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Introduction

The Open University — Mon, 15 Aug 2016 08:02:07 GMT

Natural selection acts on phenotypes that vary in survival and reproductive success, and the response to selection is a change in allele frequency resulting in evolution. In this free course, Mosquito resistance to insecticides, you will see how allele frequency can change rapidly in a population in response to selective pressure.

You will consider how alleles that arise and spread through a population because they confer resistance in that environment can have negative fitness consequences in other environments (a situation known as a trade-off). These principles can be illustrated by considering the example of the spread of mutant alleles of a gene called ester in populations of mosquitoes exposed to insecticides.

This OpenLearn course is an adapted extract from the Open University course S317 Biological science: from genes to species.

Learning outcomes

The Open University — Mon, 15 Aug 2016 08:02:07 GMT

After studying this course, you should be able to:

describe how natural selection drives changes in allele frequencies, using mosquito resistance to insecticides as an example.

1 The ester1 allele

The Open University — Mon, 15 Aug 2016 08:02:07 GMT

In the 1960s, the French government attempted to rid its Mediterranean coast of mosquitoes (Culex pipiens) in order to attract tourists. It did this by regularly spraying the mosquito breeding sites located along the coast (Figure 1) with organophosphate insecticides that kill the mosquitoes by inhibiting an enzyme called acetylcholinesterase in the nervous system. The treatment was successful initially but, by 1972, the mosquito populations had begun to recover (Raymond et al., 1998).

Figure 1 Map indicating the location where mosquitoes were sprayed (hatched area), consisting of a 20–25 km-wide belt along the Mediterranean coast.

Long description

To understand the reason for this recovery, the responses of different mosquito populations to exposure to the insecticide were examined. Researchers collected mosquito larvae from populations near the coast, where they had been sprayed with insecticide, and from populations north of the spray area, which had never been exposed to insecticide. They exposed both samples to the insecticide. In populations near the coast, which were sprayed regularly, mosquitoes were able to withstand and survive quite strong doses of the insecticide, but individuals taken from populations just north of the spraying area died when exposed to only weak doses.

Further research showed that the reason for the resistance in the coastal populations was a mutation at a locus called ester that encodes an esterase enzyme (called A1). The A1 esterase breaks down a wide range of toxins, including organophosphates. Mosquitoes that are vulnerable to the insecticide (and do not have the mutation) do not produce enough esterase to hydrolyse and thus inactivate the toxin. However, in resistant mosquitoes the mutated allele (called ester¹) alters the expression of ester by gene amplification (i.e. several copies of the same gene are found in the same genome). This causes increased production of the A1 esterase and, as a consequence, resistance to the toxin.

Scientists monitored the frequency of the ester¹ allele in populations at and near the coast for several years.

View larger image

Figure 2 Distance from the Mediterranean coast plotted against frequency of the ester¹ allele in mosquito populations in the south of France in the 1970s.

Long description

Activity 1

In which populations was the ester¹ allele most common in each of the years shown?

Answer

In all three years, the ester¹ allele frequency was highest in the populations closest to the coast.

Activity 2

Describe how the ester¹ allele frequency changes with distance from the coast.

Answer

In all three years, the ester¹ allele frequency declined sharply with distance inland. In 1973 the allele was not present in the mosquito populations that were furthest inland, but by 1975 and in 1978 it was detected in all populations, albeit at a much lower level in the inland populations.

Activity 3

What was the ester¹ allele frequency in populations at the coast in 1973?

Answer

In 1973 the frequency of the ester¹ allele was over 0.6 in populations at the coast.

Activity 4

Did the ester¹ allele reach fixation in any of the populations?

Answer

Yes, it reached fixation in the populations at the coast by 1975 (an allele frequency of 1.0 indicates fixation).

Activity 5

What was the frequency of the ester¹ allele in the furthest inland mosquito populations by 1978?

Answer

Approximately 0.2.

In areas where mosquitoes were sprayed, ester¹ conferred a higher fitness on individuals carrying it, and these individuals passed the mutation on to their offspring. This resistance also spread to areas away from the coast (probably carried by mosquitoes migrating inland); however, as seen in Figure 2, it never reached a high frequency in inland populations. The reason for this is that the ester¹ mutation carries a fitness cost to mosquitoes carrying it. The overproduction of the A1 esterase has a side effect of interfering with cholinergic synapses of the central nervous system (those in which the neurotransmitter is acetylcholine). As a result, mosquitoes carrying the allele have a high susceptibility to predation and males have low reproductive success.

A mutation that has more than one effect on an organism – such as that which gave rise to the ester¹ allele – is said to exert a pleiotropic effect on the organism. In the case of the ester¹ allele, where the allele has opposing effects on fitness depending on the context, the effect is known as antagonistic pleiotropy.

