Genetic analysis in Toxoplasma: gene discovery with expressed sequence tags and rapid mapping of natural...

Genetic analysis of the protozoan parasite Toxoplasma gondii has undergone a rapid expansion in recent years. This is due to effort in a number of laboratories that have worked on the development of molecular genetic techniques. It is also due, however, to the natural biology of this system (including a well-described sexual cycle) that makes possible genetic mapping of the F1 progeny from a cross. In this article, we present a detailed methodology for rapidly mapping natural polymorphisms between the ME49 and CEP strains for which extensive restriction fragment length polymorphism analysis has already been performed. The example we present shows that the failure to detect expression of bradyzoite-specific surface antigens in the CEP strain under conditions that promote differentiation in vitro is not a result of a general failure to express such genes; instead, it is apparently due to antigenic polymorphism in the gene products concerned. This conclusion was reached rapidly and definitively by genetic mapping, whereas molecular approaches would have taken considerably longer. We also show how the recent effort to create an extensive database of expressed sequence tags for this parasite can promote the very rapid discovery of genes that reveal much about the biology of Toxoplasma. The example presented deals with the expression of a family of closely related surface antigens in the tachyzoite stage.