Multiple nucleoli and enhanced nucleolar activity in the nurse cells of the insect ovary

The changes in chromosome 6 spatial organization during chromatin polytenization in the Calliphora erythrocephala Mg. (Diptera: Calliphoridae) nurse cells

Localization of Calliphora erythrocephala chromosome 6 in a 3D nuclear space at different stages of nurse cell chromatin polytenization was analyzed by fluorescence in situ hybridization and 3D microscopy. The obtained results suggest a large-scale chromatin relocation in the C. erythrocephala nurse cell nuclei, which is accompanied by a change in the chromosome territory of chromosome 6 associated with the change in expression activity of the nucleus and formation of reticular chromatin structure. It was revealed that the relocation of chromosome 6 (nucleolus organizer chromosome) is accompanied by fragmentation of the single large nucleolus into micronucleoli, which are spread over the entire nuclear space being associated with their nucleolar organizer regions. Presumably, the chromosome 6 material during transition to a highly polytenized structure is redistributed in the nucleus so that the inactive pericentromeric regions are displaced to the nuclear periphery, while the chromosome regions carrying rDNA sequences loop out beyond the chromosome territory. Being dispersed over the entire nuclear space, rDNA sequences are likely to be amplified, thereby providing numerous small signals from the chromosome 6-specific DNA probe. Micronucleoli are formed around the actively transcribed nucleolar organizer regions.

Polytene chromosomes in mammalian cells

This article deals with the structural and functional organization of polytene chromosomes in mammals. Based on cytophotometric, autoradiographic, and electron microscopic data, the authors put forward a concept of nonclassic polytene chromosomes, with special reference to polytene chromosomes in the mammalian placenta. In cells with nonclassic polytene chromosomes, two phases of the polytene nucleus cycle are described, such as the endointerphase (S phase) and endoprophase (G phase). The authors generalize that the main feature of nonclassic polytene chromosomes is that forces binding the sister chromatids are much weaker than in the Diptera classic polytene chromosomes. This concept is confirmed by comparative studies of human, mink, and fox polytene chromosomes. The final step of the trophoblast giant cell differentiation is characterized by a transition from polyteny to polyploidy, with subsequent fragmentation of the highly polyploid nucleus into fragments of low ploidy. Similarities and dissimilarities of pathways of formation and rearrangement of nonclassic polytene chromosomes in mammals, insects, plants, and protozoans are compared. The authors discuss the significance of polyteny as one of the intrinsic conditions for performance of the fixed genetic program of trophoblast giant cell development, a program that provides for the possibility of a long coexistence between maternal and fetal allogenic organisms during pregnancy.

Genome reorganization from polyteny to polyploidy in the nurse cells found in onion fly (Delia antiqua) and cabbage root fly (Delia radicum) ovaries (Diptera, Anthomyiidae)

The material required to ensure successful embryogenesis in the onion fly (Delia antiqua) and the cabbage root fly (Delia radicum) (Diptera, Anthomyiidae) is supplied by 15 nurse cells, while the oocyte chromosomes enter a quiescent stage during prophase I of meiosis. This level of transcription is achieved by the polyploidization of the nurse cell DNA. Elongate polytene chromosomes form in both species, but lack the banding and conspicuous puffing commonly seen in other dipteran tissues. The polytene chromosomes contract until they finally appear as small, densely staining spheres. These fragment into large numbers of endochromosomes that are much smaller than their mitotic counterparts, which then despiralize, resulting in the flocculate appearance of the nurse cell nucleus. Photodensitometry revealed a gradient of DNA values between nurse cells near the oocytes and those further away. Final DNA values 1000 times the haploid level were recorded in the nurse cell nearest to the oocyte compared with 336 times the C-value in the most distal cell. At lower temperatures (< 10 degrees C), the polytene chromosomes become banded and longer. None of the onion flies kept in these conditions produced viable eggs, though there was some reproductive success among the cabbage root flies.

