Micronuclear Variation in Paramecium bursaria

THE dimorphic condition of the nuclear apparatus of the Infusoria is one of the most interesting and characteristic specializations exhibited by the group, apparently representing a segregation of ' somatic' and ' germinal' elements into distinct macronuclei and micronuclei. Accordingly these bodies have been extensively studied during the various life-processes of the Infusoria. Furthermore, several investigators have described races of well-known species of Infusoria in which no morphologically differentiated micronucleus is present, and it is highly significant that such amicronucleate animals when carefully cultured are viable but unable to consummate conjugation. They are amicronucleate both structurally and functionally (1). In view of these facts and the importance of the structure and number of the micronuclei in determining the relationships of the several species of the genus P a r a m e c i u m (2), the following account records variations in the micronuclear number, &c, in a pedigree race of P a r a m e c i u m bursaria—variations ranging from four micronuclei to the absence of a micronucleus in genetically related animals. During July 1924, specimens of P . b u r s a r i a collected from a small freshwater pond at Woods Hole, Massachusetts, revealed the presence of two micronuclei of the 'caudatum type', instead of the single micronucleus of the same type characteristic of P . b u r s a r i a . Accordingly a pedigree race of bimicronucleate animals was started on July 26, 1924, and since that time to the present (July 1931) it has been cultured either in small mass cultures or in daily isolation cultures at