SYNOPSIS. Only well fed Paramecium aurelia, grown either monoflorally or on a mixture of 2 species of bacteria, are adequate to maintain optimal fission rates and encystment rates for Didinium nasutum. Progressive starvation of paramecia prior to their being fed to didinia leads to decreased fission rates, the appearance of abnormal cells and a loss of ability to encyst by the didinia. This depression can be fully overcome by allowing the didinia to feed again upon well nourished paramecia. A minimum of 45 well‐fed paramecia is required daily for each Didinium if maximal fission rates are to be maintained. Encystment and fission appear to be mutually exclusive processes, but encystment rates are related to fission rates and seem to be exclusive of the density of the didinial culture.
SYNOPSIS
Exconjugant clones of Paramecium aurelia stock 51S, syngen 4, which fail to separate prior to the 1st fission have numerous cytologic and mating type determination anomalies. The doublets have abnormal distribution of macronuclear anlagen, fewer macronuclear fragments per cell, and abnormalities in numbers of micronuclei. Despite apparent cell fusion and mixture of cytoplasm, the singlets arising from each side of the doublet may be of opposite mating types, and mating type determination may remain unstable for 1 or more fissions in contrast to the usual pattern of mating type determination before the 1st postconjugation fission.
SYNOPSIS. The use of axenic medium permits the study of mating type determination in stock 51 (sensitive) of syngen 4 of Paramecium aurelia. A high frequency of cytoplasmically bridged pairs was correlated with a high frequency of change of mating type following conjugation in axenic medium. The direction of change was predominantly from mating type VII to mating type VIII, suggesting a dominance of type VIII cytoplasm in the clones arising from a mixed cytoplasmic ancestry. No significant effect of either lower temperature or of NaX3 upon the pattern of mating type determination was found. The high frequency of cytoplasmic bridges between conjugants led to the formation of many double or higher multiplex clones.
Column chromatography with Biogel P2 (molecular exclusion of 1800 daltons) indicates that the transforming principle causing microstomes to become macrostomes is a small molecule. Absorbance tests show that only those fractions with high absorbance at 260 nm have biological activity, indicating that the active principle is a component of nucleic acids. Tests of purines and pyrimidines show that purines are active, with hypoxanthine having the highest activity. The combination of hypoxanthine with uridine shows a synergistic reaction. As these two compounds are the natural catabolic excretory products from nucleic acids in Tetrahymena, the fact that they induce transformation in concentrated, starving cells may be a survival mechanism allowing cannibalism to be induced when nutrients are depleted, thereby allowing the survival of the transformed cells until such time as adequate nutritional conditions are restored.
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