Abstract
Fitness costs associated with resistance to Bacillus thuringiensis (Bt) toxins critically impact the development of resistance in insect populations. In this study, the fitness costs in Trichoplusia ni strains associated with two genetically independent resistance mechanisms to Bt toxins Cry1Ac and Cry2Ab, individually and in combination, on four crop plants (cabbage, cotton, tobacco and tomato) were analyzed, in comparison with their near-isogenic susceptible strain. The net reproductive rate (R0) and intrinsic rate of increase (r) of the T. ni strains, regardless of their resistance traits, were strongly affected by the host plants. The ABCC2 gene-linked mechanism of Cry1Ac resistance was associated with relatively low fitness costs, while the Cry2Ab resistance mechanism was associated with higher fitness costs. The fitness costs in the presence of both resistance mechanisms in T. ni appeared to be non-additive. The relative fitness of Bt-resistant T. ni depended on the specific resistance mechanisms as well as host plants. In addition to difference in survivorship and fecundity, an asynchrony of adult emergence was observed among T. ni with different resistance mechanisms and on different host plants. Therefore, mechanisms of resistance and host plants available in the field are both important factors affecting development of Bt resistance in insects.
Highlights
Transgenic crops expressing the environmentally benign insecticidal toxins from Bacillus thuringiensis (Bt) are widely planted in over 70 million hectares worldwide for pest control[1]
Bt resistance associated fitness costs have been studied on artificial diet in several lepidopteran pests, including Helicoverpa armigera[4], Heliothis virescens[5], Ostrinia nubilalis[6], Pectinophora gossypiella[7], Plutella xylostella[8], Spodoptera exigua[9] and Trichoplusia ni[10], or on their primary host plants cabbage for P. xylostella[8,11,12,13], tomato, cucumber and pepper plants for T. ni[14], cotton plants for H. armigera[15], H. virescens[16] and P. gossypiella[17], maize for Busseola fusca[18], O. nubilalis[19], H. zea[20] and S. frugiperda[21] and tobacco for S. litura[22]
The broad host range of T. ni makes this insect a pest of numerous important agricultural crops, from cruciferous vegetables to the field crop cotton[23], and provides an ideal system to study the effects of crop plants with different nutritional quality and different secondary metabolites on the fitness costs associated with specific Bt resistance mechanisms
Summary
Transgenic crops expressing the environmentally benign insecticidal toxins from Bacillus thuringiensis (Bt) are widely planted in over 70 million hectares worldwide for pest control[1]. With the increasing application of Bt toxins in transgenic crops, studies on Bt resistance-associated fitness costs have been increasingly reported since the 1990s. Combining the resistance mechanisms to Cry1Ac and Cry2Ab in T. ni allows the insects to survive on the widely planted major pyramided dual-toxin transgenic Bt-cotton plants (Bollgard II)[30]. The broad host range of T. ni makes this insect a pest of numerous important agricultural crops, from cruciferous vegetables to the field crop cotton[23], and provides an ideal system to study the effects of crop plants with different nutritional quality and different secondary metabolites on the fitness costs associated with specific Bt resistance mechanisms. We used the unique near-isogenic T. ni strains[25,26,30,31] as a biological system to dissect the fitness costs associated with the mechanisms of Cry1Ac resistance, Cry2Ab resistance and a combination of both mechanisms of resistance to Cry1Ac and Cry2Ab in T. ni. on artificial diet, on their preferred host plant (cabbage) and on alternative host plants (cotton, tobacco and tomato)
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