Extra-Chromosomal DNA in Early Stages of Oogenesis in Acheta Domesticus

Structure of the germinal vesicle during oogenesis in leechGlossiphonia heteroclita (Annelida, Hirudinea, Rhynchobdellida)

Oogenesis in the glossiphoniid leech Glossiphonia heteroclita (Hirudinea, Rhynchobdellida) is nutrimental, i.e., the growing oocyte is supported by specialized germline cells, the nurse cells. The main function of the nurse cells is to provide oocytes with cell organelles and RNAs (mainly rRNA). However, in studied leech species, irrespective of the nutrimental mode of oogenesis, the germinal vesicle (GV = oocyte nucleus) seems to be very active in rRNA production. As shown in the present study, during early previtellogenesis in the GV the meiotic chromosomes and prominent primary nucleoli occur. In late previtellogenesis the chromosomes condense and occupy a limited space of nucleoplasm in close vicinity to primary nucleolus, forming a karyosome. At the onset of vitellogenesis several prominent extrachromosomal DNA bodies appear in close association with the karyosome. At the same time, the primary nucleolus is no longer visible in the GV. As vitellogenesis proceeds the extrachromosomal DNA bodies undergo fragmentation and numerous spherical, RNA- and AgNOR-positive inclusions occur in the nucleoplasm. They are regarded as multiple nucleoli. Finally, in late oogenesis numerous accessory nuclei are formed in close proximity to the nuclear envelope. They usually contain one dense body, morphologically similar to multiple nucleoli. The amplification of rDNA genes, the occurrence of extrachromosomal DNA bodies, as well as the presence of multiple nucleoli and accessory nuclei are described for the first time in the phylum Annelida.

Extrachromosomal rDNA amplification in the oocytes of Polystoechotes punctatus (Fabricius) (Insecta-Neuroptera-Polystoechotidae)

The ovary of Polystoechotes punctatus consists of several ovarioles of meroistic-polytrophic type. Histological, histochemical and ultrastructural studies revealed that the extrachromosomal amplification of rDNA takes place in the oocyte nucleus. Prior to previtellogenic growth the oocyte nucleus contains the chromosomes of meiotic prophase and a condensed extra DNA body. Initial split of extrachromosomal DNA material into several fragments coincides with the appearance of a spherical, fine granular body (referred to as primary nucleolus). Its gradual fragmentation accompanied by further dispersion of amplified DNA results in the formation of a growing number of multiple nucleoli. Until mid previtellogenesis each multiple nucleolus contains detectable amount of rDNA. In the advanced stages of previtellogenesis rDNA can hardly be visualized within the multiple nucleoli, while chromosomes form a few dense aggregates randomly disposed in the karyoplasm. At the onset of vitellogenesis the chromosomes assemble to form a karyosome. In its close vicinity DNA-positive material reaggregates. Multiple nucleoli are either found on the periphery of this aggregation or merge within it. At the final stages of vitellogenesis the number of multiple nucleoli significantly decreases.

Determination of DNA content in the nurse and follicle cells from wild type and mutant Drosophila melanogaster by DNA-Feulgen cytophotometry

DNA replication patterns in the nurse and follicle cells of wild type and a female sterile mutant, fs(1)1304, of Drosophila melanogaster have been studied by DNA-Feulgen cytophotometry, using a cell dispersal technique that allowed the measurement of DNA amounts in individual nuclei from egg chambers of known developmental stages. DNA-Feulgen values associated with various ovarian nuclei from egg chambers at different stages of development were used to assess a base line DNA content for ovarian tissues and to estimate the extent of DNA replication in the nurse cells and follicle cells of growing and mature egg chambers. Our data show that both the nurse and follicle cells undergo multiple cycles of endonuclear DNA replication and that there may be selective amplification as well as underreplication by portions of the genome in these highly polyploid, ovarian cells. Alternative models are proposed to account for the DNA replication patterns observed. Comparisons of DNA-Feulgen levels in wild type ovarian nuclei with those found for the fs(1)1304 mutant and its heterozygote in the balanced stock fs/FM3, show that equivalent DNA levels are present in follicle cell nuclei from all three types of females. Nurse cell nuclei in the homozygous fs stock, however, fail to achieve the same high DNA levels observed in both fs/FM3 and wild type nurse cell nuclei. Although the nuclei of follicle cells in ovaries from fs/fs females appear morphologically like those surrounding egg chambers in wild type ovaries, nurse cell nuclei from mutant females show a more compacted organization of their chromatin than found for nurse cell nuclei from wild type ovaries at similar developmental stages. Our findings suggest that a major effect of the fs(1)1304 mutation may be on the coiling behavior of chromatin and the conformation of DNA-protein moieties in both nurse cell and follicle cell nuclei. These changes in chromatin structure apparently are manifest by perturbations in DNA replication patterns and normal gene function in these biosynthetically active cells.

