Centromeres: moving chromosomes through space and time

Sequence, Chromatin and Evolution of Satellite DNA

Satellite DNA consists of abundant tandem repeats that play important roles in cellular processes, including chromosome segregation, genome organization and chromosome end protection. Most satellite DNA repeat units are either of nucleosomal length or 5–10 bp long and occupy centromeric, pericentromeric or telomeric regions. Due to high repetitiveness, satellite DNA sequences have largely been absent from genome assemblies. Although few conserved satellite-specific sequence motifs have been identified, DNA curvature, dyad symmetries and inverted repeats are features of various satellite DNAs in several organisms. Satellite DNA sequences are either embedded in highly compact gene-poor heterochromatin or specialized chromatin that is distinct from euchromatin. Nevertheless, some satellite DNAs are transcribed into non-coding RNAs that may play important roles in satellite DNA function. Intriguingly, satellite DNAs are among the most rapidly evolving genomic elements, such that a large fraction is species-specific in most organisms. Here we describe the different classes of satellite DNA sequences, their satellite-specific chromatin features, and how these features may contribute to satellite DNA biology and evolution. We also discuss how the evolution of functional satellite DNA classes may contribute to speciation in plants and animals.

Heterochromatin variation among the populations of Mus terricolor Blyth, 1851 (Rodentia, Muridae) chromosome type I

Twenty five to thirty specimens each from ten populations of Mus terricolor of the Terai and the Dooars regions of the Darjeeling foothills of West Bengal were cytogenetically analyzed using C-banding. Results showed intra- and inter- population variation of C-band positive heterochromatin ranging from very large blocks to minute amounts or even complete absence of heterochromatin. Large blocks of centromeric C-bands were found in Bidhan Nagar, Garidhura, Malbazar, Nagrakata and Maynaguri populations in most of the autosomes, while the rest of the populations had large blocks of C-bands on a few autosomes only. Such intra- and inter- population variation may be due to accumulation of C-positive heterochromatin, which has not got fixed homogeneously in all autosome pairs. X-chromosomes invariably possess a C-banded short arm a telomeric C-band at the distal end of the long arm in all populations. The entire Y-chromosome was C-band positive with slight population differences in staining intensity. The results suggest quantitative as well as qualitative variation of C-positive heterochromatin.

Pleistocene karyotypic divergence in Hoplias malabaricus (Bloch, 1794) (Teleostei: Erythrinidae) populations in southeastern Brazil

The lacustrine system of the middle rio Doce basin is considered a paradigm of Pleistocene geomorphology. In these lakes, two Hoplias malabaricus karyomorphs (2n = 42A and 2n = 42B) live in sintopy in Carioca Lake. Cytogenetic analyses were performed on 65 specimens from 8 lakes (including Carioca Lake) to determine the distribution and relative frequency of these karyomorphs and the degree of cytogenetic divergence caused putatively by recent geographic isolation. All fish were 2n = 42B karyomorphs, except for 1 specimen from the Marola Lake, which was 2n = 42A. Among-population variation was especially high for C-banding patterns. Other characters such as X chromosome size and CMA3/DAPI also varied among populations. Our results suggested that the karyotype of H. malabaricus is able to respond rapidly to geographic isolation, and revealed that heterochromatic variation may represent the lowest hierarchical level of chromosomal evolution.

Hybrid genome evolution by transposition

Species hybridization is reviewed focusing on its role as a source of evolutionary novelties. Contrary to the view that hybrids are lineages devoid of evolutionary value, a number of case studies are given that show how hybrids are responsible for reticulate evolution that may lead to the origin of new species. Hybrid evolution is mediated by extensive genome repatterning followed by rapid stabilization and fixation of highly adapted genotypes. Some well-documented cases demonstrate that bursts of transposition follow hybridization and may contribute to the genetic instability observed after hybridization. The mechanism that triggers transposition in hybrids is largely unknown, but coupling of hybrid transposition and demethylation has been observed in mammals and plants. A natural scenario is proposed in which marginal small hybrid populations undergo transposition mediated genome reorganizations accompanied by exogenous and endogenous selection that, in concert with drift, lead to rapid fixation of high fitness hybrid genotypes. These genotypes may represent parental introgressed species or be entirely new species.

