Cytogenetic and molecular aspects of position effect variegation in Drosophila melanogaster

Abstract

A chromosomal region subjected to position effect variegation was analysed for possible DNA under-replication. DNA clones from the vicinity of the euheterochromatin junction and from a distance of hundreds of kilobase pairs were used as probes. Formation of compact blocks of chromatin is regarded as a characteristic feature of position effect variegation. It was shown that in T (1;2) dorvar7 males undergoing position effect variegation clones representing the DNA nearest to the breakpoint in 2B7 hybridized normally in situ to the compact blocks, providing evidence against DNA underreplication. In females the same clones did not hybridize to the compact blocks. These variations in hybridization may be related to different degrees of compaction of chromosome regions in males and females. A correlation between the degree of underreplication and the level of cell polyteny was shown by Southern-blot hybridization of a DNA probe from the 2B region to DNA from an X/O strain carrying Dp (1;1)pn2b displaying position effect variegation and compaction in 94% of salivary gland cells. Almost complete underreplication of the DNA of this region was found in salivary gland cells (with a maximal degree of polyteny), intermediate underreplication was found in fat body cells (with an intermediate degree of polyteny), and replication was not disturbed in diploid cells of the larval cephalic complex.

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.…”

A reconsideration of the mechanism of position effect.

“…Another type of model supposes that the gene is a b sent from tissue in which no activity is seen (SCHULTZ 1936;PROCUNIER and TARTOF 1978;SPRADLING and KARPEN 1990). However, gene sequences are indeed present in affected tissues in several cases (HENIKOFF 1981;RUSHLOW et al 1984;H A W H I et al 1990;UMBETOVA et al 1991).…”

DNA Underreplication in the Majority of Nuclei in the Drosophila Melanogaster Thorax: Evidence from Suur and Flow Cytometry

“…The degree of underreplication is partially ameliorated in SuUR mutants, but not completely. This was anticipated by Umbetova et al (1991) who noted that the degree of underreplication was proportional to the degree of polyteny, with the maximum suppression in the highly polytenized salivary gland cells, intermediate underreplication in fat body cells that have an intermediate degree of polyteny, and no underreplication in diploid cells of the head.…”