Multiplication of nucleolar fibrillar centres and absence of rDNA amplification in mouse oocyte during meiotic prophase I

During meiotic prophase I the nucleolus of the mouse oocyte assumes a reticulate structure of 'nucleolonema' type. This change coincides with the appearance of several secondary fibrillar centres. The number of these centres at diplotene (97-113), largely exceeds that of nucleolar organizers (4c DNA = 20 NORs). The quantitative analysis of autoradiographs after hybridization in situ with 3H-uridine labelled rRNA, enabled us to demonstrate that the multiplication of the fibrillar centres in mouse oocyte nucleolus during meiotic prophase I is not the result of an amplification of the rDNA. The number of silver grains in pachytene and diplotene nuclei was twice that counted for somatic cell and oogonium nuclei (2c DNA).

Visualization of ribosomal gene activity: silver stains proteins associated with rRNA transcribed from oocyte chromosomes

Cricket oocyte chromosomes were stained with silver at pachytene when certain chromosome regions are active in rDNA amplification and rRNA transcription. The silver preferentially stained the known locations of 18S + 28S ribosomal cistrons. Cytochemical tests revealed that the silver binds neither to the rDNA nor transcribed rRNA, but rather to proteins which rapidly associate with the freshly-transcribed rRNA. As rRNA transcription proceeds, the quantity of silver stainable proteins progressively increases. The silver procedure can be used to visualize gene activity at the rDNA sites with conventional light microscopy.

Frequencies of circular units of nucleolar DNA in oocytes of two insects, Acheta domesticus and dytiscus marginalis, and changes of nucleolar morphology during oogenesis

The organization of the extrachromosomal nucleolar material in oocytes of two insect species with different ovary types, the house cricket Acheta domesticus (panoistic ovary) and the water beetle Dytiscus marginalis (meroistic ovary), was studied with light and electron microscopic techniques. Stages early in oogenesis were compared with fully vitellogenic stages (mid-to-late diplotene). The arangement of the nucleolar material undergoes a marked change from a densely aggregated to a dispersed state. The latter was characterized by high transcriptional activity. In spread and positively stained preparations of isolated nucleolar material, a high frequency of small circular units of transcribed rDNA was observed and rings with small numbers (1--5) of pre-rRNA genes were predominant. The observations suggest that the 'extra DNA body' observed in early oogenic stages of both species represents a dense aggregate ofnumerous short circular units of nucleolar chromatin, with morphological subcomponents identifiable in ultrathin sections. These apparently remain in close association with the chromosomal nucleolar organizer(s). The observations further indicate that the individual small nucleolar subunit circles dissociate and are dispersed as actively transcribed rDNA units later in diplotene. The results are discussed in relation to principles of the ultrastructural organization of nucleoli in other cell types as well as in relation to possible mechanisms of gene amplification.

DNA synthesis in the oocyte of the mature mouse: a radioautographic study

Mice were injected three times over an 8-hour period with a total of 48 muci 3H-T/gm of body weight and were sacrificed 1, 3, 5, 9 and 17 days afterwards. Radioautographs of the ovaries showed significantly higher grain counts in oocytes of follicles that are in the antrum formation stage. The radioautographic visualization of DNase digestible 3H-thymidine incorporation into the juxtanucleolar region in oocytes of mature mice occurs in association with ooctye growth in follicles that are in the antrum formation stage. The scheduled disappearance of this juxtanucleolar oocyte DNA and its label during later oocyte growth suggests a degradation or dispersion of this labeled DNA prior to ovulation.