Amplification of ribosomal DNA in Acheta. V. Hybridization of RNA complementary to ribosomal DNA with pachytene chromosomes

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.

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.

The three-dimensional organization of the amplified ribosomal DNA complex in Acheta

The major chromomere of pachytene chromosome 6 in the oocytes of Acheta (Orthoptera) is a region of amplification for ribosomal RNA cistrons (28 S and 18 S). This chromomere was isolated and studied with the scanning electron microscope. It has an outer shell that is smooth at early stages but divides into polygonal plates as indentations appear on its surface. These indentations increase in number as the DNA copies separate from each other and as the ribosomal RNA synthesis progresses. The vertices of the polygonal plates coincide with the positions at which the DNA copies reach the surface of the chromomere during their "puffing" process. The combined information gained from scanning and transmission electron microscopy, DNA measurements, DNA.RNA hybridization, and cytochemistry, leads to a three-dimensional picture of the organization of the amplified ribosomal DNA in its association with ribosomal RNA and protein.