Exercise 22.1

Which Strategy Best Impedes the Evolution of Resistance?

(This exercise is based on Zhao, J-Z., J. Cao, Y. Li, H. I. Collins, R. T. Roush, E. D. Earle, and A. M. Shelton. 2003. Transgenic plants expressing two Bacillus thuringiensis toxins delay insect resistance evolution. Nature Biotechnology 21: 1493–1497.)

(Note: The reference above links directly to the article on the journal’s website. In order to access the full text of the article, you may need to be on your institution’s network [or logged in remotely], so that you can use your institution’s access privileges.)

INTRODUCTION

The contemporary practice of agriculture owes much to the study of evolution. Evolutionary principles have been used for many decades toward improving crops and livestock through selective breeding. They have also contributed to our understanding about how agricultural pests evolve resistance to pesticides, with the goal of impeding the evolution of that resistance.

Bacillus thuringiensis is a soil bacterium that produces various toxins. Genes from this bacterium, known as Bt, have been engineered into plants, enabling the plants to produce Bt toxins that act as pesticides. These toxins are particularly effective against insect herbivore pests. With a few exceptions, the Bt toxins have little or no effect on pollinators and other beneficial insects, wildlife, or humans. The use of Bt crops also substantially reduces the use of other, less-targeted, and potentially more-harmful pesticides. As of 2006, 33% of cotton and 11% of corn planted were engineered with Bt toxins.

As with other pesticides, insect pests can and do evolve resistance against the Bt toxins. Because new toxins are difficult and expensive to produce, there is great incentive to retard the evolution of resistance to each Bt toxin.

Three strategies, all which involve two different Bt toxins, are commonly used to impede the evolution of resistance. One strategy is to grow plants with two different types of Bt toxin together in the same area; this is the mosaic strategy. Another strategy, the sequential strategy, consists of first growing plants with one type of Bt toxin, and then after a period of time, growing plants with the different type of toxin. In the third strategy, pyramiding, plants with both toxins are planted. Modeling studies suggest that pyramiding would be the most effective strategy retarding the evolution of resistance.

A group of researchers at Cornell University tested these three strategies to see which would be most effective retarding the evolution of diamondback moths to Bt-broccoli plants. Cry1Ac and Cry1C were the two types of Bt toxins used.

In the mosaic strategy, 40% of the field had Cry1Ac plants and 40% had Cry1C plants. The remaining 20% of the field was plants without any toxin.

In the sequential treatment, 80% of the field consisted Cry1Ac plants for 12 generations, with 20% with plants lacking toxins. Then at generation 13, 80% of the field was Cry1C plants, with 20% with plants without toxins.

In the pyramiding strategy, 80% of the field had plants with both the CrylC and Cry1Ac toxins, and 20% of the field had plants without any toxins.

QUESTIONS

 

Question 1. You may have noticed that in each of the three treatments, a portion (20%) of the field contained plants without any toxins. Leaving parts of the planted field pesticide-free (refuges) is common practice in biological control and is mandated in the use of Bt plants. What purpose(s) would such refuges serve in slowing down the rate of evolution of resistance? (Hint: Resistance to a pesticide often incurs a cost to the resistant individual.)

Another reason for refuges is that many of the alleles that confer resistance to Bt and other pesticides are recessive. Refuges allow for the dilution of the Bt-resistance alleles such that most individuals with one Bt-resistant allele will mate with individuals with no Bt-resistance alleles, thus ensuring that homozygotes for the Bt-resistance allele are seldom produced.

Figure 1  The figure above shows the average numbers of larvae and pupae of the moth found per Bt plant in each of the treatments. Note the logarithmic scale on the y-axis.

 

Question 2. On the logarithmic scale, an increase from 0.1 to 0.5 moths per plant is the same magnitude as an increase from 1 to _______ moths.

 

Question 3. Why does the line for the sequential, Cry1Ac plant stop at 12 generations? Why does the line for sequential, CrylC plant start at 12 generations?

 

Question 4. In the mosaic treatment, which type of plant had the higher density of moths?

 

Question 5. Assuming that the differences in moth density between the two plants in the mosaic treatment are completely due to differences in toxicity, to which toxin were the moths most susceptible?

 

Question 6. Aside from differences in toxicity, what is another possible explanation for the differences in moth density seen in the two plants in the mosaic treatment?

 

Question 7. How might one test whether the differences in moth density in the two types of plants are due to actual differences in toxicity?

 

Question 8. Assuming that the differences in moth density are due to toxicity differences, which of the different strategies was most successful at impeding the evolution of resistance in the moths between generation 12 and generation 24?