The numbers of members of three families of copia-like elements were counted on twenty X, 2nd and 3rd chromosomes collected from a natural population of Drosophila melanogaster. Theoretical predictions were computed for two models of copy number stabilization: (1) element frequencies are regulated by a simple genetic process such as copy number dependent transposition or excision, independent of chromosomal location; (2) elements are eliminated by natural selection against mutational effects of their insertion into the chromosome. Since insertions into the X can be expected to suffer more selection than autosomal insertions, due to expression of mutant phenotypes in the hemizygous state, hypothesis 2, called the disproportional model, predicts that the proportion of elements on the X will be smaller than the proportion of the genome contributed by the X, while hypothesis 1, called the equiproportional model, predicts that this proportionality will be unaffected. Two of the elements, 297 and roo, showed no evidence for deficiency of X-linked elements, but the data for a third element, 412, were consistent with the prediction based on the selective model.
These results indicate that simple selection against mutational effects of insertions of transposable elements is not generally adequate to account for their distribution within populations. We argue that a mechanism such as recombination between elements at different chromosomal sites, leading to rearrangements with highly deleterious, dominant effects could play a role in stabilizing copy number. This process would lead to a higher abundance of elements in genomic regions with restricted crossing over. We present some data indicating such an effect, and discuss possible interpretations.