Reduced meiotic fitness in hybrids with heterozygosity for heterochromatin in the speciating Mus terricolor complex

Mus terricolor I, II and III are the three chromosomal species which differ in stable autosomal short-arm heterochromatin variations established in homozygous condition. Analysis of meiosis in the laboratory-generated F1 male hybrids from crosses (both ways) between M. terricolor I and II and between M. terricolor I and III shows high frequencies of pairing abnormalities at pachytene. The backcross (N3 generation) male hybrids between M. terricolor I and II have meiotic abnormalities as in the F1 male hybrids, though to a lesser extent. They show difference in pairing abnormalities in the different karyotypic forms; the backcross hybrids heterozygous for the heterochromatic short arms have more anomalies compared to the homokaryotypic hybrids. This suggests a negative influence of the heterochromatin heterozygosity in meiotic pairing. The results indicate a role for heterochromatin variations in the development of a reproductive barrier in the speciating M. terricolor complex.

Molecular-cytogenetic characterization of a higher plant centromere/kinetochore complex

The centromeric region of a telocentric field bean chromosome that resulted from centric fission of the metacentric satellite chromosome was microdissected. The DNA of this region was amplified and biotinylated by degenerate oligonucleotide-primed polymerase chain reaction (DOP-PCR)/linker-adapter PCR. After fluorescence in situ hybridization (FISH) the entire chromosome complement of Vicia faba was labelled by these probes except for the nucleolus organizing region (NOR) and the interstitial heterochromatin, the chromosomes of V. sativa and V. narbonensis were only slightly labelled by the same probes. Dense uniform labelling was also observed when a probe amplified from a clearly delimited microdissected centromeric region of a mutant of Tradescantia paludosa was hybridized to T. paludosa chromosomes. Even after six cycles of subtractive hybridization between DNA fragments amplified from centromeric and acentric regions no sequences specifically located at the field bean centromeres were found among the remaining DNA. A mouse antiserum was produced which detected nuclear proteins of 33 kDa and 68 kDa; these were predominantly located at V. faba kinetochores during mitotic metaphase. DNA amplified from the chromatin fraction adsorbed by this serum out of the sonicated total mitotic chromatin also did not cause specific labelling of primary constrictions. From these results we conclude: (1) either centromere-specific DNA sequences are not very conserved among higher plants and are - at least in species with large genomes - intermingled with complex dispersed repetitive sequences that prevent the purification of the former, or (2) (some of) the dispersed repeats themselves specify the primary constrictions by stereophysical parameters rather than by their base sequence.

Localization of anti-CENP antibodies and alphoid sequences in acentric heterochromatin in a breast cancer cell line

Karyotype alterations are a hallmark of cancer cells. Of particular interest to our laboratory are the inactive centromeres and blocks of heterochromatin devoid of the accompanying centromere. When purified or monospecies anticentromere proteins (CENP) antibodies or the whole serum from scleroderma (crest) patients are applied to human chromosomes, the centromere region exhibits the label. When we treated MDA 435 cells with the anti-CENP-A, anti-CENP-B, or the whole serum, the label was apparent on heterochromatin pericentric to the active and inactive centromeres. Moreover, blocks of heterochromatin not associated with any centromere also exhibited the label. Anti-CENP-C, however, is more strictly confined to the centromere in discrete dots and is not detected on any region except the sites of active centromeres. Distribution of alpha sequences also shows a pattern compatible with its distribution in the heterochromatin. Apparently, the use of anti-CENP-A and anti-CENP-B antibodies or alphoid DNA may not detect either the centromere (primary constriction) or the kinetochore; CENP-C may be an exception.