Abstract
A series of pedigree lines of Didinium nasutum, consisting of the progeny of a single exconjugant, were bred continuously under practically constant environmental conditions from December 16, 1919, to September 10, 1920.
A cytological investigation of the preparations made daily from stock left after each isolation, demonstrated the absence of a morphological micronucleus in vegetative, dividing and conjugating animals. In the race from which these pedigree lines were derived by conjugation, and also in other unrelated races of Didinium, micronuclei were easily demonstrable. It is clear, therefore, that the parent race was micronucleate, while one of its progeny, a single exconjugant (the original cell of the race under investigation) was amicronucleate. Moreover, this amicronucleate condition persisted throughout the life of the race, that is, through 652 generations.
At various intervals during the life history of this culture, periods showing a tendency for encystment and conjugation occurred, but the animals which encysted or conjugated invariably died, a fact undoubtedly related to their amicronucleate condition. Rhythmical periods of depression followed by increased vitality, which resembled those shown by Woodruff and Erdmann 1 to be associated with endomixis in Paramecium caudatum were also observed. Endomixis, however, if such occurs in the free living state of Didinium, could not be carried out in this race during the rhythmical periods owing to the lack of a definitive micronucleus.
The amicronucleate state without doubt arose from some irregularity during conjugation. Prandtl 2 in his study of the cytology of conjugation in Didinium found individuals in which all the micronuclei had become transformed into macronuclei, but he thought animals so endowed were incapable of living. The present study shows that they are viable but unable to undergo processes such as encystment, conjugation, and endornixis which are dependent largely upon micronuclear activity.
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