Middle repetitive DNA: a fluid component of the Drosophila genome.

Most of the middle repetitive DNA of Drosophila melanogaster appears to be organized into families of 10-100 repeated elements that are found at scattered locations in the chromosome arms and occupy new chromosomal positions as populations of D. melanogaster diverge. These "nomadic" DNA segments can be identified by an analysis of cDm plasmids, hybrids of ColE1 and segments of randomly sheared D. melanogaster DNA. Eighty cDm plasmids were withdrawn, at random, from a library of approximately 17,000 cDm clones. Fifty-seven of these seem to contain either DNA that is not repeated in the D. melanogaster genome or DNA that has a low repetition frequency. The remaining 23 cDm plasmids contain repetitive sequences. Seventeen of these 23 plasmids contain repetitive sequences that are demonstrably scattered to many chromosomal sites that can be mapped in two D. melanogaster strains, g-1 and g-X11. The repeated elements hybridizing with each of the different Dm segments are at quite different chromosomal locations in these two strains. However, the size of each family of repeated sequences remains fairly constant in both strains. It is proposed that the number of elements in each family has been fixed by selection.