[A genetic factor, suppressing DNA underreplication in Drosophila melanogaster polytene chromosomes]

The Drosophila melanogaster line carrying the In(I)scv2 was found to exhibit unique cytological phenotype distinguished by the lack of "weak" points in the intercalary heterochromatin of the salivary gland polytene chromosomes, the absence of ectopic contacts between the chromosome regions, and the occurrence of additional intercalary heterochromatin in the centromeric regions. Southern blot hybridization revealed the absence of DNA underreplication in the intercalary heterochromatin region 39E carrying the histone gene cluster. This phenotype may have arisen under the influence of a genetic factor, Su(UR), which suppressed DNA underreplication in polytene chromosomes. Genetic analysis of the inheritance of the "suppression of DNA underreplication" phenotype showed that this factor was located in the third chromosome and was expressed in a semidominant manner. Discovery of Su(UR) suggested existence of common mechanisms regulating DNA underreplication in the centromeric and intercalary heterochromatin regions containing genes that were completely inactivated during ontogeny. These results confirm the assumption of common mechanisms of epigenetic repression of highly repetitive, somewhat repetitive, and unique sequences of the Drosophila genome.