4.3.2 Evolutionary Implications
In pyrethroid-laden environments, pyrethroid resistance genotypes confer a fitness
advantage. In the absence of a pyrethroid selective pressure, classical theory predicts
that resistance genotypes come at a cost [161]. However, the fitness costs associated
with resistance are related to the specific mechanism underlying the resistance [162],
and fitness costs have only sometimes been measured in cases of adaptive resistance
(see Ffrench-Constant and Bass [163] for a discussion). Fitness costs associated with
resistance mutations of large effect can be ameliorated by subsequent mutations in
other genes (modifiers) [162], so that even resistance driven by an apparently simple
mutation can actually be the result of a complex genetic profile [99]. Still, pyrethroid
resistance has often been documented in other insects to come at an overall fitness
cost. Boivin et al. [164] documented fitness costs including decreased fecundity and
fertility, slower development, lower weight, and shorter lifespans in deltamethrinresistant codling moths (Cydia pomonella) compared with sensitive strains.
Konopka et al. [165] showed cost of fitness through developmental and reproductive
life history traits with a population of pyrethroid-resistant C. pomonella. One study
monitored the allele frequency of a kdr mutation in houseflies (M. domestica) in
pyrethroid-free environment for 15 generations, and found a significant decrease in
frequency over time, suggesting a strong cost of having the mutation in the absence
of pyrethroids [166]. In mosquitoes (Culex quinquefasciatus), kdr resistance mutations were associated with a decreased chance of surviving to adulthood
[167]. Reduced overall fitness noted with some pyrethroid resistance mutations in
the Vgsc may be caused by reduced efficiency in mutant Vgsc or related metabolic
costs [168]. Fitness costs have been documented in homozygous recessive L1014F
mutant An. gambiae females [169]. In H. azteca, the L925I resistance mutation is
more common than the M918L mutation, suggesting that L925I is preferred, potentially because of lower fitness costs [58, 129]. Several populations of H. azteca
appear to be functionally fixed for a resistance mutation at the L925 locus, including
the population studied for tolerance to other chemicals and fitness costs
[58, 150]. One population fixed for L925I showed lower reproductive capacity,
lower thermal tolerance, and trends toward increased sensitivity to other chemicals
including DDT, copper (II) sulfate, and sodium chloride, potentially indicating some
fitness costs associated with the L925I allele, although more work will be necessary
to determine that definitively [150].
If the selective pressure is sufficiently strong, population size can be reduced to
leave only a select group of founder genotypes to continue that population, potentially leading to “genetic erosion” or a loss of genetic diversity [170]. Unlike
physiological acclimation, changes to the genetic structure of populations including
loss of sensitive genotypes and reductions in genetic diversity are permanent alterations to the population in question [87]. Losses in genetic diversity also increase
vulnerability to extinction [171]. It is noteworthy that the populations that have
evolved resistance to pyrethroids are often also harboring evolved resistance to other
pesticides. For example, many populations of H. azteca that are resistant to pyrethroids are also resistant to organophosphates through analogous target site
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K. M. Major and S. M. Brander
In pyrethroid-laden environments, pyrethroid resistance genotypes confer a fitness
advantage. In the absence of a pyrethroid selective pressure, classical theory predicts
that resistance genotypes come at a cost [161]. However, the fitness costs associated
with resistance are related to the specific mechanism underlying the resistance [162],
and fitness costs have only sometimes been measured in cases of adaptive resistance
(see Ffrench-Constant and Bass [163] for a discussion). Fitness costs associated with
resistance mutations of large effect can be ameliorated by subsequent mutations in
other genes (modifiers) [162], so that even resistance driven by an apparently simple
mutation can actually be the result of a complex genetic profile [99]. Still, pyrethroid
resistance has often been documented in other insects to come at an overall fitness
cost. Boivin et al. [164] documented fitness costs including decreased fecundity and
fertility, slower development, lower weight, and shorter lifespans in deltamethrinresistant codling moths (Cydia pomonella) compared with sensitive strains.
Konopka et al. [165] showed cost of fitness through developmental and reproductive
life history traits with a population of pyrethroid-resistant C. pomonella. One study
monitored the allele frequency of a kdr mutation in houseflies (M. domestica) in
pyrethroid-free environment for 15 generations, and found a significant decrease in
frequency over time, suggesting a strong cost of having the mutation in the absence
of pyrethroids [166]. In mosquitoes (Culex quinquefasciatus), kdr resistance mutations were associated with a decreased chance of surviving to adulthood
[167]. Reduced overall fitness noted with some pyrethroid resistance mutations in
the Vgsc may be caused by reduced efficiency in mutant Vgsc or related metabolic
costs [168]. Fitness costs have been documented in homozygous recessive L1014F
mutant An. gambiae females [169]. In H. azteca, the L925I resistance mutation is
more common than the M918L mutation, suggesting that L925I is preferred, potentially because of lower fitness costs [58, 129]. Several populations of H. azteca
appear to be functionally fixed for a resistance mutation at the L925 locus, including
the population studied for tolerance to other chemicals and fitness costs
[58, 150]. One population fixed for L925I showed lower reproductive capacity,
lower thermal tolerance, and trends toward increased sensitivity to other chemicals
including DDT, copper (II) sulfate, and sodium chloride, potentially indicating some
fitness costs associated with the L925I allele, although more work will be necessary
to determine that definitively [150].
If the selective pressure is sufficiently strong, population size can be reduced to
leave only a select group of founder genotypes to continue that population, potentially leading to “genetic erosion” or a loss of genetic diversity [170]. Unlike
physiological acclimation, changes to the genetic structure of populations including
loss of sensitive genotypes and reductions in genetic diversity are permanent alterations to the population in question [87]. Losses in genetic diversity also increase
vulnerability to extinction [171]. It is noteworthy that the populations that have
evolved resistance to pyrethroids are often also harboring evolved resistance to other
pesticides. For example, many populations of H. azteca that are resistant to pyrethroids are also resistant to organophosphates through analogous target site
132
K. M. Major and S. M. Brander