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 electr on microscopy to establish ultrastructural homologies between polytene chromosome regions of Drosophila melanogaster and Drosophila subobscu ra. Twelve probes were chosen to cover all the chromosomal elements: t he myospheroid gene, the collagen type IV gene, the collagen-like gene , the w26 homeobox gene, the beta 3 tubulin gene, the kinesin heavy ch ain gene, the tryptophan hydrolase gene, the Hsp82, Hsp22-26 and Hsp23 -28, Hsy68, Hsp70 genes and the beta unit of the F-0-F-1 ATPase gene. Most of these loci were previously undescribed in D. subobscura and im precisely 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 chromos omal regions surrounding the loci. In some cases, however, no structur ally recognisable homology can be seen either in the locus or in its f lanking regions.