Position-effect variegation and intercalary heterochromatin: a comparative study

Abstract

The behaviour of IH (intercalary heterochromatin) regions of Drosophila melanogaster polytene chromosomes was compared with that of euchromatin condensed as a result of position-effect variegation. Normally replicating regions, when subject to such an effect, were found to become among the last regions in the genome to replicate. It is shown that the factors which enhance position effect (low temperature, the removal of the Y chromosome, genetic enhancers of position effect) increase the weak point frequency in the IH, i.e. enhance DNA underreplication in these regions. We suggest that the similarity in the properties of IH, CH (centromeric heterochromatin) and the dense blocks induced by position effect is due to strong genetic inactivation and supercondensation caused by specific proteins in early development. The primary DNA structure is not likely to play a key role in this process.

Nurse cell polytene chromosomes ofDrosophila melanogaster otu mutants: Morphological changes accompanying interallelic complementation and position effect variegation

“…The Dp(1;1)pn2b duplication contains the 1A-2A1-2 euchromatic region attached to the pericentric heterochromatin of the X chromosome [Zhimulev et al, 1989;Lindsley and Zimm, 1992]. In our study this duplication was attached by crossing-over to the X chromosome bearing y, w, sn 3 , and otu 11 .…”

Su(UR)ES: A gene suppressing DNA underreplication in intercalary and pericentric heterochromatin ofDrosophila melanogasterpolytene chromosomes

“…The concept has two groups of facts to rely on: (i) Cytogenetic observations of coordinated changes in IH and PH at a varying temperature and amount of heterochromatin in the nucleus (14) and, additionally, data on a parallel reduction in DNA underreplication in IH and PH of polytene chromosomes of pseudonurse cells of the otu mutants and decreasing of position effect variegation (25,26).…”

“…Interestingly, the frequency of breaks in IH and, therefore, the degree of underreplication lowers under conditions when position effect variegation is suppressed-at higher temperature and with additional Y chromosome (14). As has been shown recently, such conditions are favorable for the ''improvement'' of the banding pattern in the eu-heterochromatin junction regions in the basements of chromosome arms so that the regions reveal a banding arrangement usually looking like ␤-heterochromatin (15).…”

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

“…The observations of Weiler and Wakimoto (1995) confi rmed that genes in ␤ -heterochromatin require the heterochromatic state for their activity. After the heterochromatic state spreads in cis into euchromatin, the effect on euchromatic loci can be observed in polytene chromosomes as a progressive loosening of the otherwise sharp banded structure accompanied by a shift to a later replication time of the polytene bands containing the variegating euchromatic loci (Wargent and Hartmann-Goldstein, 1976;Zhimulev et al, 1989;Ananiev and Gvozdev, 2003), by puff shrinkage and by inhibition of transcription in polytene puffs (Henikoff, 1981). The direction of spreading either heterochromatin into euchromatin or euchromatin into heterochromatin and the extent of the spreading are reciprocally dependent on temperature during development, the number of heterochromatic Y chromosomes added to the genome, and on suppressor of variegation (SUV) alleles in the genetic background (Gatti and Pimpinelli, 1992).…”

“…Flies have successfully introduced small regions of intercalary heterochromatin into their chromosome arms (Zhimulev et al, 1989) (2004) designed Drosophila experiments in which either a white gene or a seven-copy tandem repeat of white was inserted into a euchromatic locus at polytene band 50C. The single copy transgene was expressed while the tandem repeat was totally or partially (variegated) repressed.…”