely recessive.For mass crosses between males of resistant strain and females of susceptible strain,h values were 0.68 and 0.18 at the concentrations of 5.0 and 50.0 μg/g,suggesting the resistance was functionally incompletely dominant to incompletely recessive.However,at the concentrations of250.0 to 625.0μg/g,h was 0,indicating completely recessive resistance.3.Bioassays of the backcross of the F1 generation with the selected strain suggested that more than one locus is involved in conferring Vip3Aa19 resistance in M.separata.4.The LC50 values for Cryl Ab and Cry 1F against the resistant strain were 7.80μg/g and 20.82 μg/g with a 1.51-and 1.08-fold increase in LC50 value,respectively,indicating that no cross-resistance was detected with CrylAb and Cry 1F toxin.5.No significant differences were observed among the four insect colonies on the parameters egg stage,pupal stage,male adult longevity and pupa weight.Pupa weight of the four insect colonies varied from 367.9 mg to 396.4 mg for female and 374.9 mg to 384.2 mg for male,but no significant differences were detected among the colonies,Neonate-to-pupa developmental time for RR was significantly longer(1.5d)from that of SS.The average number of eggs laid per female of these three insect genotypes(SS,R♀S♂ and R♂S♀)were 1193.8,1054.2,and 983.9,respectively.However,the average number of eggs laid per female of RR(847.9)was significantly lower than that of SS.Mean length of a generation(T)was higher for the RR(54.32 d)than the heterozygotes derived from the RR♀×SS♂ cross(51.76 d)and SS(51.39 d).No significant differences in the net reproductive rate(R0),intrinsic rate of population increase(rm),and finite rate of pop increase(λ)were observed among the four insect genotypes.