The causes and consequences of genome reduction in eukaryotes

Genome size in eukaryotic (nucleus-containing) microorganisms varies more than 200,000 fold. At the lower end of this spectrum are the reduced genomes of microorganisms that have become symbionts or intracellular pathogens, such as apicomplexans (e.g., Plasmodium, the causative agent of malaria) and microsporidian parasites (e.g., Encephalitozoon, an opportunistic pathogen of AIDS patients). The most extreme examples of eukaryotic genome reduction are the "nucleomorph" genomes of cryptophytes and chlorarachniophytes. Nucleomorphs are the remnant nuclei of eukaryotic phototrophs that established themselves as endosymbionts inside a non-photosynthetic host cell¿at well under a megabase in size, nucleomorph genomes are the smallest and most compact nuclear genomes known. This research proposes to use the miniaturized nucleomorphs of cryptophytes and chlorarachniophytes as a model system with which to elucidate the process of genome reduction and quantify its impact on protein structure and function. These results will contribute to a comprehensive understanding of the universal principles governing genome size change in eukaryotes and provide insight into the genetic and biochemical changes that occur when free-living eukaryotic microorganisms become intracellular parasites.