Transmission analysis of B chromosomes in Rattus rattus from Northern Africa.

Traditionally, B chromosomes have been classified as parasitic or heterotic, depending of whether or not they show selfish behaviour. Nevertheless, experimental evidence has been found supporting the idea that supernumerary chromosomes may evolve from parasitism to neutrality. In this work, B chromosome transmission in Rattus rattus has been analysed by performing several crosses between individuals carrying different numbers of supernumerary chromosomes. Our results demonstrated a Mendelian transmission rate through males, but slight accumulation of the Bs through females. This parasitic behaviour is shared in populations as distant as Asia and Africa, and even in a related species in Australia, suggesting the possibility of an ancient origin of these supernumerary chromosomes.

Silent ribosomal cistrons are located at the pairing segment of the postreductional sex chromosomes of Apodemus sylvaticus (Rodentia, Muridae).

We analysed the karyotype of the rodent species Apodemus sylvaticus by G- and C-banding, Ag-NOR-staining and in situ hybridization, with special attention to the sex chromosomes. NOR-bearing chromosome pairs were identified with simultaneous staining of G bands and NORs. In situ hybridization with an rDNA probe revealed the presence of silent ribosomal cistrons in both sex chromosomes. Studies of meiosis demonstrated that these inactive ribosomal cistrons are located in the pairing segment, which occupies the proximal portion of the telocentric sex chromosomes, and may thus be involved in postreduction caused by an obligatory chiasma in this position.

Sex-chromosome pairing through heterochromatin in the African rodent Lemniscomys barbarus (Rodentia, Muridae). A synaptonemal complex study.

Giemsa-stained spread preparations and microspread preparations of Lemniscomys barbarus spermatocytes were made to investigate the meiotic behaviour of the peculiar sex chromosomes of this species. A typical sex vesicle is absent, as the X and Y chromosomes appear unfolded at zygotene and pachytene. In most cells, the sex chromosomes are associated at distal segments at metaphase I, probably as a consequence of a distal chiasma. The pairing segment is located in the heterochromatic regions of both sex chromosomes, which include silent ribosomal cistrons interspersed throughout the heterochromatin. This may suggest a possible involvement of ribosomal genes in both pairing and recombination processes. X-Y pairing proceeds beyond the pseudoautosomal region, thus involving heterologous segments of the differential regions, a fact that is clearly evident at the Y centromeric region.

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.

Isolation of a species-specific satellite DNA with a novel CENP-B-like box from the North African rodent Lemniscomys barbarus.

A species-specific satellite DNA (Lb-MspISAT) was isolated from the North African rodent Lemniscomys barbarus. This DNA is highly homogeneous in the sequence of different repeats and shows no internal repetitions. Filter and in situ hybridizations demonstrated that it is tandemly repeated at the centromeres of all chromosomes of the complement. A 19-bp CENP-B-like motif was found in Lb-MspISAT which conserves 12 of the 17-bp of the human CENP-B box, but only 5 of the 9-bp of the canonical sequence that is necessary to bind the CENP-B protein. Compared with the human CENP-B box, nucleotide substitutions and insertions increase the palindromic structure of this motif. The possibilities that it may be involved in centromeric function or in homogenization of the Lb-MspISAT sequence are discussed.

Recent evolution of NOR-bearing and sex chromosomes of the North African rodent Lemniscomys barbarus.

The karyotype and meiotic phases of Lemniscomys barbarus from Morocco were extensively studied with G- and C-banding, Ag-NOR and fluorochrome staining, in situ hybridization with an rDNA probe, and synaptonemal complex analysis. Comparison of the data with those previously published for an Algerian specimen revealed in the Moroccan specimens the presence of large heterochromatic segments in the sex chromosomes, a new nucleolar organizer at chromosome pair 1, and silent NORs on both sex chromosomes - features that are not present in the Algerian specimen. These findings demonstrate that during the very recent evolution of the karyotype of this species a new NOR was acquired by pair 1, possibly by amplification of ribosomal genes after a translocation event. This new NOR changed the preference of activation of the NORs in these individuals and became the preferentially activated NOR. Another autosome-sex chromosome translocation led to the presence of NORs on the sex chromosomes, which were then inactivated by the invasion of repetitive sequences. These silent NORs may be involved in the pairing of the two sex chromosomes.

Cytogenetic peculiarities in the Algerian hedgehog: silver stains not only NORs but also heterochromatic blocks.

Hedgehogs belong to one of the several mammalian taxa in which karyotype differences are based on variations in heterochromatin content. Furthermore, the number and location of nucleolar organizer regions (NORs) can also vary widely. In the present study these cytogenetic features were investigated in the Algerian hedgehog, Erinaceus (Aethechinus) algirus. The heterochromatin and NOR distribution patterns in the karyotype of this species are new among hedgehogs, whereas the euchromatic regions, including their G-band pattern, are similar to those reported by others. In addition, silver staining revealed a cytogenetic feature exclusive to the heterochromatic blocks of E. algirus: their silver staining with standard cytogenetic procedures. Because no similar phenomenon has been described previously in a mammalian species, several hypotheses about the significance and specificity of silver staining to NOR sites are discussed. Finally, the existence of different types of heterochromatin in the species analysed here, lead us to propose that what hedgehogs have inherited from their common ancestor is a mechanism which permits the accumulation of heterochromatin on specific chromosomes, rather than the heterochromatin itself.