Backcrossed populations reduced confound effects of heterosis on the measurements of fitness costs and dominance levels of Bt resistance in Helicoverpa zea

Heterosis is a common phenomenon in which the heterozygous offspring produced by crossing two inbred populations with dissimilar genetic backgrounds exhibit higher fitness compared to their parents. This phenomenon might result in overestimated dominance levels and underestimated fitness costs of the resistance. However, this issue has not been carefully addressed yet. In this study, a series of Helicoverpa zea populations with different genetic similarities were generated by multiple cross-and-backcrosses between a Cry1A/Cry2A-resistant (RR) and a susceptible (SS) populations. The fitness of the genetic populations after 3 to 6 times of cross-and-backcrosses was evaluated on artificial diet and corn leaves. The results showed that the F1 resistant heterozygotes (RS) generated by the crosses between the backcrossed RR and SS showed an average fitness index (RS fitness/SS fitness) of 0.87, which is 10.3% lower than the RS generated by crosses between non-backcrossed RR and SS (0.97). The estimated dominance level based on crosses between non-backcrossed RR and SS was 0.59, which was 2.5 times greater than that (0.24) measured from crosses between backcrossed RR and SS. The information generated from this study will be beneficial for improving experimental designs to measure fitness costs and dominance levels of resistance, leading to more accurate predictions of Bt resistance evolution in the field