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

摘要

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 underreplicated, which leads to breaks and ectopic contacts emerging at a certain rate. Almost no underreplication, breaks or ectopic conjugation are present in mutants lacking the normal Su(UR)ES gene product. It could be expected that an increase in the number of the Su(UR)ES~+ would, in turn, drastically increase ectopic conjugation and breakage. To test this hypothesis, a strain of D. melanogaster was obtained with two additional doses of Su(UR)ES~+ introduce into its genome. the flies with four gene doses exhibited a considerable increase inn ectopic conjugation: both the proportion of regions participating in conjugation and the number of chromosomes with numerous contact nodes were increased. As a result, chromosomes that were straight and well-stretch n homozygotes for the mutation in Su(UR)ES became twisted 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 only 3 weak points in strains with two doses of Su(UR)ES~+ and as 22 weak points in the strain with four doses. In transgenic strain, the frequency of breaks in previously known weak points increased, and new breaks appeared in 19 additional sites. All new break points appeared in the regions that were earlier described as regions of late replication the S phase.