Cytogenetic and molecular aspects of position effect variegation in Drosophila melanogaster

Abstract

The peculiarities of compact blocks appearing as a consequence of position effect variegation were studied in male polytene chromosomes. In T(1;2)dorvar7/Y males the frequency of nuclei with a block in the 2B region was lower at all temperature and the chromosome region involved in compaction was shorter than in T(1;2)dorvar7/FM6 females. The fraction of nuclei with blocks was considerably increased in dorvar7/0 males, especially at 18 degrees C when the viability of these males is sharply reduced. The following features distinguish the blocks in males from those in females: (i) compaction of the 2B region in the males results in genetic inactivation only to a very small extent; (ii) the structure of the blocks in males is diffuse; and (iii) the male blocks still maintain some transcriptional activity as indicated by 3H-uridine incorporation. The temperature-sensitive period of both block formation and genetic inactivation was found to be during the first 3 h of embryonic development.

Position-Effect Variegation, Heterochromatin Formation, and Gene Silencing in Drosophila

“…The consequence is an altered packaging with concomitant silencing of genes normally packaged in a euchromatic form. Visual inspection of the polytene chromosomes of larvae carrying such a rearrangement shows that the region carrying the reporter gene is packaged in a dense block of heterochromatin only in the cells in which the gene is inactive (Zhimulev et al 1986). Patterns of variegated expression, observed as a consequence of rearrangement of white, vary in the number of pigmented cells, the size of the pigmented patches, and the level of pigment in the two different cell types observed, one with a high level of expression, and one with a low level or no expression (Fig.…”

Repetitive Sequences in Plant Nuclear DNA: Types, Distribution, Evolution and Function

“…Heterochromatin has been associated with several functions ranging from regulating gene expression to protecting chromosomal integrity. Heterochromatin can incur different levels of expression of adjacent genes during inheritance, because of either a position effect or juxta-position of heterochromatin and highly active genes, as observed in Drosophila and Saccharomyces [49–52] .…”

Genomic Organization of H2Av Containing Nucleosomes in Drosophila Heterochromatin

“…Most genes reside in euchromatin, and are thought to exist in a more open state than those found in compact heterochromatin. Genes that are ectopically placed within heterochromatin tend to be repressed [4], [5], [6], [7], which reflects a possible spreading of the heterochromatin structure into such genes. Heterochromatic regions are gene poor.…”