Loss of Stages after Continuous Passage of Toxoplasma gondii and Besnoitia jellisoni*

SYNOPSIS. Toxoplasma gondii, passed from mouse to mouse in the tachyzoite stage for 30–35 generations, developed cysts, which, when fed to cats, failed to produce oocysts. Besnoitia jellisoni, passed similarly for 20 generations, lost the capacity to form cysts. These phenomena are explained by a loss of genomes or gene products during the rapid passage selecting for tachyzoites.