Identification and function of sex pheromones and their receptors in Drosophila

Insects not only use their senses of smell and taste to find the animal or plant hosts from which they feed and to find sites for laying eggs, but also to communicate with each other. Mating is the outcome of successful courtship, and courtship depends on communication between males and females. Recently, we found that a large part of the olfactory system in the fruit fly Drosophila, an insect that is used widely in biological research, appears to be dedicated to smelling fly odours. This suggests olfactory sex pheromones may play an important role in its reproductive behaviour. The aim of this project is to identify new olfactory sex pheromones in Drosophila and to analyze their function in courtship. In the flies' antennae, odours interact with odor receptors, encoded by the Odor receptor gene (Or) family. These odor receptors are expressed in the membranes of sensory neurons. The neurons encode information about the quality, intensity and temporal characteristics of odours in electrical signals. The olfactory sensory neurons are located in the antenna of the fly in structures called sensilla, and we can record the electrical activity of the neurons through an electrode placed into the sensilla. We found that trichoid sensilla, which are one morphological type, hold neurons that respond to fly odours. There are twelve Or genes expressed in the trichoids. We will use a previously developed assay system to test each of the receptors' sensitivity to fly odours. We will then identify the components in the fly odours that activate the receptors. Finally in behavioural experiments, we will explore how the flies use their receptors to distinguish suitable from unsuitable mates. Fruit flies belong to the order of the Diptera. The Diptera also include biting flies and mosquitoes that transmit serious diseases such as malaria, dengue fever and trypanosomiasis ('sleeping sickness' in humans). An understanding of sex pheromone communication in one fly species may elucidate general principles of Dipteran courtship, and may lead to new strategies of insect control by giving us the tools to disrupt the insects' reproductive behaviour.

Although it is well established that Diptera such as flies and mosquitoes use their sense of smell to find hosts for feeding, very little is known about the role of odours in their sexual communication. The fruit fly Drosophila melanogaster is experimentally tractable and we will use it initially as a model to understand principles of courtship that may also operate in medically and economically important Diptera. Using a new stimulus method, whereby the stimulus is loaded onto a capillary that is brought into the vicinity of the fly to mimic the proximity of interacting insects, we recently showed that a large proportion (~20%) of its olfactory sensory neurons (OSNs) respond specifically to fly odours and we identified several receptors that mediate these responses. The aim of this project is to identify the components in fly odour that stimulate the receptors, and then to determine the contribution of each candidate sex pheromone component to the courtship process. Each receptor will be expressed in an in vivo expression system. Extracts of flies will be used as a source of candidate sex pheromone components. Extracts and their chromatographically separated fractions will be assayed on each receptor to identify ligands. We will test a model of how the receptors might be used to distinguish suitable from unsuitable partners, by changing the amount of their ligands on the partners' bodies in courtship assays or by interfering with the perception of the ligands through the expression of tetanus toxin light chain under control of the Or promoters in the OSNs. Outcomes are the identities of novel pheromones and improved models of the olfactory basis of mate recognition in Drosophila.