Field evolved resistance to Bt insecticidal toxins has occurred in the diamondback moth, Plutella xylostella, and despite being widely studied, the fitness costs associated with resistance remain unclear. In the absence of selection, neutral polymorphisms with no fitness costs can be maintained in large populations through genetic drift. However, if the selection coefficient is high, deleterious alleles should be removed from populations under purifying selection. Insecticide resistance alleles without fitness cost may therefore pose a serious threat in agriculture, as they could persist in populations over time and undergo rapid re-selection.
A Hawaiian population of the diamondback moth, NO-QA, has evolved resistance to Bt insecticides through a 30bp deletion in the ATP-dependent Binding Cassette transporter gene abcc2. To assess fitness costs associated with resistance, NO-QA was crossed with a Bt susceptible strain, Waite, and their progeny used to found four replicate population cages with an initial resistance frequency of 0.5. Caged populations were maintained for 19 generations without insecticide exposure then genotyped at generation 10 (n=192) and 19 (n=384) to assess changes abcc2 resistance frequency over time. The frequency of resistance alleles reduced over time from 0.5 to and average of 0.31, yet replicates were highly variable, indicating genetic drift may have had a substantial influence. Selection coefficients for each replicate population ranged from 0 to less than -0.5, which supported a fitness cost. Reduced genome representation libraries (RAD-seq) for NO-QA, Waite, and four population cage replicates were then sequenced to assess potential effects of strain bias across the genome. After 19 generations, the average genome composition was biased towards the Bt susceptible Waite strain. Nevertheless, this data demonstrates this type of Bt resistance comes with a fitness cost in diamondback moth.