Ultrastructure of regions containing homologous loci in polytene chromosomes of Drosophila melanogaster and Drosophila subobscura.

We have used a new approach involving in situ hybridisation and electron microscopy to establish ultrastructural homologies between polytene chromosome regions of Drosophila melanogaster and Drosophila subobscura. Twelve probes were chosen to cover all the chromosomal elements: the myospheroid gene, the collagen type IV gene, the collagen-like gene, the w26 homeobox gene, the beta3 tubulin gene, the kinesin heavy chain gene, the tryptophan hydrolase gene, the Hsp82, Hsp22-26 and Hsp23-28, Hsp68, Hsp70 genes and the beta unit of the F0-F1 ATPase gene. Most of these loci were previously undescribed in D. subobscura and imprecisely located in D. melanogaster. We have demonstrated here, by an ultrastructural analysis of each chromosomal region, that homologous genetic loci tend to show a similar ultrastructure in the two species. With a few exceptions, the structural homology extends to the chromosomal regions surrounding the loci. In some cases, however, no structurally recognisable homology can be seen either in the locus or in its flanking regions.

Electron micrograph map of the Drosophila melanogaster polytene chromosome 3R divisions 91 through 100.

The banding pattern of the distal half of the polytene salivary gland 3R chromosome of Drosophila melanogaster was studied by means of the thin section electron microscopy. Bands were identified according to the revised light microscopic map of Bridges. Bridges' map contains 332 single bands and 137 double bands within the region 91 through 100. This makes a total of 606 bands when the doublets are counted as two bands each, but 469 bands when the doublets are counted as one band. In the electron micrographs we found a total of 443 bands within this region. 109 Bridges' singlets were easily detected in almost all thin sections, while 144 mainly faint bands could be seen only in some micrographs. 79 Bridges' single bands and one doublet (94D7-8) could not be found. 42 Bridges' doublets were made up of two separate bands each, 87 Bridges' doublets looked single, and three pairs of Bridges' doublets formed dark complexes in the thin sections. The telomere region with the most distal band 100F4-5 was gray. A total of 15 new bands, which are not drawn on Bridges' map, were detected. Most of the new bands were in the divisions 96 and 99.

Electron micrograph map of the Drosophila melanogaster polytene chromosome 3R divisions 81 through 90.

The banding pattern of the proximal half of the polytene salivary gland 3R chromosome of Drosophila melanogaster was studied with thin section electron microscopy. Bands were identified according to Bridges' revised light microscopic map, which contains 330 single and 121 double bands within the regions 81 through 90. We found a total of 447 bands in this region. 97 Bridges' single bands were easily detected in almost all thin sections, while 177 faint bands could be seen only in some micrographs. 56 Bridges' single bands could not be found. 32 Bridges' doublets were made up of two separate bands each in thin sections. The other 89 Bridges' doublets seemed to be either single bands or remained obscure. A total of 20 small new bands, which were not drawn on Bridges' map, were detected.

Electron microscopic analysis of the banding pattern in the salivary gland chromosomes of Drosophila melanogaster. Divisions 11 through 20 of X.

The banding pattern of the proximal half of the polytene salivary gland X chromosome of Drosophila melanogaster was studied using thin section electron microscopy. The bands were identified according to Bridges' revised light microscopic map. On Bridges' map, the divisions 11 to 20 contain 112 single bands and as many as 181 double bands. A majority of Bridges' single bands were identified in the thin sections. A total of 23 Bridges' single bands (and 4 bands of division 20) could not be found; in particular, bands were missing from the difficult regions 11DE, 15A, 15C and 16A. Electron microscopy showed the existence of 18 additional faint bands, 4 at the region 18D and 7 at 19EF. Bridges' faintest single bands and the new bands were best seen in formaldehyde fixed material. About 1/4 of Bridges' double bands were found to be made up of two separate bands each. The remaining Bridges' doublets include all kinds of bands: broad, narrow, dark, faint, puffed. Many of them look single in thin sections.

The faint band/interband region 28C2 to 28C4-5(-) of the Drosophila melanogaster salivary gland polytene chromosomes is rich in transcripts.

Urate oxidase mRNA and five other transcripts map along 38 kb of DNA in the region 28C on the Drosophila melanogaster second chromosome. Three biotinylated restriction fragments from this 38 kb of DNA, one from each end and one from the middle, were individually hybridized in situ to slightly stretched salivary gland polytene chromosomes. The data from these in situ hybridizations in combination with the transcription map of the 38 kb of DNA indicate that: (i) there are six discrete RNA species encoded along the 38 kb of DNA and (ii) these six transcripts map to the faint band/interband region which includes the proximal edge of 28C1, the three faint bands, 28C2, 28C3 and 28C4-5(-), and the adjacent interband chromatin. Our data are consistent with the few published studies directly demonstrating that faint band/interband regions of the Drosophila melanogaster salivary gland polytene chromosomes code for a high density of transcripts.

Electron micrograph maps of divisions 51 through 60 of thin sectioned polytene 2R chromosome of Drosophila melanogaster.

The banding pattern of the distal half, i.e., the divisions 51 through 60, of the salivary gland 2R chromosome in Drosophila melanogaster was studied using thin section electron microscopy. This chromosome region contains 280 single and 157 double bands on the Bridges' revised light microscopic map. Most of the single bands were identified in thin sections; 34 bands were regarded as missing or they could not be reliably shown. In addition, about 20 new, mainly very faint single bands were found. 21 Bridges' double bands were found to be made up of two separate bands each in thin sections.

Ultrastructure of E1 + 2 + 9 + 12 inversion breakpoints in Drosophila subobscura.

The ultrastructure of the Drosophila subobscura chromosome regions around the breakpoints of the complex E1 + 2 + 9 + 12 gene arrangement was analyzed. This overlapping inversion is formed by the association of the E1, E2, E9, and E12 simple inversions. Ultrastructure of sections involving 58D/59A, 61C/D, 62D/63A, 64B/C, 67A/B, and 68B/C breakpoints on Est chromosomes were compared with the ultrastructure of sections involving chromosomes were compared with the ultrastructure of sections involving 58D/68B, 62D/64C, 59A/63A, 64B/68C, 67B/61C, and 67A/61B breakpoints on E1 + 2 + 9 + 12 chromosomes. No detectable changes of structural organization on banding patterns induced by the E1 + 2 + 9 + 12 inversion were found. Ultrastructural analysis of the two E12 breakpoints has, however, facilitated the analysis of the left boundary of E12 inversion. Accordingly, we propose 61B/C as a new breakpoint instead of 61C/D.

Electron microscopic photomap of divisions 64 through 70 of the salivary gland 3L chromosome in Drosophila melanogaster.

The study is a part of an electron microscopic mapping project on the salivary gland chromosomes of D. melanogaster. Chromosomes were fixed with acetic methanol and, alternatively, with formaldehyde-Ringer solution, squashed and thin sectioned for electron microscopy. The banding pattern was compared with the drawn light microscopic map of Bridges, which shows 209 single and 93 double bands within the divisions 64 through 70. Thirteen Bridges' singlets could not be consistently documented in the electron micrographs. About 50 new, mainly very faint bands were found, most of them following formaldehyde fixation. Again, about 40 bands marked double by Bridges were interpreted to be two separate single bands. This paper completes the electron microscopic mapping of the 3L chromosome.