Genomic exclusion is an abnormal form of conjugation occurring between cells with defective micronuclei and normal cells with diploid micronuclei. The progeny are heterocaryons; each cell has an old macronucleus but a new diploid micronucleus derived from one meiotic product of the normal mate. Such cells express genes found in the old macronucleus, are sexually mature, and can be specifically selected. When inbred, they give rise to lines genetically homozygous at all known loci.
Our measurements have bearing on the resolution with which maps can be constructed and abnormalities can be detected by studying the proximity of DNA sequences in metaphase and interphase chromosomes. The results of our analyses are summarized in Figure 8. Metaphase chromosomes are compacted sufficiently that it is impractical to order sequences separated by less than approximately 1 Mbp. In contrast, 100-kbp resolution can be obtained in interphase chromosomes. Distance measurements reveal that interphase chromatin behaves as a random polymer over distances up to 1-2 Mbp. At greater distances, higher order constraints, perhaps the dimensions of the individual chromosome domains, come into play. A caveat remains: Because the effect of the FISH procedure on native chromosome organization is not well understood, these conclusions may not be applicable to native chromatin. We have illustrated that FISH, with appropriately chosen probes, can supplement conventional cytogenetics in the study of chromosome abnormalities. The technique is increasingly being applied in research laboratories to detect and characterize chromosome abnormalities and point the way to the location of genes involved in human disease.
The focus of this review is on the micronucleus and macronucleus in the ciliated protozoa and the organization and function of the DNA molecules within them. We present (1) some of the structural and functional differences which are known, (2) the genetic evidence for macronuclear units, (3) two hypotheses for the organization of the DNA molecules in the macronucleus to explain these units, and (4) experiments designed to discriminate between these hypotheses. We conclude that the size of the genome is not reduced in the macronucleus and that there are 45 copies of the haploid genome present in the macronucleus of normal strains ofTetrahymena pyriformis and 800 copies in the macronucleus of Paramecium aurelia. The ciliate genome is relatively simple in terms of repeated sequences. However, not all copies of the genes present in the macronucleus may be identical since fractions of differing thermal stability appear after renaturation.
The esterase isozymes were surveyed in axenic stocks of syngens 1, 2, 4, 5, 6, and 8 of Paramecium aurelia by starch gel electrophoresis. In paramecia there appear to be four types of esterases which are clearer in axenie than in baeterized stocks. Each type differs in its substrate specificity and/or its response to the inhibitor eserine sulfate. Minor variations in type D esterases sometimes occur in different extracts of the same stock and may result from changes in the temperature of growth of the cells or growth cycle differences. Differences in the mobility of the A, B, or C (eathodal) types of esterases may occur in different syngens. They also occur for the A and B types among stocks within a syngen, but the frequency is low, except in the case ofsyngen 2. Since each of the types of esterases varies independently, at least four and possibly more genes appear to specify the esterases in the species complex. Some pairs of syngens vary in their eleetrophoretic positions for all types of esterases. Other pairs have identical zymograms. This observation suggests that some syngens may differ from each other by as many as four esterase genes, while others may not differ at all. The difference between P. aurelia and Tetrahymena pyriformis in the degree of intrasyngenie variation observed for enzymes is discussed in relation to other types of characters, the organization of the genetic material in the macronueleus, the presence of symbionts, and their breeding systems. It is suggested that enzyme variation is achieved by the action of different selective forces in these two groups of ciliated protozoa.
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