So why did the ester¹ allele increase in coastal populations despite it increasing the likelihood of predation and lowering reproductive success? The reason is that the selection coefficient of a particular allele depends on the net effect of that allele on an organism’s fitness. In the case of the mosquitoes, the fitness gains of resistance were greater than the losses due to predation (or other fitness deficits), but only in coastal regions where the mosquitoes were sprayed. Further inland, the ester¹ allele conferred no benefit because mosquitoes were not sprayed there. In these populations the ester¹ allele was disadvantageous because it raised the probability of predation and/or decreased reproductive success.

2 The ester4 allele

The Open University — Mon, 15 Aug 2016 08:02:07 GMT

Although ester¹ was the prevalent allele in the 1970s and early 1980s, a second mutation at the ester locus, ester⁴, emerged in 1984 and replaced ester¹ in the 1990s. The replacement of ester¹ with ester⁴ over the period 1986 to 1996 is depicted in Figure 3.

View larger image

Long description

Activity 6

Describe the change in frequency with distance from the coast of both alleles over the three years.

Answer

In 1986, the frequency of the ester¹ allele was nearly 0.6 at the coast and dropped to near zero at 50 km from the coast. Ester⁴ occurred at a low frequency at the coast and increased very slightly with distance from the coast. In 1993 and 1996 the frequency of the ester¹ allele had declined in coastal regions while the ester⁴ allele had increased in frequency at the coast and also away from the coast.

Laboratory experiments indicate that the reason for these changes in the frequency of the two alleles is that although ester⁴ confers slightly less resistance to insecticides than ester¹, its negative effects on fitness are also less dramatic than those of ester¹. This is reflected in the slope of the curves shown in Figure 3 – there is no dramatic drop-off in the frequency of the ester⁴ allele with increasing distance from the coast. This suggests that the allele is favoured at the coast but selection against it is less strong than was found for ester¹ further inland; that is, mosquitoes that migrate inland do not pay the price of carrying the ester⁴ allele to the same extent as those carrying ester¹.

Conclusion

The Open University — Mon, 15 Aug 2016 08:02:07 GMT

As in the case of the mosquitos on the Mediterranean coast, many instances of insecticide resistance in insects are due to a single allele that is partially or fully dominant over the allele that determines susceptibility. In this free course, Mosquito resistance to insecticides, you have learned that these resistance alleles increase in frequency very rapidly when insecticide is applied because susceptible genotypes suffer very high mortality. However, in the absence of insecticides, resistant genotypes are 5% to 10% less fit than susceptible ones and so decline in frequency. Resistance to insecticides therefore illustrates a trade-off – traits that are advantageous in one environment can have effects that are disadvantageous in other environments.

Glossary

The Open University — Mon, 15 Aug 2016 08:02:07 GMT

alleles: Variants of a gene that may be followed genetically, usually through their phenotypic effect, but also using a molecular assay, such as PCR.
antagonistic pleiotropy: Refers to a situation in which a single gene creates multiple opposing effects, such that beneficial effects of a trait created by the gene are offset by 'losses' in other traits.
phenotypes: The observable traits or characteristics of an organism, such as its biochemical, morphological, physiological or behavioural characteristics.
pleiotropic: Describes a cellular process, signal or gene that has more than one effect, or more than one phenotypic outcome. At the genetic level, describes a situation where a single mutation affects two or more apparently unrelated phenotypic traits.

References

The Open University — Mon, 15 Aug 2016 08:02:07 GMT

Raymond, M., Chevillon, C., Guillemaud, T., Lenormand, T. and Pasteur, N. (1998) ‘An overview of the evolution of overproduced esterases in the mosquito Culex pipiens’, Philosophical Transactions of The Royal Society B, vol. 353, no. 1376, pp. 1707–11.

Acknowledgements

The Open University — Mon, 15 Aug 2016 08:02:07 GMT

This free course was written by Mandy Dyson.

Except for third party materials and otherwise stated (see terms and conditions), this content is made available under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 Licence.

The material acknowledged below is Proprietary and used under licence (not subject to Creative Commons Licence). Grateful acknowledgement is made to the following sources for permission to reproduce material in this free course:

Course image: Alamy

Figures

Figure 1: Lenormand, T. et al. (1999) ‘Tracking the evolution of insecticide resistance in the mosquito Culex pipiens’, Nature, vol. 400, pp. 861–4, Nature Publishing Group.

Figures 2 and 3: Raymond, M. et al. (1998) ‘An overview of the evolution of overproduced esterases in the mosquito Culex pipiens’, Philosophical Transactions of The Royal Society B, vol. 353, no. 1376, pp. 1707–11.

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