Cytological localization and organization of dispersed middle repetitive DNA sequences of Drosophila subobscura

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Abstract
To study the middle repetitive fraction of the Drosophila subobscura genome, 26 phage clones containing repetitive sequences were examined by Southern DNA blot analysis and by in situ hybridization to polytene chromosomes. These results led to a classification of the clones according to five different types of hybridization patterns. Two types, each containing seven clones, are characterized by hybridization at 100 to 300 sites dispersed over the euchromatic parts of the chromosomes, and in addition by one prominently labelled chromosome band. One of these two classes also showed strong labelling of the chromocentre. The remaining types of hybridization pattern lacked a prominent band but showed hybridization either to the euchromatic regions or to the chromocentre or both. Chromosome A (=X) was the preferred location of prominently labelled bands and it also showed an excess of labelling by some clones. Some of the cloned dispersed sequences were localized cytologically on chromosomes of larvae from crosses between different strains of D. subobscura and between two closely related species, in order to detect heterozygosity at hybridization sites. Comparisons of the chromosomal distribution of labelling sites showed differences in number and location, indicating the possibility of transposition events.