Fitness Costs Of A Disrupted Cadherin Allele Conferring Cry1Ac Resistance Of Helicoverpa Armigera

Cotton bollworm, Helicoverpa armigera (Hiibner) (Lepidoptera:Noctuidae), is one of the most important agricultural pests in the world. Outbreak of cotton bollworms in cotton caused serious economic losses in China. Widespread commercialization of transgenic Bt cotton has already resulted in effective control of its main target lepidopteran pests including cotton bollworm. But continuous expression of Bt toxins in the whole development time of transgenic Bt cotton will impose the cotton bollworm under a long-term and strong selection pressure by Bt toxins and it could increase the risk of Bt resistance evolved by cotton bollworm. Mutations on cadherin gene were confirmed as one of the main mechanisms responsible for resistance to Bt toxin Cry1Ac in H. armigera. To ascertain fitness costs conferred by the mutated alelle of cadherin gene in H. armigera will be valuble for understanding the evolutionary laws of resistance to Bt cotton and optimizing resistance manage strategies.1. Life timetables and relative fitness of a pair of near-isogenic lines of H.armigera (SCD:wild type cadherin allele; SCD-r1:mutated r1 cadherin allele)A selected resistant strain GYBT displays a>500-fold resistance to Bt toxin CrylAc which was linked with a cadherin mutation (r1allele). The mutated r1allele was introgressed into a susceptible strain SCD to establish a Cry1Ac-resistant strain (SCD-r1) by repeated backcrossing and family seletion. SCD-r1 and SCD is a pair of near-isogenic strains, which can avoid effects of different genetic backgrounds to fitness costs. Life timetables of SCD-r1 and SCD were constructed and no significant difference in net reproductive rate (R0) was observed between these two strains. The development time of resistant SCD-r1 larvae is 2.2d (about 13.1%) longer than that of the susceptible SCD larvae. The resistant strain shows no reproductive disadvatages without competition compared with the susceptible SCD strain. 2. Fitness costs in mating competition and sperm transfer associated with a mutated cadherin allele in H. armigeraMale and female moths of H.armigera are polyandrous which could cause mating and sperm competition in field conditions. We studied mating behaviors and sperm competition between the SCD-r1 and SCD strains of H. armigera with mating competition. Different mating trios were set up (two males competing for one female, or two females competing for one male), and the results showed that there were no significant differences in mating durations and mating times between resistant and susceptible individuals, which indicated similar mating chances under competition conditions. Offsprings of the mating trios were genotyped, and we found that the resistant moths showed disadvantages in paternity competition. Considering no differences in mating behavior and mating chance between strains, it is suggested that the resistant moths may have deficits in sperm competiton. To confirm this hypothesis, single-pair matings were made to count sperms transferred from males to females. The results demonstrated that both eupyrenes and apyrenes transferred from the resistant male moths were significantly decreased than from the susceptible males. PCR detection showed that cadherin mRNA was expressed in male reproductive system of H.armigera. Given the role of eupyrenes and apyrenes played in mating competition and reproduction, we proposed that disruption of the cadherin gene(r1 allele) results in decreased sperm number, and fewer sperms for resistant males to transfer to females in mating. The fitness costs in sperm competition will be beneficial to delaying resistance evolution by H.armigera to Bt cotton.