How do female insects get switched on to reproduction?

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Insects are highly adept at reproducing. The females of many insects lay huge numbers of eggs meaning that populations can increase very rapidly. This helps explain how insects have become the group of animals containing the most species, and also why some insects are so problematic for humans. Pest species that eat crops or carry human diseases can be extremely hard to control thanks to their phenomenal reproductive ability.But female insects don't typically lay eggs all the time. To avoid laying unfertilized eggs, females switch to high egg laying only after they've mated and received sufficient sperm from males. Alongside switching on egg laying, many female insects also avoid mating with males again until they need to obtain more sperm, in order to focus their energy on feeding and producing offspring.The molecules that cause these switches in behaviour in females are of great interest because they are key to insect reproductive success. If we know what the responsible molecules are we might be able to develop new pesticides that disrupt them, and control pest species in a very targeted way. Such knowledge is also important for our basic understanding of how evolution works, because we expect that genes that code for molecules that increase reproduction will typically become more common over successive generations.There are currently very few species for which we know the genes and molecules involved in switching on reproduction in females. The fruit fly, Drosophila melanogaster, is the best understood. Males in this species make a protein called 'Sex Peptide' (SP) which they transfer to females in the seminal fluid. After mating, SP activates a protein in the females called the 'Sex Peptide Receptor' (SPR) and this causes females to boost egg laying and reject further mating attempts from males. However, we don't know if SP and SPR do the same thing in other insect species. This is a big problem because it means we don't know whether SP and SPR switch on reproduction for lots of insect species, or whether different genes and molecules have evolved for this purpose elsewhere. If we knew the answer it would help us understand how insects evolve to reproduce so efficiently, and how we might control this in pest species using novel chemicals or improved biological control methods.We will address this problem by testing how SP and SPR work in a different insect species, Drosophila subobscura. This species is also a fly, it also has SP and SPR, but it is quite distantly evolutionarily related to Drosophila melanogaster. Drosophila subobscura females also show a very strong behavioural switch after mating: they boost egg laying and never mate again (because one mating provides all the sperm they ever need). This makes Drosophila subobscura species useful candidate for testing whether SP and SPR serve the same purpose over a fairly broad range of species.We will create genetically modified Drosophila subobscura that lack SP and SPR. We will then test them to see whether removing these molecules abolishes the normal boost to egg laying and rejection of males. The results will unequivocally answer the question of whether SP and SPR control the reproductive switch in Drosophila subobscura in the same way they do in Drosophila melanogaster, hence telling us how commonly these molecules play the same role. These results will thus create a step-change in our understanding of evolution of insect reproduction, and will pave the way for further studies across more species (including pest species), and for exploring other important reproductive molecules. The genetically modified flies we will generate will become an invaluable tool for our own future studies, and for other researchers with whom will share the flies freely.