The Size and Internal Structure of a Heterochromatic Block Determine Its Ability to Induce Position Effect Variegation in Drosophila melanogaster

The Size and Internal Structure of a Heterochromatic Block Determine Its Ability to Induce Position Effect Variegation in Drosophila melanogaster

Eugene V. Tolchkov^a, Vanya I. Rasheva^a, Silvia Bonaccorsi^b, Thomas Westphal^a, and Vladimir A. Gvozdev^a
^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 segmentis transposed to the vicinity of X heterochromatin (Xh), resultingin position effect variegation (PEV) of the genes in the 2BEregion. Practically the whole X-linked heterochromatin is situatedadjacent to variegated euchromatic genes. Secondary rearrangementsshowing weakening or reversion of PEV were obtained by irradiationof the In(1LR)pn2a. These rearrangements demonstrate a positivecorrelation between the strength of PEV of the wapl locus andthe sizes of the adjacent heterochromatic blocks carrying thecentromere. The smallest PEV-inducing fragment consists of ablock corresponding to ~10% of Xh and containing the entireXR, 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 wasalso observed as a result of an internal rearrangement of theXh blocks where the centromere is moved away from the eu-heterochromatinboundary but the amount of X heterochromatin remaining adjacentto 2E is unchanged. We propose a primary role of the X pericentromericregion in PEV induction and an enhancing effect of the otherblocks, positively correlated with their size.

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