Polytene chromosomes

Genome reorganization from polyteny to polyploidy in the nurse cells found in onion fly (Delia antiqua) and cabbage root fly (Delia radicum) ovaries (Diptera, Anthomyiidae)

The material required to ensure successful embryogenesis in the onion fly (Delia antiqua) and the cabbage root fly (Delia radicum) (Diptera, Anthomyiidae) is supplied by 15 nurse cells, while the oocyte chromosomes enter a quiescent stage during prophase I of meiosis. This level of transcription is achieved by the polyploidization of the nurse cell DNA. Elongate polytene chromosomes form in both species, but lack the banding and conspicuous puffing commonly seen in other dipteran tissues. The polytene chromosomes contract until they finally appear as small, densely staining spheres. These fragment into large numbers of endochromosomes that are much smaller than their mitotic counterparts, which then despiralize, resulting in the flocculate appearance of the nurse cell nucleus. Photodensitometry revealed a gradient of DNA values between nurse cells near the oocytes and those further away. Final DNA values 1000 times the haploid level were recorded in the nurse cell nearest to the oocyte compared with 336 times the C-value in the most distal cell. At lower temperatures (< 10 degrees C), the polytene chromosomes become banded and longer. None of the onion flies kept in these conditions produced viable eggs, though there was some reproductive success among the cabbage root flies.

The promoter of DNA puff gene II/9-1 of Sciara coprophila is inducible by ecdysone in late prepupal salivary glands of Drosophila melanogaster

DNA puffs occur in Sciarid salivary gland chromosomes; they are sites of DNA amplification and intense transcription and they appear to encode secreted structural proteins needed for pupation. In this report we have used P-element transformation of Drosophila to study regulation of a Sciara DNA puff gene. We found that a 718-bp promoter fragment of DNA puff gene II/9-1 from Sciara coprophila directs expression of the bacterial reporter gene CAT in late prepupal salivary glands of transgenic Drosophila melanogaster. The identical tissue and analogous stage specificity indicate that some aspects of the ecdysone response are evolutionarily conserved between Drosophila and Sciara. When transgenic salivary glands are cultured in vitro, CAT activity is rapidly induced by ecdysone, suggesting direct control of gene expression by the ecdysone receptor. Putative stage-specific factors limit expression of the chimeric Sciara-CAT gene in transgenic Drosophila to late prepupae but not to third instar larvae when ecdysone titers are also high.

Regulatory sequences of the Sgs-4 gene of Drosophila melanogaster analysed by P element-mediated transformation

The X chromosomally located allele Sgs-4c for a larval secretion protein of Drosophila melanogaster is normally expressed in female larvae of the strain Oregon R and is hyperexpressed in male larvae exhibiting dosage compensation; the allele Sgs-4d in the strain Samarkand is weakly expressed and is not hyperexpressed in male larvae showing a dosage effect. P element-mediated transformation of upstream DNA sequences from both alleles combined with Sgs-4d coding and downstream sequences was performed to localize sequences which are responsible for the level of gene expression and for hyperexpression of Sgs-4c in male larvae. Our results demonstrate that weak expression and dosage effect are inherited with the upstream region from -1 to -838. This Samarkand fragment differs from the homologous Oregon R region only by a C to T transition at -344 which lies within an assumed binding sequence for the ecdysone receptor complex of dyad base symmetry. Replacing the Samarkand upstream region from -1 to -838 by the Oregon R region restores normal Sgs-4 expression and dosage compensation. Hyperexpression in male larvae displays high sensitivity to position effect and is nearly completely inhibited in one transformed line under heterozygous conditions. The integration of an Sgs-4d transposon into a weak spot of polytene chromosome 2L results in a decrease in gene expression. The GTT- and GT-rich regions at -1.2 and -2.0 kb do not obviously influence Sgs-4 expression but possibly play a role in induction of stage-specific chromosome puffing.

Upstream sequences of dosage-compensated and non-compensated alleles of the larval secretion protein gene Sgs-4 in Drosophila

The X chromosomal gene Sgs-4 coding for a larval secretion protein of Drosophila melanogaster is expressed stage and tissue specifically and is hyperexpressed in male larvae of most Drosophila stocks which show dosage compensation. We analysed three Sgs-4 alleles which differ in the size of their coding region, in the intensity of their expression and in the level of dosage compensation. The size and amount of Sgs-4 proteins directly reflect those of RNAs. Different RNA sizes result from different numbers of 21 bp repeats within the structural genes. We sequenced 2.8 kb of DNA upstream of the transcription initiation site of the three alleles. Sequences known to be essential for correct gene expression were located. The only difference within DNA sequences from -1 to -1200 between two alleles with different degrees of expression and differing in dosage compensation is a C to T transition at -344 within a supposed consensus sequence for ecdysone receptor complex binding (ECR). This mutation is partly located within a region of dyad symmetry. Alleles with identical expression show identical mutations within a GTT-rich region at -1.2 kb, but differ within a GT-rich region at -2.0 kb. A polyadenylated 0.5 kb RNA was found to be transcribed from the GTT-rich region of the strand opposite to that of Sgs-4. The corresponding gene is active only in larval salivary glands and, therefore, is named gland specific gene, gsg.