Amitosis and endocycles in early cultured mouse trophoblast

The possible occurrence of amitosis has been studied in nuclei of trophoblast outgrowths of mouse embryos placed in culture at the two-cell stage. By day 7 of culture, the inner cell mass has usually floated away, while the trophoblast outgrowth remains attached. Of 591 trophoblast cells from 25 embryos, 469 were uninucleate, 87 binucleate, 4 trinucleate, and in 31 the nuclei were attached to each other. In our interpretation, these come about through a process in which the nucleus stretches, then the nuclear membrane invaginates and finally constricts the nucleus into two parts. The resulting nuclei, asymmetric in size and in amount and arrangement of heterochromatin and nucleoli, lie side-by-side. We conclude that these cells with two or more attached or separate nuclei represent stages in true amitosis. In Table 1, amitosis is compared with mitosis without cytokinesis and with cell fusion, both of which can also give rise to binucleate and multinucleate cells. Mitosis without cytokinesis does not agree in any respect with the characteristics of amitosis, whereas at least a few similarities exist between cell fusion and amitosis. However, amitosis may give rise to near-haploid nuclei, which cannot be produced by mitosis or cell fusion. Simultaneously with amitosis, the nuclei grow through endocycles. In many nuclei, the heterochromatin is clearly underreplicated, while the nucleoli are numerous and often of enormous size, probably reflecting amplification of the rRNA genes.

A stochastic mechanism controls the relative replication of equally competent ribosomal RNA gene sets in individual dipteran polyploid nuclei

The endoreplication of the two nucleolar organizers (NOs) of the diploid genome has been examined in individual polyploid nuclei of the dipteran Calliphora erythrocephala. Crosses between two strains with diagnostic nontranscribed spacer polymorphisms in their rRNA genes were used to provide progeny with distinguishable NOs, and single nuclei of two highly polyploid cell types--salivary gland and nurse cells--were examined from individual F1 animals. Initially the representation of the two NOs in total polyploid tissue DNA was determined. This revealed that, although the NO regions present in one of the strains (Tom) were very similar in spacer composition, they displayed two types of behavior in the hybrids containing the single NO region typical of the second strain (Karla). In TW phenotype F1 progeny, very little replication of the Tom NO relative to the Karla NO occurred, whereas in TS phenotype progeny replication of the Tom and Karla NOs was approximately equivalent. When individual polyploid nuclei of the TS phenotype animals were examined, however, the relative replication of the Tom and Karla NOs was found not to be a fixed genetic property but to vary dramatically from cell to cell. This was true even for the nurse cell nuclei within a single ovarian follicle, which are the products of only four mitotic divisions of a single germ-line cell. These findings indicate that for NOs of similar replicative competence, a stochastic mechanism governs the relative usage of each NO for endoreplication and that the relative activity of the two NOs is not stably determined through the mitotic divisions preceding polyploidization. Stochastic selection after mitotic DNA replication could be a general phenomenon governing the relative usage (transcription) of different, but equally competent, alleles of any gene in individual cells, if the required factors are in short supply.

Homologous banding patterns in the polytene chromosomes from the larval salivary glands and ovarian nurse cells of Anopheles stephensi Liston (Culicidae)

A comparison of the banding patterns of two homologous polytene chromosome arms from the larval salivary gland and ovarian nurse cell complement of Anopheles stephensi is presented. The homologous chromosomes from the somatic larval salivary glands and germ-line derived ovarian nurse cells have essentially the same band-interband organisation. An analysis of the 3H-uridine labelling patterns of a small chromosome segment from the two tissues indicates that germ-line polytene chromosomes are not radically different from somatic polytene chromosomes in their patterns of gene expression.

Chromomeres and puffing in experimentally induced polytene chromosomes of Calliphora erythrocephala

In the most advanced types of meroistic ovaries the synthesis of RNA for the growing oocyte in each follicle is taken over by nurse cells, i.e., sister cells of the definite egg cell. In calliphora, the highly polyploid nurse cells (NC) develop a polytene karyotype under conditions of strict brother-sister inbreeding and using a controlled selection technique. A comparison of the polytene NC-chromosomes with those from the pupal bristle forming cells reveals an unexpected discrepancy: while both chromosome complements exhibit a constant banding pattern it is not possible to homologize the two tissue specific patterns by identifying homologous band-sequences. Puffing in NC likewise turns out to be unusual in its extent as well as in that is remains constant during long periods of oogenesis. In a more detailed discussion an interpretation and evaluation of these findings will be attempted.

