SU(UR)ES - A GENE SUPPRESSING DNA UNDERREPLICATION IN INTERCALARY ANDPERICENTRIC HETEROCHROMATIN OF DROSOPHILA-MELANOGASTER POLYTENE CHROMOSOMES

A genetic locus suppressing DNA underreplication in intercalary hetero chromatin (IH) and pericentric heterochromatin (PH) of the polytene ch romosomes of Drosophila melanogaster salivary glands, has been describ ed. Found in the In(1)sc(v2) strain, the mutation, designated as Su(UR )ES, was located on chromosome 3L, at position 34.8 and cytologically, mapped to region 68A3-B4. A cytological phenotype was observed in the salivary gland chromosomes of larvae homozygous and hemizygous for Su (UR)ES: (i) in the LH regions, that normally are incompletely polyteni zed and so they often break to form ''weak points,'' underreplication is suppressed, breaks and ectopic contacts disappear; (ii) the degree of polytenization in PH grows higher. That is why the regions in chrom osome arm basements, normally beta-heterochromatic, acquire a distinct banding pattern, i.e., become euchromatic by morphological criteria; (iii) an additional bulk of polytenized material arises between the ar ms of chromosome 3 to form a fragment with a typical banding pattern. Chromosome 2 PH reveals additional alpha-heterochromatin, Su(UR)ES doe s not affect the viability, fertility, or morphological characters of the image, and has semidominant expression in the heterozygote and dis tinct maternal effect. The results obtained provide evidence that the processes leading to DNA underreplication in III and PH are affected b y the same genetic mechanism.