Determining the genetic basis of sexual conflict in Drosophila melanogaster: the role of the sex peptide.

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Sexual conflict is a revolutionary way of viewing what happens between males and females over reproductive decisions. Rather than co-operating over who to mate with, how often to mate, how many offspring to produce and how much parental care to give, the sexes instead often disagree, and try to exploit one another for their own ends. Examples of this 'sexual conflict' have been described in many different organisms, but have been most intensively studied in the insects. In the fruitfly Drosophila melanogaster, there is strong conflict between the sexes over how often to mate. A high frequency of mating best suits males, as they provide no parental care and thus each extra mating results in them fathering more offspring. The interests of females are best served by a lower frequency of mating - they have a limited number of eggs to lay and beyond a certain level of remating, they gain no further increases in offspring numbers, Furthermore, high frequencies of mating are costly to females, shortening their lifespans and lowering their overall lifetime reproduction. Our previous work has revealed that these 'mating costs' in females are caused by the actions of molecules in the male semen, which are transferred to females during mating. We have recently identified a substance known as the 'sex peptide' which is responsible for the female mating costs, and matings with males that lack this peptide are 'cost-free' for females. Males benefit from the transfer of the sex peptide because one of its functions is to render females temporarily unwilling to mate again, hence males who transfer it father more offspring. The sex peptide therefore simultaneously harms females and benefits males and is the first gene product shown to underlie sexual conflict in any species. Our discovery prompts many questions: e.g. how does the SP harm females?, are females equally sensitive to it throughout their lives? and is the damage caused by the sex peptide permanent? We address these questions in the work proposed here. We have three broad aims and these are (1) to quantify the extent to which the sex peptide underlies sexual conflict, (2) to reveal how sex peptide exerts harm on females and (3) to measure how the costs and benefits of receiving the sex peptide are affected by mating rate and by variations in food supply. We will do large-scale experiments in the laboratory, using healthy flies. This will allow us to detect subtle differences and to estimate accurately 'fitness' (a combination of survival and reproductive output). To manipulate the level of sex peptide that females receive we will mate them to normal males or those which lack the sex peptide. The results will provide an in depth analysis, for the first time, of the basis of sexual conflict at the genetic level. The general benefits of the research will be to deepen our understanding of the evolution of sexual conflict, by understanding the nature and dynamics of the conflicts between males and females. This will allow us to better understand how reproductive strategies evolve in males and females. There are also wider potential applications in insect pest control. For example, in many parts of the world the true' fruit flies are pests. The females lay their eggs in ripe fruit, which is spoiled as the larvae develop. These pests are often controlled using the 'sterile insect technique' (SIT). In this, males are reared on a large scale in factories, sterilised and then released into the environment to mate with the wild females. Such matings are sterile and the pest population size is thus reduced. SIT works best if females do not mate again, and sex-peptide-like factors appear to play a crucial role in this. Hence manipulating the level of sex peptide-like factors could provide improvements in pest suppression in insects subjected to SIT.