Compartmentalization of the developing macronucleus following conjugation in stylonychia and euplotes
TL;DR: The evidence presented here suggests that the "chromatin granules" seen in the mature vegetative macronucleus represent the material of single bands of the polytene chromosomes seen during the earlier stages of macronuclear development.
Abstract: The development of the macronucleus following conjugation in the hypotrichous ciliates Euplotes and Stylonychia has been examined with the electron microscope. Banded polytene chromosomes can be seen in thin sections of the macronuclear anlagen during the early periods of exconjugant development. As the chromosomes reach their maximum state of polyteny, sheets of fibrous material appear between the chromosomes and transect the chromosomes in the interband regions. Individual bands of the polytene chromosomes thus appear to be isolated in separate compartments. Subsequently, during the stage when the bulk of the polytenic DNA is degraded (1), these compartments swell, resulting in a nucleus packed with thousands of separate spherical chambers. Individual chromosomes are no longer discernible. The anlagen retain this compartmentalized condition for several hours, at the end of which time aggregates of dense material form within many of the compartments. The partitioning layers disperse shortly before replication bands appear within the elongating anlagen, initiating the second period of DNA synthesis characteristic of macronuclear development in these hypotrichs. The evidence presented here suggests that the "chromatin granules" seen in the mature vegetative macronucleus represent the material of single bands of the polytene chromosomes seen during the earlier stages of macronuclear development. The possibility is also discussed that the degradation of DNA in the polytene chromosomes may be genetically selective, which would result in a somatic macronucleus with a different genetic constitution than that of the micronucleus from which it was derived.
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TL;DR: Vegetatively growing ciliates appear to possess a mechanism for adjusting copy numbers of individual genes, which corrects gene imbalances resulting from random distribution of DNA molecules during amitosis of the macronucleus.
590 citations
TL;DR: In this paper, DNA was isolated from macronuclei and micronucleis of the ciliated protozoan, Stylonychia mytilus under conditions that minimize the possibility of DNA degradation.
Abstract: DNA was isolated from macronuclei and micronuclei of the ciliated protozoan, Stylonychia mytilus under conditions that minimize the possibility of DNA degradation Macronuclear DNA has an S value of 10 to 11 in sucrose gradients Macronuclear DNA has an average molecular weight of 115×106 daltons and a range of molecular weights of 10×106 to 195×106 daltons The average length of macronuclear DNA, measured by electron microscopy, is 080 microns and the range is 02 to 22 microns Almost all micronuclear DNA pieces are too long to be measured by electron microscopy The shortest piece of micronuclear DNA found was 150 microns in length
380 citations
TL;DR: Ciliated protozoa are characterized by generative micronuclei and vegetative polyploid macron nuclei and their kinetic complexity can be determined by comparison with the kinetic complexity of E. coli DNA.
Abstract: Ciliated protozoa are characterized by generative micronuclei and vegetative polyploid macronuclei. Micronuclei of Stylonychia mytilus contain 1 600 times as much DNA per haploid genome as E. coli. Most of this DNA is shown to be repetitive. The development of the macronucleus involves, as demonstrated by cytology, only 1/3 of the chromosomes which in a first replication phase are polytenized in probably 5 replication steps and appear as giant chromosomes. At this developmental stage considerable amounts of repetitive DNA are still present in the chromosomes. During the subsequent disintegration phase more than 90% of the DNA are eliminated from the macronucleus anlage. The remainder is further replicated five times and composes the final macronucleus. Since this DNA reassociates with a reaction rate almost identical to an ideal second order reaction its kinetic complexity can be determined by comparison with the kinetic complexity of E. coli DNA. Macronuclear DNA reassociates with a kinetic complexity of 26 times the kinetic complexity of E. coli DNA (corrected for GC content) which indicates that macronuclear DNA sequences exist at a ploidy level of 4 096 C. We assume that macronuclear DNA may be present only once per haploid genome. In this case it represents only 1.6% of the DNA in micronuclei or 10% of the DNA in the giant chromosome stage.
277 citations
Cites background from "Compartmentalization of the develop..."
...The decrease in content of DNA of macronucleus anlagcn during Stage 5 is accompanied by a large scale degradation of the giant chromosomes as earlier shown by cytology and electron microscopy (Kloetzel, 1970; Ammermann, 1971)....
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TL;DR: The germline genomes of ciliated protozoa are dynamic structures, undergoing massive DNA rearrangements during the formation of a functional macronucleus, and recent work suggests that chromatin differentiation plays a role in specifying DNA segments either for rearrangement or for elimination.
