Abstract
The genomic concentrations of certain middle repetitive DNA sequences vary considerably among closely related species of Drosophila. In fact, the chromosomes of D. melanogaster appear to carry approximately 3 times as much middle repetitive DNA as those of the sibling species D. simulans. Although most of the middle repetitive DNA of D. melanogaster consists of segments of "nomadic" DNA that occupy different dispersed chromosomal locations in different strains of flies, repeated DNA sequences recovered from the D. simulans genome are most often restricted to single chromosomal positions. Apparent differences in the total concentrations of middle repetitive DNA in the two species are most easily attributed to an approximately sevenfold difference in their dispersed repetitive and nomadic DNA contents. These differences may affect the relative mutation rates of these species or contribute to their reproductive isolation or both.