Localisation polymorphism of mdg-1, copia, I and P mobile elements in genomes of Drosophila melanogaster, from data of inbred lines

Using data from in situ hybridisation of the giant chromosomes from salivary glands, mdg-1, I, copia, and P mobile element polymorphism was studied in 17 highly-inbred lines of Drosophila melanogaster and in the expected hybrids obtained from the theoretically-crossed inbred lines. The mean copy numbers of each element on the inbred lines were close and equal to 16·8 for mdg-1, 17·5 for I, 17·8 for copia and 18·6 for P (these values included the insertions in the centromeric regions). The P element differed from the other three for variance in copy number and distribution of the number of insertion site occurrences. A low frequency of X-linked copia element insertions (as compared with frequency of insertions in the other chromosomes) was reported, suggesting that natural selection acts against insertional deleterious mutations of this element. Such a low frequency of X-linked insertions was not observed for the other three elements. The mdg-1, I and copia element copy numbers were positively correlated, thus leading to lines with high or low copy number. No correlation was observed between the P element copy number and the numbers of mdg-1, I and copia elements. Moreover, a genomic control of copy number was detected for mdg-1, I and copia, but not for P. The number of labelling sites, the number of copies, the number of heterozygous insertion sites, and various indices of genetic variability were estimated from the expected hybrids obtained from the theoretically-crossed inbred lines; the calculated numbers of labelling sites for mdg-1 and I did not differ from those obtained directly from the initial wild-derived population (we had no estimate of the number of labelling sites available in this population for copia and P). An average proportion of heterozygous sites of 0·86, 0·92, 0·90 and 0·94, and an average number of copies of 33, 35, 38 and 37 were found in the theoretical hybrids for the mdg-1, I, copia and P elements, respectively.