Fitness Costs Of Recessive And Non-Recessive Resistance To Cryiac In Helicoverpa Armigera (Hubner)

Cotton bollworm, Helicoverpa armigera, is one of the most important cotton pests. The main target of Bt cotton, cotton bollworm, has been effectively controlled since Bt cotton was planted extensively in China. Bt toxin Cry1Ac is expressed during the whole development time of cotton, and selection pressure by Bt toxin will result in resistance evolution, and threat the sustainable adoption of Bt cotton. Fitness cost of Bt rsistance in target pests is one of the key factors determing resistance evolution, and knowledge of fitness costs of Bt resistance is important for designing adaptive resistance management tactics. The strain SCD-423of H, armigera has non-recessive resistance to Cry1Ac, which is not linked with the cadherin gene. The strain SCD-r1has recessive resistance to CrylAc, which is tightly linked with a mutant allele (r1) of the cadherin gene. SCD-423, SCD-rl and SCD are near-isogenic lines. In this study, we investigated relative fitness between SCD-423and SCD under laboratory condition, impacts of the cadherin deletion mutation (r1allele) on sperm competition, and effects of Bt cotton and non-Bt cotton leaves on larval development of the two resistant strains and the susceptible SCD strain of H, armigera.1. Relative fitness of the SCD-423strain of H. armigrea with non-recessive resistance to CrylAcThe SCD-423strain has about500-fold resistance to CrylAc activated toxin compared with the susceptible SCD strain, and CrylAc resistance in SCD-423is non-recessive. SCD-423and SCD are near-isogenic lines with similar genetic background. Life tables of SCD-423, SCD and their F1progeny (SCD♀×SCD-423♂, SCD-423♀×SCD♂) strains on artificial diet in the laboratory were constructed. The results showed that the pupation rate, adult emergence rate and egg hatching rate in SCD-423were significantly lower than in SCD. However, larval development time of the SCD-423strain was1.21d shorter than SCD strain, average pupal weight increased by7mg and mean eggs laid per female of the SCD-423increased by322eggs compared with SCD, which suggests the resistant SCD-423has fitness advantage in these biological traits over the susceptible SCD. The net replacement rate (R0) was calculated to determine relative fitness for SCD-rl, SCD and their F1progeny, and the relative fitness of SCD-rl and the F1heterogotes were0.88and0.94compared respectively to SCD. Our results indicated fitness costs of non-recessive resistance to Cry1Ac in SCD-423were limited, which suggests resistance risk could be high regarding this kind of non-recessivel resistance allele in H. armigera.2. Effects of a deletion mutation of cadherin on sperm competition in H. armigeraThe SCD-rl strain has about500-fold resistance to Cry1Ac activated toxin compared with the susceptible SCD strain, and recessive resistance to Cry1Ac in SCD-rl is conferred by a deletion mutation in the cadherin gene of H. armigera. SCD-rl and SCD are near-isogenic lines with similar genetic background. Because H. armigera females can mate with several males throughout their reproductive lives, sperm competition exists. Four combinations of matings between SCD-rl (designated as r1) and SCD (designated as s) were placed as:s♀×s♂/r1♂; s♀×rl♂/s♂; r1♀×rl♂/s♂; r1♀×s♂/r1♂, thus a female was sequentially mated with males from rl and s. Effects of the cadherin deletion mutation on sperm competition were assessed by genotyping their offspring with a molecular marker. The results demonstrated:(1) Although a female was mated effectively with two males, only one male sired their offspring among the80twice-mated females tested.(2) Among the four mating combinations, SCD-r1males obtained significant advantages in sperm competition. Based on our results, it can be predicted that sperm precedence in males with homozygous r1allele of cadherin may promote resistance heritability and evolution in the field. The underlying mechnisms for such sperm precedence deserve further investigation.3. Effects of Bt cotton and non-Bt cotton leaves on larval development of the CrylAc-resistant and susceptible strains of H. armigeraThe effects of Bt cotton and non-Bt cotton leaves on the larval development were compared in the laboratory among the AY423strain of non-recessive CrylAc resistance, SCD-rl strain of recessive and cadherin-based CrylAc resistance, and the sceptible SCD strain. After feeding with Bt cotton leaves from neonates for10days, larval survivals of AY423, SCD-rl and SCD were50%,37%and4%respectively,31%,31%and2.4%larvae reached respectively3rd instar among these3strains; no larvae developed into4th instar. After feeding with non-Bt cotton leaves from neonates for10days, larval survivals of AY423, SCD-rl and SCD were54%,49%and53%respectively;31%,38%and9.2%larvae respectively developed to3rd instar, and11%,12%and43%larvae respectively developed to4th instar among these3strains. Our results showed that the two Cry1Ac-resistant strains of H. armigera had significantly higher fitness when fed with Bt cotton leaves compared with SCD strain, whereas the SCD strain had higher fitness when fed with non-Bt cotton leaves compared with the two Cry1Ac-resistant strains. Considering fitness costs such as lower development and survival of the Cry1Ac-resistant larvae under non-Bt cotton, it is pratical to delay resistance evolution through planting non-Bt cotton as refuge.