Struktur und Entwicklung des Makronucleus von Paramecium bursaria

Abstract

Summary Macronuclei, macronuclear anlagen and micronuclei have been isolated from living cells with tungsten needles. In preparations under the cover slip the macronuclear gel may be solvatisated with 0,0007- to 0,001-molar solutions of Na2HPO4. Pressure in the moment of swelling results in sufficiently spreaded macronuclear contents. It is mainly composed of numerous feulgenpositive granular threads, here interpreted as fragments of chromosomes. The granules are identical with the “small bodies” described by former authors. Even the nucleoli contain such feulgenpositive chains of granules. In the anlagen development in two strains of American provenience (mt II and III) only short segments detached from the kinetochore region of the chromosomes are amplificated and incorporated into the new macronucleus. The major parts of the chromosomes undergo resorption. In Tubingen strains of P. bursaria the degrading parts of the chromosomes are condensed to a massive body (Binnenkorper). The loss of an important fraction of the chromosome complement provoces reservations against the denomination of this nucleus as a polyploid one. “Ampliploidy” as a new term could better refer to the given state. A condensation cycle of the macronuclear chromosome fragments as well as aggregation and disaggergation of nucleolar subunits (the latter apparently identical with the “large bodies” in P. aurelia) in the cell cycle have been deduced from electron micrographs. Two phases of substances production of the nucleolar subunits require further elucidation. There was a total failure to find any duplicational stages of the macronuclear chromosome fragments in fission as well as in interphase. Chromosome counts revealed about 52 tetrads in a cross of the mating types II and III. The Tubingen strains G2 and G3 have polyploid micronuclei. Estimates resulted in tetrad numbers between 120 and 180. These figures comprise almost the extremes of the range of variation found by Chen. The results and interpretations about macronuclear structure and development in combination with hypotheses by Allen and Gibson (master-slave hypothesis) and Jurand, Beale and Young (large bodies beefing identical with macronuclear subunits sensu Sonneborn) have been used to sketch a concept of macronuclear development, which eventually helps to understand aging processes like DNA-decline in growing young clones and the period of unripeness in the life cycle. If the macronuclear chromatin were multiplicated in the cell cycle from the nucleolar subunits only, than the original spectrum of DNA sequences, as present after anlagen development, should be progressively diluted out in the course of fissional cycles in favor of the special sequences contained in the nucleolar chromosome (or chromosomes).