Effect of four doses of gene Su(UR)ES on intercalary heterochromatin in Drosophila melanogaster

Polytene chromosomes of salivary glands of various Drosophila melanogaster strains containing two doses of the normal Su(UR)ES allele have a constant set of intercalary heterochromatin (IHC) sites. Their DNA is underreplicate d, which leads to breaks and ectopic contacts emerging at a certain rate. A lmost no underreplication, breaks, or ectopic conjugation are present in mu tants lacking the normal Su(UR)ES gene product. It could be expected that a n increase in the number of the Su(UR)ES+ gene doses would, in turn, drasti cally increase ectopic conjugation and breakage. To test this hypothesis, a strain of D. melanogaster was obtained with two additional doses of Su(UR) ES+ introduced into its genome. The flies with four gene doses exhibited a considerable increase in ectopic conjugation: both the proportion of region s participating in conjugation and the number of chromosomes with numerous contact nodes were increased. As a result, chromosomes that were straight a nd well-stretched in homozygotes for the mutation in Su(UR)ES became twiste d and wound and contained many loops or nodes. Many chromosomes were wound too tightly for cytological analysis. Four doses of Su(UR)ES+ considerably increased the number of weak "points." For example, the 2R chromosome has o nly 3 weak points in strains with two doses of Su(UR)ES+ and as many as 22 weak points in the strain with four doses. In the transgenic strain, the fr equency of breaks in previously known weak points increased and new breaks appeared in 19 additional sites. All new break points appeared in the regio ns that were earlier described as regions of late replication in the S phas e.