Position-effect Variegation in Drosophila: Recent Progress

Abstract

Chromosomes of higher eukaryotes are cytologically differentiated into euchromatin, which decondenses at telophase, and heterochromatin, which remains visibly condensed (Heitz 1929). In Drosophila, heterochromatin forms at blocks of repetitive DNA, both simple sequence repeats (Mayfield and Ellison 1975; Lohe et al. 1993) and moderately repetitive sequences apparently derived from transposable elements (Carmena and Gonzalez 1995; Le et al. 1995; Pimpinelli et al. 1995). When euchromatin and heterochromatin are juxtaposed, mosaic gene silencing can occur, a phenomenon known as position-effect variegation (PEV). Although PEV has been familiar to geneticists for decades, mechanistic questions have only recently been addressed: What differences between euchromatin and heterochromatin are responsible for silencing? How do trans-acting factors participate in silencing? How are genes silenced from a distance? Do repeats play a direct role? Answering these questions is becoming increasingly important, given the mounting evidence that some of the same questions arise for PEV-like phenomena in a wide variety of systems.

The focus of this brief critical review is recent work that seems most relevant to epigenetic phenomena, especially those involving repeats. Other aspects of PEV are discussed in excellent comprehensive reviews by Lewis (1950), Spofford (1976), and Weiler and Wakimoto (1995).

DIFFERENT TYPES OF PEV
Genes are subject to PEV in different situations that represent different vantage points from which interactions between euchromatin and heterochromatin can be observed. The most common situation is when a euchromatic gene is moved next to heterochromatin by chromosomal rearrangement, i.e., classic PEV. Figure 1a illustrates classic PEV on the famous...