Abstract: The germline genomes of ciliated protozoa are dynamic structures, undergoing massive DNA rearrangement during the formation of a functional macronucleus. Macronuclear development involves chromosome fragmentation coupled with de novo telomere synthesis, numerous DNA splicing events that remove internal segments of DNA, and, in some ciliates, the reordering of scrambled gene segments. Despite the fact that all ciliates share similar forms of DNA rearrangement, there appears to be great diversity in both the nature of the rearranged DNA and the molecular mechanisms involved. Epigenetic effects on rearrangement have also been observed, and recent work suggests that chromatin differentiation plays a role in specifying DNA segments either for rearrangement or for elimination.
183 citations
TL;DR: The results lead to the following hypothesis: the macronuclei of the two hypotrich ciliates contain unconnected chromomeres or small aggregates which are distributed at random to the two daughter nuclei during the divisions.
Abstract: Some stages of macronuclear anlagen development, known from earlier investigations (see Fig. 1), were studied in detail. The results are: a) The giant chromosomes of Stylonychia mytilus are not somatically paired, but are connected end-to-end to form one or a few composite chromosomes. When they later disintegrate, the bands become isolated granules. b) Spectrophotometric measurements show that during the DNA-poor stage which follows the disintegration of the chromosomes, the macronuclear anlagen of Euplotes have a DNA content of 21 c, while the syncaryotic (deriving from syncarya) and hemicaryotic (deriving from haploid hemicarya) anlagen of Stylonychia have the DNA content of diploid micronuclei (2c). Nevertheless the syncaryotic anlagen of Stylonychia and Euplotes initially develop two nucleoli at the end of this stage, the hemicaryotic anlagen of Stylonychia only one. From this it is concluded that the genes of one giant chromosome band stay together in one granule, c) Labeled DNA from the giant chromosomes which remains in the anlagen during the DNA-poor stage is distributed approximately equally to the daughter nuclei during the first few fissions of the exconjugants.-Autoradiographic experiments showed that the DNA of the macronuclei of Stylonychia that is duplicated at one time in a replication band is not duplicated simultaneously during the next DNA-duplication. The DNA duplications during the second polyploidization stage of the macronuclear anlagen development are exceptions, because the mixing of the macronuclear DNA which occurs before every fission does not occur during the second polyploidization stage.—The pseudomicronuclei which sometimes are formed from the macronuclei in emicronucleated strains of Stylonychia contain numerous elements which are much smaller than the chromosomes.—The macronucleus of Stylonychia is very insensitive to irradiation with X-rays.—The results lead to the following hypothesis: The macronuclei of the two hypotrich ciliates contain unconnected chromomeres or small aggregates which are distributed at random to the two daughter nuclei during the divisions.
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References
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Journal Article•
1,900 citations
TL;DR: This chapter focuses on P. Aurelia, a species that has been extensively investigated by both the older and the newer methods, and the results illustrate the genetic phenomena known in the earlier work on the Ciliates.
Abstract: Publisher Summary The genetics of the ciliated Protozoa entered a new phase with the discovery of mating types in Paramecium aurelia (P. Aurelia). It became possible to cross-breed genetically diverse stocks as easily as in higher organisms, and thus pursues genetic studies by means of Mendelian methods. The genetic advances that have followed from this discovery, in every species in which mating types are now known, form the subject of this chapter. This chapter focuses on P. Aurelia that has been extensively investigated by both the older and the newer methods. The results on this species illustrate the genetic phenomena known in the earlier work on the Ciliates. Therefore, familiarity with the cytology and genetics of P. aurelia provide an excellent introduction to the cytogenetics of the ciliated Protozoa. The chapter also presents accounts of the mating types and breeding system, of the basic cytogenetic processes forming the indispensable groundwork for discussion of the genetics and of the genetic results themselves.
289 citations
01 Jan 1964
TL;DR: This chapter discusses the protozoan nucleus, which is found in interphasic and in chromosomal resting nuclei of euglenoids and foraminiferans and one frequently encountered type is the karyosome nucleus, characterized by a single centrally located nucleolus.
Abstract: Publisher Summary This chapter discusses the protozoan nucleus. The nuclei of tissue cells of multicellular organisms are of rather uniform appearance. As a rule, chromosomes are not recognizable in them. In contrast, chromocentric nuclei are absent in protozoa. Aside from two exceptions, heterochromatin, including sex chromosomes, is unknown in unicellular organisms. The appearance of resting nuclei in protozoa is determined to a high degree by the variability of the chromosomal material. In protozoa, the chromosomes undergo changes of form and structure in connection with the coiling cycle similar to those of higher organisms. The chromosomes are often quite uncoiled and extended in the interphasic type of resting nuclei and are more or less tightly coiled in the non-interphasic or chromosomal type. the quantity and distribution of the nucleolar material may also be responsible for the appearance of protozoan nuclei. One frequently encountered type is the karyosome nucleus, which is characterized by a single centrally located nucleolus. It is found in interphasic and in chromosomal resting nuclei of euglenoids and foraminiferans.
161 citations
01 Jan 1969
84 citations