The molecular mechanisms that control growth appear to be conserved across the animal kingdom, with nitric oxide regulation of cell proliferation and growth being found to be very significant. Indeed, in Drosophila larval development and mammalian systems NO has been shown to be particularly important in these processes, and previous work in our laboratory has identified the Drosophila forkhead transcription factor dFOXO as a critical target through which NO signalling exerts its regulatory effects on growth, although little is currently known concerning the precise mechanisms involved. Accordingly, in this thesis, we investigate the processes through which NO may modulate growth and demonstrate that targeted expression of a constitutively active NO Synthase to whole larval salivary glands or clones of cells within the glands, results in reduced endoreplication and growth as measured by nuclear size. Targeted over expression of dFOXO itself is shown to result in similar phenotypes, and subsequent molecular analysis of potential signalling targets required for this inhibition of growth reveals that dFOXO, Thor and Myc expression are regulated in vivo by NO. To elucidate if NO acts directly on dFOXO, the genetic interaction of components of the insulin signalling pathway is analysed, exploiting RNA interference to assay what components are necessary for the NO signal to be effectively transduced, and it is demonstrated that NO control of growth is not through sGC, one of the most significant known targets for NOmediated regulation in other organisms. We subsequently investigated the roles of Thor, a Drosophila 4E-binding protein, and the kinase, Lk6, homologues of which are known to be important in growth regulation in other organisms, and thus potential effectors of NO and dFOXO. However our data demonstrated that neither Thor nor Lk6 are required for the inhibition of growth by NO. Interestingly a potential anti-oncogenic effect of NO signalling was also revealed following analysis of interactions between NO and Ras or Myc induced growth in which NO was able reduce the overgrowth produce by both these oncogenes. Overall this research confirms dFOXO as an essential target for NO induced inhibition of growth. The work also eliminates two dFOXO transcription targets, Thor and Lk6, as necessary for NO to regulate growth.

The hemiparasite Rhinanthus minor is a common component of many northern temperate grasslands. It can have major impacts on ecosystem processes, and is often present at very high densities, therefore constituting an important potential food source for invertebrate herbivores. Thus, the aim of this thesis is to investigate the interaction between this hemiparasite and its invertebrate herbivores, and to explore the various ecological factors which are likely to affect this interaction. In the first series of experiments the thesis explores how the density of the hemiparasite affects the composition of the vegetation, the performance of the hemiparasite and the levels of invertebrate herbivore damage it receives. The results of a field experiment and a greenhouse study demonstrated that hemiparasite density can adversely affect its own performance and survivorship and dramatically change the composition of the vegetation, but surprisingly appeared to have no impacts on the levels of herbivore damage the hemiparasite receives. The second series of experiments investigated the impacts if host identity on the performance of the hemiparasite and how this affects its invertebrate herbivores. The results demonstrated that host identity can have a major impact on the performance of the hemiparasite and its herbivores; however, the indirect effects on the invertebrates appear to be species specific. Thirdly, the thesis examines the effects of multiple host plants on the performance of R. minor and the knock-on effects for its invertebrate herbivores. Experiments demonstrated that multiple hosts are beneficial to R. minor, and that the antiherbivore defensive properties conferred to the hemiparasite by certain host plants are maintained in the presence of a second host species. Finally, the impact of nutrient addition and host plant damage on the performance of the hemiparasite and on the performance of its invertebrate herbivores was examined. The experiments showed that while certain host plants have highly contrasting effects on the performance of the hemiparasite‟s herbivores, the addition of nutrients and impact of host plant damage largely remove these differences, while neither factor appeared to affect the performance of the hemiparasite.

The interaction between hosts and parasites in bird populations has been studied extensively. I use game theoretic methods to model this interaction. This has been done previously but has not been studied taking into account the detailed sequential nature of this game. I introduce models allowing the host and parasite to make a number of decisions which will depend on a number of natural factors. A sequence of events follows, which is broken down into two key stages; firstly the interaction between the host and the parasite adult, and secondly that between the host and the parasite chick. The final decision involves the host choosing whether to raise or abandon the chicks that are in the nest. There are certain natural parameters and probabilities which are central to these various decisions; in particular the host is generally uncertain whether parasitism has taken place, but can assess the likelihood of parasitism based upon certain cues (e.g. how many eggs remain in its nest). I have taken elements of games which have been previously created and constructed my own models to fully describe this interaction. These parasites have different methods of parasitizing the nests of their hosts, and the hosts can in turn have different reactions to these parasites. This is later built into a model where there is more than one host nesting over a breeding season. We have a number of nesting sites and different time points in which the host can begin to nest. In the previous models the host was given the opportunity to abandon the nest. In this game the host is allowed to abandon and then restart the nesting process. The probability that the host is parasitized can be decided using a number of factors including the number of hosts laying during a given time period, the nesting site or the number of parasites during the course of the season. Using these models we are able to find situations which match those which we have seen in nature. Also the models are able to predict what natural changes such as parasitism rate or mimicry will do to the interaction. Overall I believe these models to give as good an indication of the key elements of the interaction and how they can change over time.