Frequency and distribution of chiasmata in Syrian hamster spermatocytes studied by the BrdU antibody technique

Chromosome organization and chromatin modification: influence on genome function and evolution

Histone modifications of nucleosomes distinguish euchromatic from heterochromatic chromatin states, distinguish gene regulation in eukaryotes from that of prokaryotes, and appear to allow eukaryotes to focus recombination events on regions of highest gene concentrations. Four additional epigenetic mechanisms that regulate commitment of cell lineages to their differentiated states are involved in the inheritance of differentiated states, e.g., DNA methylation, RNA interference, gene repositioning between interphase compartments, and gene replication time. The number of additional mechanisms used increases with the taxon's somatic complexity. The ability of siRNA transcribed from one locus to target, in trans, RNAi-associated nucleation of heterochromatin in distal, but complementary, loci seems central to orchestration of chromatin states along chromosomes. Most genes are inactive when heterochromatic. However, genes within beta-heterochromatin actually require the heterochromatic state for their activity, a property that uniquely positions such genes as sources of siRNA to target heterochromatinization of both the source locus and distal loci. Vertebrate chromosomes are organized into permanent structures that, during S-phase, regulate simultaneous firing of replicon clusters. The late replicating clusters, seen as G-bands during metaphase and as meiotic chromomeres during meiosis, epitomize an ontological utilization of all five self-reinforcing epigenetic mechanisms to regulate the reversible chromatin state called facultative (conditional) heterochromatin. Alternating euchromatin/heterochromatin domains separated by band boundaries, and interphase repositioning of G-band genes during ontological commitment can impose constraints on both meiotic interactions and mammalian karyotype evolution.

Variation in chiasma frequency among eight accessions of Arabidopsis thaliana

Natural variation in meiotic recombination frequency in Arabidopsis thaliana has been assessed by analyzing chiasma frequency variation among a range of geographically and ecologically diverse accessions. Fifty pollen mother cells at metaphase I of meiosis were analyzed from each of eight accessions and fluorescence in situ hybridization was applied to enable identification of all 10 chromosome arms. There was no significant variation in mean chiasma frequency between plants within accessions, but there was significant variation between accessions. Further analysis confirmed this finding and identified two particular accessions, Cvi and Ler, as having chiasma frequencies significantly lower than those of the other accessions. The analysis also revealed that the pattern of chiasma distribution between arms and among chromosomes is not consistent over accessions. Further detailed analyses were conducted on each individual chromosome (1-5) in turn, revealing that chromosome 4, one of the acrocentric chromosomes, is the least variable while the other acrocentric chromosome (2) is the most variable. These findings indicate the existence of recombination regulatory elements in Arabidopsis and we conclude that it may be possible in the future to identify these elements and determine their mode of action. The practical implications of such developments are considerable.

Comparative analysis of crossover exchanges and chiasmata in Allium cepa x fistulosum after genomic in situ hybridization (GISH)

Genomic in situ hybridization (GISH) successfully differentiated homoeologous genomes in the inter-specific hybrid Allium cepa x fistulosum, thus allowing the detection of reciprocal crossover events as label exchanges in separating anaphase I chromosomes. Three of the eight chromosome pairs were positively identified by fluorescence in situ hybridization (FISH) to rDNA sequences. There was a general similarity of the GISH-based label exchange frequencies and metaphase I chiasma frequencies, but with a 20% deficit of chiasmata. Reasons for this apparent deficit are discussed. The locations of chiasmata and label exchanges are in broad agreement.

Bromodeoxyuridine: a diagnostic tool in biology and medicine, Part III. Proliferation in normal, injured and diseased tissue, growth factors, differentiation, DNA replication sites and in situ hybridization

This paper is a continuation of parts I (history, methods and cell kinetics) and II (clinical applications and carcinogenesis) published previously (Dolbeare, 1995 Histochem. J. 27, 339, 923). Incorporation of bromodeoxyuridine (BrdUrd) into DNA is used to measure proliferation in normal, diseased and injured tissue and to follow the effect of growth factors. Immunochemical detection of BrdUrd can be used to determine proliferative characteristics of differentiating tissues and to obtain birth dates for actual differentiation events. Studies are also described in which BrdUrd is used to follow the order of DNA replication in specific chromosomes, DNA replication sites in the nucleus and to monitor DNA repair. BrdUrd incorporation has been used as a tool for in situ hybridization experiments.

Do chiasmata disappear? An examination of whether closely spaced chiasmata are liable to reduction or loss

The questions of whether closely spaced crossovers could be misidentified as single chiasmata or could cancel out each other and whether exchange could occur without chiasma formation were examined in Locusta migratoria. Monochiasmate bivalents that showed differential sister chromatid staining following bromodeoxyuridine incorporation were screened for their patterns of label distribution. Half of the chiasmata were associated with an exchange between dark and lightly stained chromatids, as expected if recombination involved any two non-sister chromatids chosen at random. Two variant types of label distribution were also seen: approximately 10% of all monochiasmate bivalents had an anomalous distribution of dark and light chromatids around the chiasma, and in three of the 1365 bivalents screened a second type of anomalous pattern was observed for the first time, in which all four chromatids had a label exchange at the chiasma. The observed incidence of the latter was considerably less than expected if they originated through a four-strand double crossover with closely positioned exchanges. Analysis of label distribution patterns in monochiasmate bivalents did not produce evidence of the other configurations expected if two closely spaced exchanges could be misidentified as single chiasmata or could cancel one another out so as not to form a chiasma. We conclude that analysis of chiasma frequencies and distributions offers an accurate means of assessing recombination in organisms with favourable cytology.

Evaluation of sister-chromatid exchanges in mouse spermatogonia: a comparison between the classical fluorescence plus Giemsa staining and an immunocytochemical approach

Sister-chromatid exchanges (SCE) were analyzed in mouse spermatogonia using two different protocols for bromodeoxyuridine (BrdU) exposure and detection. With the classical approach, based on subcutaneous implantation of agar-coated BrdU tablets and fluorescence plus Giemsa (FPG) staining a satisfactory differentiation of spermatogonial metaphases was obtained with 50 or 25 mg BrdU per mouse (two or one tablets respectively). Alternatively, the immunodetection of BrdU was carried out after exposure to a very low BrdU concentration (three injections i.p., 3 mg/kg b.w. each, at 5-h intervals), and after exposure to one BrdU tablet; SCE frequencies evaluated in this way were lower than those found after classical FPG staining, even when the mice were exposed to the same BrdU concentration (one tablet, 25 mg BrdU). We concluded that the two methodologies may have different sensitivities with respect to SCE detection. In addition, when the effect of a treatment with mitomycin C was tested (1 mg/kg b.w., at time intervals ranging from 24 h to 5 days), no sister-chromatid differentiation was obtained with the multiple injection protocol, or with one BrdU tablet. By contrast, with two BrdU tablets and FPG, well differentiated metaphases were found at any time interval tested after MMC treatment, and the peak frequency of SCE (3.4 times the baseline) was observed at 55 h after treatment, as expected on the basis of cell cycle duration in spermatogonia. In summary, even though the use of medium-low concentrations of BrdU was successful in untreated animals, these protocols appeared inadequate to detect SCE induction by MMC. It is possible that, in the presence of cell cycle delay induced by the treatment, interferences with the rate of BrdU uptake produce an unsatisfactory differentiation of sister chromatids.