Genetics, Vol. 154, 1611-1626, April 2000, Copyright © 2000
The Size and Internal Structure of a Heterochromatic Block Determine Its Ability to Induce Position Effect Variegation in Drosophila melanogaster
Eugene V. Tolchkova,
Vanya I. Rashevaa,
Silvia Bonaccorsib,
Thomas Westphala, and
Vladimir A. Gvozdeva
a Department of Molecular Genetics of Animals, Institute of Molecular Genetics, Russian Academy of Sciences, Moscow 123182, Russia
b Centro di Genetica Evoluzionistica del CNR, Universitá di Roma "La Sapienza," Rome, Italy I-00185
Corresponding author:
Eugene V. Tolchkov, Institute of Molecular Genetics, Kurchatov Sq. 2, Moscow 123182, Russia., teugene{at}img.ras.ru (E-mail)
Communicating editor: S. HENIKOFF
In the In(1LR)pn2a rearrangement, the 1A-2E euchromatic segment is transposed to the vicinity of X heterochromatin (Xh), resulting in position effect variegation (PEV) of the genes in the 2BE region. Practically the whole X-linked heterochromatin is situated adjacent to variegated euchromatic genes. Secondary rearrangements showing weakening or reversion of PEV were obtained by irradiation of the In(1LR)pn2a. These rearrangements demonstrate a positive correlation between the strength of PEV of the wapl locus and the sizes of the adjacent heterochromatic blocks carrying the centromere. The smallest PEV-inducing fragment consists of a block corresponding to ~10% of Xh and containing the entire XR, the centromere, and a very proximal portion of XL heterochromatin. Heterochromatic blocks retaining the entire XR near the 2E region, but lacking the centromere, show no PEV. Reversion of PEV was also observed as a result of an internal rearrangement of the Xh blocks where the centromere is moved away from the eu-heterochromatin boundary but the amount of X heterochromatin remaining adjacent to 2E is unchanged. We propose a primary role of the X pericentromeric region in PEV induction and an enhancing effect of the other blocks, positively correlated with their size.
