Ribosomal gene amplification does not occur in the oocytes of Locusta migratoria

Preferential occupancy of R2 retroelements on the B chromosomes of the grasshopper Eyprepocnemis plorans

R2 non-LTR retrotransposons exclusively insert into the 28S rRNA genes of their host, and are expressed by co-transcription with the rDNA unit. The grasshopper Eyprepocnemis plorans contains transcribed rDNA clusters on most of its A chromosomes, as well as non-transcribed rDNA clusters on the parasitic B chromosomes found in many populations. Here the structure of the E. plorans R2 element, its abundance relative to the number of rDNA units and its retrotransposition activity were determined. Animals screened from five populations contained on average over 12,000 rDNA units on their A chromosomes, but surprisingly only about 100 R2 elements. Monitoring the patterns of R2 insertions in individuals from these populations revealed only low levels of retrotransposition. The low rates of R2 insertion observed in E. plorans differ from the high levels of R2 insertion previously observed in insect species that have many fewer rDNA units. It is proposed that high levels of R2 are strongly selected against in E. plorans, because the rDNA transcription machinery in this species is unable to differentiate between R2-inserted and uninserted units. The B chromosomes of E. plorans contain an additional 7,000 to 15,000 rDNA units, but in contrast to the A chromosomes, from 150 to over 1,500 R2 elements. The higher concentration of R2 in the inactive B chromosomes rDNA clusters suggests these chromosomes can act as a sink for R2 insertions thus further reducing the level of insertions on the A chromosomes. These studies suggest an interesting evolutionary relationship between the parasitic B chromosomes and R2 elements.

Unusually high numbers of ribosomal RNA genes in copepods (Arthropoda: Crustacea) and their relationship to genome size

We report on copy numbers of 18S ribosomal RNA genes in three species of copepods (Crustacea: Copepoda), two of which possess an unusual arrangement in which 5S genes are included within the 18S-5.8S-28S repeat unit. Slot blots of genomic and standard DNA were hybridized with an 18S rRNA gene probe constructed from one of the marine species and hybridization was quantified using chemiluminescence. Diploid 18S rRNA gene copy numbers are estimated as ca. 15 300 and 33 500 in the marine species Calanus finmarchicus (13.0 pg DNA in 2C adult nuclei) and C. glacialis (24.2 pg DNA), respectively, and ca. 840 and 730 in two freshwater populations of Mesocyclops edax (both ca. 3 pg DNA) from Virginia and Nova Scotia, respectively. The roughly proportional relationship between 2C somatic nuclear DNA contents and rRNA gene copy number in the sibling species C. finmarchicus and C. glacialis may reflect polytenic replication of entire genomes during abrupt speciation events. Copy numbers may also reflect differential losses during embryonic chromatin diminution.

Chromosomal localization and copy number of 18S + 28S ribosomal RNA genes in evolutionarily diverse mosquitoes (Diptera, Culicidae)

In situ hybridization using 3H-labeled 18S and 28S ribosomal DNA (rDNA) probes from Aedes albopictus was performed on the mitotic chromosomes of 20 species of mosquitoes belonging to 8 genera of subfamilies Culicinae and Anophelinae. In all but one species examined, the rDNA family was localized to a single chromosome per haploid genome. Aedes triseriatus was the only exception, with the rDNA cistrons present on chromosome 1 and on chromosome 3. The ribosomal RNA genes were located on chromosome 1 in Ae. albopictus, Ae. aegypti, Ae. flavopictus, Ae. seatoi, A. polynesiensis, Ae. alcasidi, Ae. annandalei, Ae. mascarensis, Ae. hendersoni, Ae. atropalpus, Ae. epactius, Culex pipiens quinquefasciatus, Wyeomyia smithii, and Sabethes cyaneus; chromosome 2 in Ae. mediovittatus and Haemagogus equinus; chromosome 3 in Armigeres subalbatus and Tripteroides bambusa; and the heteromorphic X and Y chromosomes in Anopheles quadrimaculatus. The variation in the location of ribosomal RNA genes on the different chromosomes and at different positions on the chromosome arm among the mosquito species examined is suggestive of considerable chromosome repatterning through translocations and inversions in the karyotypic evolution of mosquitoes. Dot-blot hybridization was used to estimate copy number of rRNA genes; the copy number per haploid genome ranged from 39 +/- 3.27 in Sa. cyaneus to 1023 +/- 68.14 in Ae. flavopictus.

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.

Extrachromosomal amplification of rDNA in oocytes of Hemerobius spp. (Insecta, Neuroptera)

In previtellogenic oocytes of the neuropteran, Hemerobius spp., two distinct, DNA-positive intranuclear structures have been observed. Chromosomes of meiotic prophase assemble in the center of the oocyte nucleus forming a highly polymorphic karyosphere, which persists in this position until the very late stages of vitellogenesis. The extrachromosomal DNA body, containing amplified ribosomal genes, undergoes fragmentation and dispersion in the nucleoplasm. At the onset of previtellogenic growth, transcription of extra rDNA starts, which is accompanied by the appearance of dense, granular material (multiple nucleoli). Arising nucleoli gradually fill the nucleoplasm. At the electron microscopic (EM) level two electron dense structural forms of the granular material have been described. Together with general histological and ultrastructural analysis the amplification of rDNA genes in Hemerobius spp. oocytes has been demonstrated by means of the spreading technique, which has shown that extra rDNA is organized in rings containing various numbers of active ribosomal genes. The transcription activity of amplified genes is manifested in the form of typical "Christmas tree" structures.

The 28S ribosomal RNA-encoding gene of Hymenoptera: inserted sequences in the retrotransposon-rich regions

The genomes of two parasitoid wasps, Diadromus pulchellus and Eupelmus vuilleti, and the honey bee, Apis mellifera, contain few interspersed repeated sequences corresponding to transposons (Tn). This suggests that the genomic organisation of Hymenoptera could be due to the elimination of deleterious Tn in haploid males. We have used restriction-fragment length polymorphism analysis to show that nondeleterious Tn are present in the DNA (rDNA) encoding ribosomal RNA of twelve species of Hymenoptera. Sequence analysis of the 28S rDNA type-I and type-II insertion-rich regions of 80 species showed that this region is very highly conserved (95.8%). A consensus sequence and restriction map of the rDNA region were established. These sequence data were used to develop a strategy for detecting inserted elements in the rDNA fragments containing type-I or type-II insertion sites, and this strategy was used to screen twelve hymenopteran species and four non-Hymenoptera control species. The rDNA fragments from the Hymenoptera and control species contained inserted sequences in the area where type-I and type-II elements are inserted in the 28S rDNA retrotransposon-rich region of Diptera and Lepidoptera. The hymenopteran genomes therefore appear to contain repeated elements, the mobility and nature of which remain to be determined.

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.