Actually the common yeast falls into this category also. There are very few microbes that don't have some sort of sexual cycle and acquiring it or losing it happens all the time.
I just chose the paramecium because there was such a nice web page explaining how it works.
Wait until someone brings up parthenogenesis and we get to discuss lizards that sometimes mate the two sexes, but mostly the female lizards reproduce with no need of males. Now that one often creates a real problem of understanding among people ignorant of the variety of animal reproductives strategies.