Independent replication of the ribosomal RNA genes in the polytrophic-meroistic ovaries of Calliphora erythrocephala, Drosophila hydei, and Sarcophaga barbata

By filter saturation hybridizations the ribosomal (r)DNA contents of the ovaries Calliphora erythrocephala, Drosophila hydei, and Sarcophaga barbata have been measured in comparison to the rDNA percentages of their diploid brains. The measurements of the ovarian rDNA have been carried out on ovaries where the nurse cells in the distal egg chamber of the ovarioles had reached their highest ploidy level. The diploid rDNA content of each of the respective species was chosen as a 100% standard and the rDNA amounts of the ovaries were related to this 100% level. The results show that the ovaries of C. erythrocephala contain 135% rDNA whereas the rDNA contents in the ovaries of D. hydei and S. barbata are only 51% and 47%, respectively. Measurements carried out on isolated nuclei of the nurse cells and follicle cells in D. hydei show that both have a reduced rDNA content in comparison to the brains (45% and 70%, respectively). The data are discussed in relation to the problem of an rDNA amplification in the germ cells and an rDNA underreplication in polyploid nuclei.

Absence of ribosomal DNA amplification in the meroistic (telotrophic) ovary of the large milkweed bug Oncopeltus fasciatus (Dallas) (Hemiptera: Lygaeidae)

In the typical meroistic insect ovary, the oocyte nucleus synthesizes little if any RNA. Nurse cells or trophocytes actively synthesize ribosomes which are transported to and accumulated by the oocyte. In the telotrophic ovary a morphological separation exists, the nurse cells being localized at the apical end of each ovariole and communicating with the ooocytes via nutritive cords. In order to determine whether the genes coding for ribosomal RNA (rRNA) are amplified in the telotrophic ovary of the milkweed bug Oncopeltus fasciatus, the percentages of the genome coding for ribosomal RNA in somatic cells, spermatogenic cells, ovarian follicles, and nurse cells were compared. The oocytes and most of the nurse cells of O. fasciatus are uninucleolate. DNA hybridizing with ribosomal RNA is localized in a satellite DNA, the density of which is 1.712 g/cm(-3). The density of main-band DNA is 1.694 g/cm(-3). The ribosomal DNA satellite accounts for approximately 0.2% of the DNA in somatic and gametogenic tissues of both males and females. RNA-DNA hybridization analysis demonstrates that approximately 0.03% of the DNA in somatic tissues, testis, ovarian follicles, and isolated nurse cells hybridizes with ribosomal RNA. The fact that the percentage of DNA hybridizing with rRNA is the same in somatic and in male and female gametogenic tissues indicates that amplification of ribosomal DNA does not occur in nurse cells and that if it occurs in oocytes, it represents less than a 50-fold increase in ribosomal DNA. An increase in total genome DNA accounted by polyploidization appears to provide for increasing the amount of ribosomal DNA in the nurse cells.

[Autoradiographic studies on the oosome in the oocyte ofPimpla turionellae L. (Hymenoptera)]

During the longitudinal growth of the oocytes ofPimpla the voluminous oosome area has considerable amounts of newly synthetized proteins (= 1st stage of oosome formation). Short time incubations result in labelling the oosome area as distinctly as the plasm in the nurse cells, in spite of the considerable distance between nurse cells and oosome area. Thus there is no evidence of trophocytic proteins being transported towards the oosome. Consequently during the 1st stage of oosome formation more proteins are synthetized in the oosome area than in the ooplasm.All over the oocyte, oocytic and extra-oocytic components are adding to the propagation of ooplasm (intussusceptive growth). During the 1st stage of its formation, the oosome could possibly contain an additional system for appositional growth within the posterior part of the oocyte, or/and the proteins synthetized herein could play a part in the synthesis of fats. Further questions in connection with these preliminary hypotheses can be answered by more refined methods only. During the oocytic stages following the degeneration of the nurse cell chamber (= 2nd stage of oosome formation), the ever more ball-shaped oosome accumulates trophocytic RNA, which is present all over the rest of the egg space as well. This stable RNA is meant to serve either for the somatic embryogenesis or the germ line.Further autoradiographic observations of the ovarioles essentially confirm known facts concerning the extra-oocytic supply of the oocyte as stated for meroistic ovarioles. This includes the supply with stable and unstable RNA from the nurse cell chamber and proteins from haemolymph for vitellogenesis as well as nurse cell plasm rich in RNA, available during the degeneration of the nurse cell chamber.