Su(UR)ES: a gene suppressing DNA underreplication in intercalary and pericentric heterochromatin of Drosophila melanogaster polytene chromosomes

Abstract

A genetic locus suppressing DNA underreplication in intercalary heterochromatin (IH) and pericentric heterochromatin (PH) of the polytene chromosomes of Drosophila melanogaster salivary glands, has been described. Found in the In(1)scV2 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 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 beta-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 alpha-heterochromatin. Su(UR)ES does 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.

Figure 1

Morphology of pericentric regions of chromosome 2 in Oregon-R (a) and Su(UR)ES(b). Brackets in a indicate weak points in regions 39DE and 42B of normal chromosomes. In mutants these regions are seen as large dense bands. The segment between 40A and 41D is diffuse β-heterochromatin in Oregon-R chromosomes (a), while the mutants display distinct bands in 40B-F and 41A-C (b).

Figure 2

Frequencies of ectopic conjugation of IH regions (a) and telomere associations (b) in sc^V2 (1, ▪), sc⁴ (2, ░⃞) and sc^L8 (3, □) strains. Abscissa: IH regions (a) telomere regions (b). Ordinates: frequency of participation of the region in ectopic pairing (%).

Figure 3

Morphology of pericentric heterochromatin of chromosome 3 in Oregon-R (a) and Su(UR)ES strains (b). Su(UR)ES chromosome carries three subblock banded fragments between regions 80C and 81F that replace diffuse β-heterochromatin of the normal chromosome. The subblocks are denoted by brackets 1, 2, and 3.

Figure 4

Additional dense material in centromeric regions of polytene chromosomes. Salivary glands were stained with acetic orsein and squashed in 55% lactic acid, then kept at room temperature. Stain wears off after treatment, only the densest structures remain stained, which are in a very small sector of the chromocenter in Oregon-R (arrow in a). This heterochromatic material is much more abundant in Su(UR)ES strain: region 20BC of the X chromosome (b); large blocks of the pericentric region of chromosome 2 (c); the dense bands of “Plato Atlantis” (d); 4, chromosome 4.

Figure 5

Southern blot analysis of DNA in wild-type (Oregon-R) and Su(UR)ES strains. DNA from adult heads and larval salivary glands (SG) was extracted as described in Materials and Methods and hydrolized by HindIII. The same blot was hybridized with probes containing 359-bp satellite (a), histone gene (b), and DNA of 28S rRNA gene (c). Two probes from the 3′Ubx and 5′Ubx exons were hybridized to other blot. DNA of the rosy gene was used as a standard (d).