Distributions of three transposable elements, P, 297, and copia, in natural populations of Drosophila melanogaster

Two Japanese natural populations and a Canary Island cage population of Drosophila melanogaster were analyzed with respect to three transposable elements P, 297, and copia, using chromosomal in situ hybridization, copia was distributed more or less equally over all chromosomes. Its copy number on the X chromosome was slightly less than that on autosomes of equivalent size, possibly indicating the near neutrality or slightly deleteriousness of the transposable element actually present in the chromosomes. The distribution of P elements was a little different from that of the other two transposable elements, as it was not concentrated around chromocenter, in contrast to that of 297 and copia. No evidence of threshold was observed with respect to the copy number of transposable elements in a genome in any of the three transposable elements. The variance of copy number of these transposons between different strains were generally larger than those expected under the poisson distribution, especially in the P element. These observations possibly indicate near neutrality of these transposable elements (in their present positions) with respect to natural selection or non-equilibrium state in populations.