"Protozoan parasites from diverse evolutionary lineages including Trypanosoma and Leishmania, Giardia, Entamoeba, Plasmodium and the microsporidia have a major impact on human health, especially in developing countries and among immunocompromised individuals. During their evolution as parasites these organisms can both lose pathways if they can steal host metabolites and gain genes that may help them to manipulate their hosts. It is this balance of genome reduction and genome expansion, and the potential consequences for parasites and their eukaryotic hosts, that I will investigate in my project. Although the genome evolution of bacterial pathogens is relatively well studied, much less is known about how eukaryotic parasites - which are much more similar to their hosts - evolve at the genomic level. A sophisticated analytical approach will be used to compare genome reduction and expansion events in different groups of parasitic eukaryotes and their free-living relatives, to identify lineage specific and common evolutionary features. This framework will be used to identify and characterize new, parasite-specific protein families that may be involved in pathogenesis and, as an important part of my training, I will collaborate with bench scientists in the host lab to test these hypotheses. By identifying the gene families that are gained and lost during the evolution of parasitic protozoa, I will identify cases when bacteria and eukaryotes evolve in similar and different ways and hence gain insight into how parasites evolve more generally. Moreover, since parasites may lose genes that are strongly conserved among free-living model organisms, they also represent “natural experiments” for understanding which features of eukaryotic cells are essential and which are lineage specific elaborations. As a test case to explore this principle I will investigate the evolution and conservation of proteins involved in a fundamentally important eukaryotic pathway - the cell cycle."