Genetics of Toxoplasma gondii tachyzoite to bradyzoite differentiation

Asexual development is a critical aspect of the parasitic lifestyle of Toxoplasma gondii, allowing transmission and persistence of the parasite. Development of the rapidly-dividing, disease-causing tachyzoite and the quiescent, encysted bradyzoite involves significant morphological and metabolic changes Chapter 1 provides a brief overview of parasite biology focusing specifically on asexual development and the host and parasite factors known to influence asexual development.Identification of the genes necessary for tachyzoite-to-bradyzoite conversion is needed to understand the genetic controls of differentiation in Toxoplasma. Chapter 2 describes an insertional mutagenesis designed to obtain mutants unable to differentiate to bradyzoites and identify the disrupted genes responsible for the phenotype. The mutagenesis identified pseudouridine synthase 1 (PUS1) as a critical gene for bradyzoite development.Varying levels of differentiation efficiency of the different strains of T. gondii suggest that genetic polymorphism between the strains is responsible for these differences. In the studies described in Chapter 3, the role of PUS1 polymorphism in differentiation was investigated through sequence analysis and experiments interrogating the different alleles of PUS1. Investigation of PUS1 polymorphism found high levels of PUS1 polymorphism to reside in an N-terminal domain of unknown function and not in the PUS domain. The results argue that PUS1 polymorphism plays a significant role in the differences in switch efficiency observed for different Toxoplasma strains.Bradyzoite development is controlled by underlying genetic regulatory networks in Toxoplasma. Chapter 4 describes experiments based on combinatorial transcript profiling by microarray to identify common genes and pathways regulated by differentiation. A differentiation time course of wild-type Type II parasites identified &sim300 developmentally regulated transcripts. Analysis of multiple mutant and transgenic parasite lines identified sets of commonly dysregulated genes and pathways as potential mechanisms for regulating differentiation.