SYNOPSIS. Doublet Paramecium tetraurelia contain either a single macronucleus which is substantially larger than that in a singlet cell, or 2 smaller macronuclei. Doublets have approximately twice the DNA content and twice the total protein content of singlets. The cell cycle of doublets is 164% as long as that of singlets, but the relative position of the macronuclear DNA synthesis period within the cell cycle is the same as in singlets. The DNA content of doublets is regulated so that differences in the number of macronuclei do not produce corresponding changes in DNA content; bimacronucleate doublets have only 27% more DNA than unimacronucleate doublets.
SYNOPSIS. Doublet Paramecium tetraurelia would be expected to contain 2 macronuclei if their nuclear complement were strictly analogous to that of singlets. However, most doublets are unimacronucleate. It is shown in this study that dimacronucleate cells are present only in young clones. Unimacronucleate cells arise either through abnormalities in the determination and distribution of macronuclear anlagen during the first cell cycle after conjugation, or from dimacronucleate cells through abnormal division and segregation of macronuclei during the fission process.When a change in the number of macronuclei occurs through abnormalities in the division and segregation of daughter macronuclei, the daughter cells produced typically have DNA contents more similar than those expected from either random segregation of daughter macronuclei, or from the normal segregation pattern in ciliates in which changes in the number of macronuclei in progeny cells do not occur. This suggests that part of the regulation process of macronuclear DNA content in Paramecium may occur through control of the segregation pattern of daughter macronuclei.
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