Funktionsstrukturen im Oocytenkern von Locusta migratoria

Looking at Christmas trees in the nucleolus

We describe novel nucleolar structures, observed by thin section electron microscopy in oocyte nuclei of the grasshopper Locusta migratoria, which we interpret, based on morphological and compositional criteria, as rDNA transcription units. Morphologically they resemble the condensed and foreshortened "Christmas trees" seen in Miller spreads of nucleolar chromatin prepared from the same biological material. They contain DNA and rRNA as shown by immunocytochemistry and in situ hybridization and are concentrated in several intranucleolar cavities. The presumptive rDNA transcription units extend throughout the interior of these nucleolar pockets or are selectively enriched at their outermost zones in close contact with the surrounding fibrillarin-positive dense component. We suggest that the nucleolar pockets of Locusta oocytes are equivalent to the fibrillar centers of somatic nucleoli and discuss possible implications for the current understanding of the functional organization of nucleoli.

Structural organization of Acheta rDNA. Evidence for differential amplification of soma and germ-line-specific rDNA sequences

Amplification is one of the mechanisms whereby the expression of genes can be specifically reinforced. Ribosomal gene amplification in amphibian and insect oocytes is a particularly well documented case. We studied heterogeneity, amplification and size of Acheta domesticus (insects; Orthoptera) ribosomal DNA and characteristics of male and female somatic or germ line rDNAs by analysis of genomic clones from a conventional and a microclone library. The length of the Acheta rDNA repeat unit (transcription unit and non-transcribed spacer (NTS] varied from 47 x 10(3) to 60 x 10(3) base-pairs, with highest variability within the NTS region. Deletions, fragment length heterogeneity and size variability in small steps of individual NTS segments are responsible for the observed size variation. The number of rDNA repeat units per haploid genome of Acheta was determined as 190(+/- 10%). The rDNA is amplified 14(+/- 10%)-fold in the oocyte, producing about 10,000 gene copies per cell. Our results show that the amplification mechanism does not favor individual fragments within the repeat unit. Thus, it can be concluded that amplification does not change the chromosomal characteristics of the rDNA pool. Two fragments specific for oocyte rDNA suggest that the rearrangements accompanying amplification are preferentially located in one distinct EcoRI fragment. Certain regions of Acheta rDNA contain cell-type-specific fragments: it was thus possible to characterize one purely male fragment and a second one specific for male and female soma but not for germ line rDNA. We show that Acheta rDNA reveals a combination of many features reported from different organisms and novel tissue-specific alterations on an extremely large repeat unit. The tissue-specific alterations indicate sexual and soma/germ line differentiation events that are derived by as yet unknown mechanisms.

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.