“Paramecium aurelia and mutation”

Abstract

The ordinary species is Paramecium caudatum; superficially, it resembles P. aurelia. The latter differs from the former in smaller size, in rounded instead of attenuated posterior end, and in the possession of two instead of one micronucleus. The last is generally regarded as the most important difference between the two species. In March, 1905, a pair of conjugating Paramecium caudatum was isolated from a culture in an epidemic of conjugations. The ex-conjugates had all of the characteristics of P. aurelia. One died before many generations in culture, the other is still living and is now in the 346th generation. This one retained the characteristics of P. aurelia until about the 45th generation after conjugation, when it lapsed again into the P. caudatum form, with one micronucleus, and other characteristics of P. caudatum. The latter characters are still maintained.

The observation indicates one of two things. Either this is an interesting case of mutation of species with lapse into the parent form after several generations, or the specific characteristics are inadequate and P. caudatum and P. aurelia are but variants of one species. The latter is the more reasonable hypothesis and on grounds of priority, the common forms of paramecium should be called Paramecium aurelia.

Physiologically the form known as P. caudatum is more vigorous in culture than is P. aurelia During the time that the cultures were in the P. aurelia phase the division rate was relatively low (four divisions in five days), but soon after the change to the caudatum form the division-rate rose to two and a half divisions per day on the average for forty days, which is the highest rate on record. With this physiological difference there was a marked difference in the relative volumes of micronucleus and cell-body but no difference in the relative volumes of macronucleus and body.