Supernumerary heterochromatic segments associated with the nucleolar chromosomes of Pyrgomorpha conica (Orthoptera) contain methylated rDNA sequences

High-throughput analysis of satellite DNA in the grasshopper Pyrgomorpha conica reveals abundance of homologous and heterologous higher-order repeats

Satellite DNA (satDNA) constitutes an important fraction of repetitive DNA in eukaryotic genomes, but it is barely known in most species. The high-throughput analysis of satDNA in the grasshopper Pyrgomorpha conica revealed 87 satDNA variants grouped into 76 different families, representing 9.4% of the genome. Fluorescent in situ hybridization (FISH) analysis of the 38 most abundant satDNA families revealed four different patterns of chromosome distribution. Homology search between the 76 satDNA families showed the existence of 15 superfamilies, each including two or more families, with the most abundant superfamily representing more than 80% of all satDNA found in this species. This also revealed the presence of two types of higher-order repeats (HORs), one showing internal homologous subrepeats, as conventional HORs, and an additional type showing non-homologous internal subrepeats, the latter arising by the combination of a given satDNA family with a non-annotated sequence, or with telomeric DNA. Interestingly, the heterologous subrepeats included in these HORs showed higher divergence within the HOR than outside it, suggesting that heterologous HORs show poor homogenization, in high contrast with conventional (homologous) HORs. Finally, heterologous HORs can show high differences in divergence between their constituent subrepeats, suggesting the possibility of regional homogenization.

Comparative analysis of chromosomal localization of ribosomal and telomeric DNA markers in three species of Pyrgomorphidae grasshoppers

The karyotypes of three species of Pyrgomorphidae grasshoppers were studied: Zonocerus elegans (Thunberg, 1815), Pyrgomorpha guentheri (Burr, 1899) and Atractomorpha lata (Mochulsky, 1866). Data on karyotypes of P. guentheri and Z. elegans are reported here for the first time. All species have karyotypes consisting of 19 acrocentric chromosomes in males and 20 acrocentric chromosomes in females (2n♂=19, NF=19; 2n♀=20, NF=20) and X0/XX sex determination system. A comparative analysis of the localization of C-heterochromatin, clusters of ribosomal DNA, and telomere repeats revealed inter-species diversity in these cytogenetic markers. These differences indicate that the karyotype divergence in the species studied is not associated with structural chromosome rearrangements, but with the evolution of repeated DNA sequences.

Inactive ribosomal cistrons are spread throughout the B chromosomes of Rattus rattus (Rodentia, Muridae). Implications for their origin and evolution

In-situ hybridization with a rDNA probe has demonstrated the presence of non-transcribed ribosomal genes in the B chromosomes of the black rat Rattus rattus. To test whether methylation of ribosomal DNA present in the B chromosomes could account for their inactivation, we performed in-situ digestions and Southern analyses of DNA digested with the isoschizomers MspI and HpaII. Our results suggest that the accessory chromosomes of this species have originated from one of the smaller NOR-carrying chromosome pairs. In the course of evolution, repetitive sequences invaded this supernumerary element and its ribosomal DNA content was dispersed throughout the chromosome and inactivated by heterochromatinization and methylation.

Unusually high amount of inactive ribosomal DNA in the grasshopper Stauroderus scalaris

Fluorescence in-situ hybridization (FISH) was employed to determine the chromosomal location of the ribosomal DNA cistrons in spermatocytes of two populations of the grasshopper Stauroderus scalaris. The results showed that paracentromeric C-bands, which in this species constitute about 50% of the total chromatin, contain substantial amounts of rDNA in all chromosomes. However, silver impregnation showed the presence of a single active nucleolus organizing region (NOR) in chromosome 3 of primary spermatocytes, indicating an extremely high amount of silent rDNA across the whole genome of this species in the two geographically distant populations analysed. The significance of such an unusual phenomenon is discussed.

Fluorochromes for detection of callose in meiocytes of olive (Olea europaea L.)

