Scer\GAL4Sgs3.PD-mediated expression of SuURScer\UAS.cZa leads to formation of large chromosomal swellings in the intercalary heterochromatin regions, such as the 75C1-2 region.
Expression of SuURScer\UAS.cZa under the control of Scer\GAL4AB1 results in a dramatic reduction in the size of larval salivary gland nuclei compared to controls.
Expression of SuURScer\UAS.cZa under the control of Scer\GAL4AB1 results in the formation of miniature salivary glands. The nuclei of the salivary glands cells are also significantly smaller than normal. Weak points in the polytene chromosomes are still seen in these animals.
Expression of SuURScer\UAS.cZa under the control of Scer\GAL4Sgs3.PD results in the appearance of swellings in intercalary heterochromatin regions of the salivary gland polytene chromosomes.
The polytene chromosomes of the salivary glands of SuURScer\UAS.cZa; Scer\GAL4Sgs3.PD larvae grown at 25'C develop a characteristic pattern of swellings beginning at mid-third instar. By late (wandering) third instar stage, some of these swellings become very large. The size and number of swellings peaks in mid prepupal stages (4-8 hours APF). This phenotype is enhanced at 29'C, suppressed at 18'C and is reversible by switching larvae displaying these swellings at 29'C to 18'C. The number (about 190) and cytological pattern of swellings is highly reproducible, but is completely different from the pattern of chromosome puffs seen in wild-type: swellings arise in chromosome regions where puffs never appear ; the sites of their formation are normally tightly condensed, solid bands and some, such as the region of the Antp and bithorax complexes, are well known examples of silenced regions in intercalary heterochromatin. Analysis of all 190 polytene chromosome bands where swellings appear shows that they represent the regions of intercalary heterochromatin characterized by late replication or underreplication. 3Huridine labelling shows that, unlike chromosome puffs, these swellings are not sites of transcription.