A deficiency screen for dominant suppressors of telomeric silencing in Drosophila

Abstract

Heterochromatin is a specialized chromatin structure in chromosomal regions associated with repeated DNA sequences and low concentrations of genes. Formation of heterochromatin is determined in large part by enzymes that modify histones and structural proteins that bind to these modified histones in a cooperative fashion. In Drosophila, mutations in genes that encode heterochromatic proteins are often dominant and increase expression of genes placed into heterochromatic positions. To find components of telomeric heterochromatin in Drosophila, we screened a collection of autosomal deficiencies for dominant suppressors of silencing of a transgene at the telomere of chromosome 2L. While many deficiency chromosomes are associated with dominant suppressors, in the cases tested on chromosome 2 the suppressor mapped to the 2L telomere, rather than the deficiency. We infer that background effects may hamper the search for genes that play a role in telomeric heterochromatin formation and that either very few genes participate in this pathway or mutations in these genes are not dominant suppressors of telomeric position effect. The data also suggest that the 2L telomere region plays a major role in telomeric silencing.

Figure 1.—

Eye color phenotypes of suppressors of telomeric silencing. All photographs show hemizygous P{w^var}KR3-2. Equivocal suppressors, labeled “+” in Table 1, have a phenotype that overlaps the nonsuppressed phenotype and are not shown here. (A) The absence of a suppressor of TPE, labeled “−” in Table 1. (B) A weak suppressor of TPE, labeled ++ in Table 1. (C) A moderate suppressor of TPE, labeled +++. (D) A strong suppressor of TPE, labeled ++++.

Figure 2.—

Identification of 2L TAS on deficiency chromosomes. Arrows point to TAS on the balancer chromosomes. Arrowheads point to the site where TAS should be on the deficiency chromosomes. (A) Df 20/Gla. (B) Df 157/SM1. Deficiency designation is as in Table 1.

Figure 3.—

Deficiencies for chromosome 2 and their ability to suppress telomeric silencing. Open rectangles below the polytene chromosome represent deficiencies without the ability to suppress TPE. Solid rectangles show deficiencies on chromosomes that suppress TPE. The minimum number of nonsuppressing deficiencies covering the full genetic distance is shown; i.e., redundant deficiencies are not shown. False positive results are not shown. Symbols shown above the chromosome are genes whose mutations exhibit effects on gene expression or genes that are homologous of yeast genes that encode telomeric proteins. Numbers above the chromosome indicate sites identified by the deficiencies as potential sites of suppressors of TPE.

Figure 4.—

Deficiencies for chromosome 3 and their ability to suppress telomeric silencing. Symbols are the same as for Figure 3.