Chromatin Structure and “DNA Sequence View”: The Role of Satellite DNA in Ectopic Pairing of the Drosophila X Polytene Chromosome

Chromatin 3D structure plays a crucial role in regulation of gene activity. Previous studies have envisioned spatial contact formations between chromatin domains with different epigenetic properties, protein compositions and transcription activity. This leaves specific DNA sequences that affect chromosome interactions. The Drosophila melanogaster polytene chromosomes are involved in non-allelic ectopic pairing. The mutant strain agnts3, a Drosophila model for Williams–Beuren syndrome, has an increased frequency of ectopic contacts (FEC) compared to the wild-type strain Canton-S (CS). Ectopic pairing can be mediated by some specific DNA sequences. In this study, using our Homology Segment Analysis software, we estimated the correlation between FEC and frequency of short matching DNA fragments (FMF) for all sections of the X chromosome of Drosophila CS and agnts3 strains. With fragment lengths of 50 nucleotides (nt), CS showed a specific FEC–FMF correlation for 20% of the sections involved in ectopic contacts. The correlation was unspecific in agnts3, which may indicate the alternative epigenetic mechanisms affecting FEC in the mutant strain. Most of the fragments that specifically contributed to FMF were related to 1.688 or 372-bp middle repeats. Thus, middle repetitive DNA may serve as an organizer of ectopic pairing.

Comparative study of Insecticide potential of Methyl parathion and diazinon in Drosophila melanogaster

Several reports indicate that many chemical pollutants which are widely spread in the environment, such as insecticide, pesticide and drugs are mutagenic in various test system. These findings reflect an urgent need to draw more attention to the possible genetic hazards of such pollutants to public health. The present investigation of working hypothesis deals with the effects of two insecticides viz. Methyl Parathion (MeP) and Diazinon (DZ) on non-target organism Drosophila melanogaster. We have carried out the chromosomal aberration test with various concentration for insecticides (MeP and DZ) which infer properties like ectopic pairing, inversion loop, puffing, fusion, and asynapsis. Chromosomal aberration result shows significant effects with DZ even in less concentration (0.02ppm) when compared with MeP (0.2ppm). The present study proposes that diazinon is more cytotoxic than methyl parathion in Drosophila melanogaster.

Evaluation of the mutagenic potential of propoxur and methyl parathion using polytene chromosomes of Anopheles stephensi

The mutagenicity of two pesticides, propoxur and methyl parathion was evaluated by using polytene chromosomes of Anopheles stephensi. The results were based on the frequency of various structural aberrations encountered in the polytene chromosomes of the larvae treated with LC20 of propoxur and methyl parathion separately. Propoxur induced a total of 67 aberrations as against 15 in the controls while methyl parathion induced 53 aberrations as against 13 in the controls. These aberrations were dominated by inversions, translocations, deletions, ectopic pairing, asynapses, breaks, fusions and induced puffing. The frequency of propoxur induced aberrations was highest in chromosome 3R followed by 2R, 3L, 2L and X-chromosome. Methyl parathion induced highest number of aberrations in 2R followed by 2L, 3R, 3L and X-chromosomes. This study suggests that larval polytene chromosomes are sensitive indicators of pesticide genotoxicity in which both propoxur and methyl parathion are significantly chromotoxic for the genome of a mosquito taken as an experimental insect.

Identical megabase transgenes on mouse chromosomes 3 and 4 do not promote ectopic pairing or synapsis at meiosis

To investigate ectopic interactions at the chromatin level, we examined the meiotic organization of 1–2 mb phage λ transgenes on mouse chromosomes 3 and 4 by fluorescence in situ hybridization in combination with immunocytology of meiotic chromosomes. At early meiotic prophase, the transgenes are sufficiently dispersed in the nuclear volume to permit potential DNA–DNA interactions, but no synaptonemal complexes form between the sites of transgenes residing on different chromosomes. At later stages, when the chromatin is more condensed, the transgenes on different chromosomes are not preferentially associated as they are when they are on the same chromosome. At diplotene and metaphase I, no formations were observed that could be interpreted as reciprocal crossovers or chiasmata between the transgenes located on chromosomes 3 and 4. It appears that in normal fertile mice, a 1- to 2-mb homology is insufficient to initiate synapsis between nonhomologs, and it is concluded that homology is assessed within the broader context of the chromosome to initiate synapsis at meiotic prophase.Key words: transgenes, ectopic pairing, meiosis, synaptonemal complex, immunocytology, FISH.