The arrangement of transposable elements in the polytene chromosomes of Drosophila melanogaster

Abstract

The location of different cloned transposable elements was determined in the polytene chromosomes of gt waand Oregon RC inbred Drosophila melanogaster stocks. Each stock was found to have about 300 hybridization sites for 12 different clones. The arrangement of the clones in the chromosomes was identical in individuals of the same stock but differed significantly between the two stocks studied. Only 10% of the localisation sites of the clones did not change their location in the chromosomes. Calculations showed that each band of a polytene chromosome may contain transposable elements. Thus these elements provide a high level of structural heterozygosis in the chromosomes of hybrid individuals. There was a slight but statistically significant tendency of transposable elements to form clusters in certain chromosomal regions. The number of chromosomal regions that they occupied was 80% of the number expected for a random distribution. A concentration of transposable elements occurred in the chromosomal regions adjacent to the chromocenter, in histone and 5S RNA loci. The banding pattern of homologous chromosomal regions as revealed by electron microscopy was similar in both D. melanogaster stocks although they differed greatly in the arrangement of transposable elements. It is likely that the structural heterozygosis provided by transposable elements in heterozygous individuals influences the frequency of genetic recombination in affected chromosomal regions, disturbs the time course of DNA synthesis, and changes the efficiency of transcription.