Studies on the Macronuclei of Doublet Paramecium tetraurelia: Distribution of Macronuclei and DNA at Fission*

Abstract

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.

Nuclear and Cortical Regulation in Doublets of Paramecium: II. When and How do Two Cortical Domains Reorganize to One?

“…In agreement with Berger and Morton (1980), the great majority of doublets had reduced the number of macronuclei from two to one by about 20 fissions after electrofusion, often while maintaining near‐perfect cortical balance. However, in all three lines of P. undecaurelia , unusually frequent autogamy continuously generated a small proportion of doublets with two macronuclei, as well as variable numbers of micronuclei (Prajer et al 1999).…”

“…This clearly is true in Paramecium , in which the doublets can persist for extended periods only if cortical balance is preserved. This cortical balance is not dependent upon maintenance of two macronuclei in the progeny of the fused cells, since the number of macronuclei in most doublets is rapidly reduced from two to one (Berger and Morton 1980). Although dual macronuclei may spontaneously recur as a result of abnormal autogamy (Prajer et al 1999), their recurrence is neither necessary nor sufficient for prolonged maintenance of the doublet condition.…”

What Do Genic Mutations Tell Us about the Structural Patterning of a Complex Single-Celled Organism?

“…There is good reason to believe that the same conclusion applies to doublets induced in other ciliates, including Tetrahymena thermophila (103,109). This structural constraint within the cell cortex does not influence the number of macronuclei in doublets; these cells typically reverted from possession of two macronuclei to one, while the cortex continued to propagate its duality (14,109).…”

“…There is good reason to believe that the same conclusion applies to doublets induced in other ciliates, including Tetrahymena thermophila (103,109). This structural constraint within the cell cortex does not influence the number of macronuclei in doublets; these cells typically reverted from possession of two macronuclei to one, while the cortex continued to propagate its duality (14,109).The third form of structural inheritance was discovered by Fauré-Fremiet (31). He noted that whereas the great majority of the doublets that he studied manifested a twofold rotational symmetry in the organization of their two normal sets of cortical structures, one exceptional clone of a ciliate named Urostyla trichogaster displayed a mirror image symmetry in the arrangement of its two sets of cortical structures.…”

Measurement of Unequal DNA Partitioning in Tetrahymena pyriformis Using Slit‐Scanning Flow Cytometry

“…Euplotes eurystomus was foundto haveameandifferenceof 11.6% (Witt, 19771, while values of 33.8% and 37.3% have been found for Bursaria truncatella (Ruthman, 1964;Zech, 1966). In studies of Paramecium tetraurelia, mean difference values from 9 to 20.4% were found in one study (Berger and Morton, 1980) and values of 5.8 and 10 9% in another, where values of up to 63 % were found for a mutant strain (Berger and Schmidt, 1978). Values in other Paramecia were also within this range (Kimball, 1967;Kudrjavtsev, 1966).…”