Sex ratio distorters and resistance management

Pest insects cause the loss of billions of pounds worth of crops every year, and insect disease vectors spread diseases that kill millions of people. We rely on "biocides" - chemical pesticides, biological pest control agents and so on to control these unwelcome animals, but they often evolve resistance to the agents that we have available, causing a most serious problem throughout the globe. An understanding of the mechanisms that lead to the evolution of resistance is necessary to allow us to manage pests in order to avoid this. One aspect of an animal's biology that might have important effects on the evolution of resistance is the presence of sex ratio distorters - either symbiotic microorganisms or specific kinds of genes which cause (usually) female-biased sex ratios in many species of insect and other arthropods. If the sex ratio is biased towards females the speed of evolution might be reduced, and we can predict further effects which should slow the speed of evolution which will be caused by, for example, the increased possibility that "low quality" males will father offspring when there is a scarcity of males. A further effect of sex ratio distorters could arise when resistance has already evolved: in this case, if the selection for resistance is removed, by ceasing use of a pesticide for example, then we would predict that populations with a sex ratio distorter present should lose their resistance more slowly.

Despite these potentially very important consequences of the presence of these sex ratio distorters, and despite the fact that they are extremely common and found in many pest species, these effects have never been studied experimentally or theoretically. We propose a project which would address this knowledge gap both experimentally and also by the use of mathematical simulation models. For the laboratory experiments would would expose fruit fly populations to a pyrethroid pesticide called permethrin for twenty generations, and compare how rapidly "ordinary" populations without a sex ratio distorter develop resistance with populations with either a symbiont sex ratio distorter or a genetic one. We would also compare the speed by which resistance is lost once a population is no longer exposed to a pesticide between these populations.

For the modelling work, we would use the results from our laboratory experiments to help build simulation models of pest species being challenged with biocides. These would then be used to examine the potential effects on resistance evolution under a variety of management schemes with either an existing sex ratio distorter present, the introduction of one to the pest population or the loss of one. This would allow us to generate a theoretical framework for the impacts of sex ratio distorters on resistance evolution and management which would then be of use to pest and resistance management practitioners.

Sex ratio distorting agents are widespread in the arthropods, with both endosymbiont microorganisms and selfish genetic elements causing female biased sex ratios in many species. The presence of these agents is likely to cause changes to the adaptive capacity of the affected populations, with potential effects on both the rate of evolution and the stability of biocide (chemical pesticides, biocontrol agents) resistance being the most important applied consequence of sex ratio distortion. These ideas have never been tested experimentally and have hardly been addressed by theoreticians either. We propose to test the potential effects of the presence of sex ratio distorters using laboratory experimental evolution experiments. Both the effects of selfish genetic elements, in Drosophila pseudoobscura, and endosymbionts, in D. melanogaster, will be tested by experimentally selecting for resistance to the pyrethroid pesticide permethrin, and the rate at which the populations become resistant will be compared with controls with no sex ratio distorters. The rate of loss of resistance in populations where selection has been relaxed will also be tested.

Based on the results from the laboratory experiments, we will construct mathematical models of resistance evolution in pest populations and use these to examine resistance evolution and management in three scenarios: pests with sex ratio distorters present, pests which experience the introduction of a sex ratio distorter and pests which evolve suppression of a sex ratio distorter. This will produce a theoretical framework for the assessment of the potential impact of sex ratio distorters in practical pest management and also allow us to assess the potential benefits of introducing sex ratio distorters to pest populations in order to aid in resistance management.