The callosic wall which covers microsporocyte mother cells during meiotic division has been studied using different fluorochromes as alternatives to the widely used aniline blue. We have confirmed that both acridine orange and 4', 6' diamidino-2-phenylindole (DAPI) produce a fluorescent response to callose which is comparable in specificity and intensity to that of aniline blue; therefore, they can be used to study callose wall formation. Staining properties of these fluorochromes, as well of those of curcumin and sirofluor, reported earlier as fluorescent stains for callose, are discussed. We also discuss the efficacy of the combined use of sirofluor and DAPI to study particular aspects of the deposition of callose.

Ribosomal RNA genes specific to the B chromosomes in Brachycome dichromosomatica are not transcribed in leaf tissue

Ribosomal RNA genes are present near the end of the short arm and, to a lesser extent, near the centromere of the B chromosomes of some populations of Brachycome dichromosomatica. The internal transcribed spacer (ITS2) was amplified by PCR from total leaf DNA using primers within the conserved regions encoding the 5.8S and 25S stable rRNA species. Comparison of PCR amplified ITS2 sequences from several individual plants without B chromosomes with corresponding sequences derived from microdissected B chromosomes revealed two consistent differences between the rDNA of A and B chromosomes. One of these differences produced an SfcI restriction site that was present only in the ITS2 of the B-chromosome rDNA. Amplification by PCR of ITS2 from total genomic DNA from plants with and without B chromosomes showed an additive relationship between the amount of PCR product containing the SfcI site and the number of B chromosomes present. Quantitative analysis indicated that the proportion of total nuclear rDNA present on a single B chromosome varied between 2 and 4% in different A chromosome backgrounds. Similar experiments, with appropriate positive and negative controls, using reverse transcriptase PCR of the equivalent region within the 40S precursor rRNA, suggested that the B-chromosome rDNA was not transcribed. Similarly, PCR of reverse transcribed total RNA from plants containing B chromosomes using primers specific for the B chromosome ITS2 was unable to detect a transcript from the B chromosome.

Meiosis in holocentric chromosomes: kinetic activity is randomly restricted to the chromatid ends of sex univalents in Graphosoma italicum (Heteroptera)

We have determined the number and location of the nucleolar organizing regions in spermatocytes of Graphosoma italicum (2n = 12A + XYmale/XXfemale) by means of silver impregnation, chromomycin A3/distamycin A staining and fluorescence in situ hybridization. The identification of only one nucleolar organizing region located at one of the X chromosome ends has provided a suitable cytological marker to analyse the segregation of this univalent and that of the XY pseudobivalent during the first and second meiotic divisions respectively. Our results clearly show that at first meiotic metaphase the chromatids of the X chromosome are orientated with their long axes perpendicular to the polar axis. Although the kinetic activity is restricted to only one end in both X chromatids during the first meiotic division, both ends of the same chromatid have the same probability of showing such kinetic activity. In this sense, we also report that the chromatid segregation may be initiated either at the same sister chromatid ends or at opposite ends in each chromatid. Thus, this indicates a sex chromatid independence as regards to the chromatid segregation during the first meiotic division. Throughout the second meiotic division both ends of the X chromatid are involved with the same probability in the end-to-end association to conform the XY pseudobivalent. This also implies a random localization of the kinetic activity at the ends opposite to those involved in the end-to-end association.

Ribosomal RNA genes and the B chromosome of Brachycome dichromosomatica

Fluorescence in situ hybridization (FISH) with biotinylated rDNA revealed the presence of an rRNA gene cluster on both the A and B chromosomes of Brachycome dichromosomatica, an Australian native ephemeral plant of the arid regions of south-eastern Australia. This species contains only two pairs of A chromosomes and up to three B chromosomes. The regular attachment of the B chromosome to a nucleolus suggests that these ribosomal RNA genes are transcribed. Southern hybridization of DNA from 0B and +B plants digested with a variety of restriction enzymes indicates that the rRNA genes on the A and B chromosomes are the same in sequence and methylation status.