Su(UR)ES: A gene suppressing DNA underreplication in intercalary and pericentric heterochromatin ofDrosophila melanogasterpolytene chromosomes

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Abstract
A genetic locus suppressing DNA underreplication in intercalary heterochromatin (IH) and pericentric heterochromatin (PH) of the polytene chromosomes ofDrosophila melanogastersalivary glands, has been described. Found in theIn(1)scV2strain, the mutation, designated asSu(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 forSu(UR)ES: (i) in the IH regions, that normally are incompletely polytenized 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 chromosome arm basements, normally β-heterochromatic, acquire a distinct banding pattern, i.e., become euchromatic by morphological criteria; (iii) an additional bulk of polytenized material arises between the arms of chromosome 3 to form a fragment with a typical banding pattern. Chromosome 2 PH reveals additional α-heterochromatin.Su(UR)ESdoes not affect the viability, fertility, or morphological characters of the imago, and has semidominant expression in the heterozygote and distinct maternal effect. The results obtained provide evidence that the processes leading to DNA underreplication in IH and PH are affected by the same genetic mechanism.