Genetic of Insecticide Resistance

Alvaro RomeroDepartment of Entomology

BIO508 -EVOLUTION Fall-2006

University of Kentucky

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).

Resistance Mechanisms in Insects

Increased enzymatic detoxification • Monooxygenase-mediated resistance (P450s)• Esterases • Glutathione S-transferases

Target site insensitivity• Voltage-gated ion channels • Acetylcholinesterase • Ligand-gated ion channels

opm.phar.umich.edu/images/proteins/1e6e.gif

P450

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

How Many Resistance Genes are Selected to Confer Resistance?

Field populations under intensive selection(selection for rare mutations-with major effect- Monogenic).

Lab populations(Selection for several resistant traits-polygenic)

ffrech-Constant, R. H. et al. (2004).

More than one gene in laboratory- selected populations

Genetics 130: 613-620

Single genes-examples

ffrech-Constant, R. H. et al. (2004).

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).

Paradigms (1)

Genes of major effect play a key role in field–evolved resistance because intensity

of selection is extremely high

Simulation modelGeneral 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)

Characteristic of six-locus models simulated

Polygenic model Effect of each allele is Small and nearly equally

Monogenic model Effect of one allele (80% of the total resistance)is larger than the others

This is genotypic effect of resistant alleles (Genotypic effect of susceptible alleles: 0)

Intermediate model

Initial frequency also varies: Alleles of smaller effect Alleles of greater effect

High frequency

Low frequency

Groeters and Tabashinik (2000).

Results Selection at 50% (weakest), 10% (weak), and 1% (strong)

Groeters and Tabashinik (2000).

Results

Resistance evolved fasterin model C and D (monogenic)

Groeters and Tabashinik (2000).

Paradigm (1)

• Genes of major effect play a key role in field – evolved resistance because intensity of selection is extremely high

Major genes dominated responses to selection for resistance across a wide range of simulated selection intensities

Rebuttal

Paradigm (2)

Monogenically-based field resistance, caused by intensive field selection, is followed by the appearance of polygenetic variation after

selection in the lab.

Intensive selection (10%) for 40 generations fixes the major and intermediate alleles

Further weak selection (50%) fixes the minor allele (polygenetic resistance)

Intensive field selection …then…. weak laboratory selection

Groeters and Tabashinik (2000).

Weak field selection …then…. Strong laboratory selection

Weak selection (50%) for 40 generations fixes the major effect allele (monogenetic resistance)

Further strong selection (10%) fixes the intermediate and eventually the minor allele(polygenetic resistance)

Groeters and Tabashinik (2000).

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.

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

Refuge Strategy

Groups of susceptible individuals that are not exposed to insecticides which then

mate resistant ones to keep a vulnerable population

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

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).