Genetic of Insecticide Resistance

1. Genetic of Insecticide Resistance Alvaro Romero Department of Entomology BIO508 -EVOLUTION Fall-2006 University of Kentucky

2. Resistance-definition “The inherited ability of a strain of some organism to survive doses of a toxicant that would kill the majority of individuals in a normal population of the same species” (WHO, 1957).

3. Resistance Mechanisms in Insects • Increased enzymatic detoxification • Monooxygenase-mediated resistance (P450s) • Esterases • Glutathione S-transferases P450 opm.phar.umich.edu/images/proteins/1e6e.gif • Target site insensitivity • Voltage-gated ion channels • Acetylcholinesterase • Ligand-gated ion channels

4. Key Questions in Insecticide Resistance • Resistance phenotype controlled by one or more genes? • How many mutations are within resistance genes? • How many independent origins do they have in the field population? Ffrech-Constant, R. H. et al. (2004). The genetics and genomics of insecticide resistance. TRENDS in Genetics. Vol. 20 (3): 163-170

5. How Many Resistance Genes are Selected to Confer Resistance? Lab populations (Selection for several resistant traits-polygenic) Field populations under intensive selection (selection for rare mutations- with major effect- Monogenic). ffrech-Constant, R. H. et al. (2004).

6. More than one gene in laboratory- selected populations Genetics 130: 613-620

7. Single genes-examples ffrech-Constant, R. H. et al. (2004).

8. Identification of an insecticide resistance gene via transcriptional analysis using DNA microarray Single genes with major effects… even in complex multigene enzyme systems (P450) ffrech-Constant, R. H. et al. (2004).

9. Paradigms (1) Genes of major effect play a key role in field–evolved resistance because intensity of selection is extremely high

10. Simulation model General conditions • Empirical estimates of selection intensity in the field (nine species examined) • Six unlinked loci, each with two alleles • Starting population: 10,000 individuals • Individual’s genotype was estimated by summing genotypic effect across alleles at each locus and across the six loci • Populations were resistant when 50% of the individuals had tolerance values greater than a threshold value that would kill 95% of individuals with susceptible genotypes (G = 0)

11. Characteristic of six-locus models simulated This is genotypic effect of resistant alleles (Genotypic effect of susceptible alleles: 0) Polygenic model Effect of each allele is Small and nearly equally Intermediate model Monogenic model Effect of one allele (80% of the total resistance) is larger than the others Initial frequency also varies: Alleles of smaller effect Alleles of greater effect High frequency Low frequency Groeters and Tabashinik (2000).

12. ResultsSelection at 50% (weakest), 10% (weak), and 1% (strong) Groeters and Tabashinik (2000).

13. Results Resistance evolved faster in model C and D (monogenic) Groeters and Tabashinik (2000).

14. Paradigm (1) • Genes of major effect play a key role in field – evolved resistance because intensity of selection is extremely high Rebuttal Major genes dominated responses to selection for resistance across a wide range of simulated selection intensities

15. Paradigm (2) Monogenically-based field resistance, caused by intensive field selection, is followed by the appearance of polygenetic variation after selection in the lab.

16. Intensive field selection …then…. weak laboratory selection Further weak selection (50%) fixes the minor allele (polygenetic resistance) Intensive selection (10%) for 40 generations fixes the major and intermediate alleles Groeters and Tabashinik (2000).

17. Weak field selection …then…. Strong laboratory selection Further strong selection (10%) fixes the intermediate and eventually the minor allele (polygenetic resistance) Weak selection (50%) for 40 generations fixes the major effect allele (monogenetic resistance) Groeters and Tabashinik (2000).

18. Paradigm (2) Monogenically-based field resistance, caused by intensive field selection, is followed by the appearance of polygenetic variation after selection in the lab. Rebuttal The situation may be more complex The frequency of major and minor alleles for resistance depends on the population’s selection history and the moment that genetic basis is studied.

19. Practical Implications of this Simulation Model • Use of intensive selection at the lab to mimic field evolution may be invalid (there is a wide range of intensity of selection in the field) • Contribute to the investigation of more general evolutionary phenomena of adaptation • An excellent tool to study the ability of refuges to delay the evolution of resistance

20. Refuge Strategy Groups of susceptible individuals that are not exposed to insecticides which then mate resistant ones to keep a vulnerable population

21. Refuge model • Percentage of the population (r) avoid selection. • Selection applied to the remaining (100-r) % of the population • Adults surviving selection and refuges were combined randomly • Refuge sizes of 10 and 25% Resistant delay # of generations for resistance to evolve with a refuge Minus # of generations for resistance to evolve without a refuge

22. Results-refuges • Delay of resistance depends mainly on selection intensity but not by the distribution of allelic effects among major and minor loci • Low selection (50 and 10%), little delay at a refuge rate of 10 and 25% • High selection (1%), a greater delay, especially at a refuge rate of 25%. Groeters and Tabashinik (2000).