a1 Department of Ecology and Evolution, The University of Chicago, 1101 E. 57th St, Chicago IL 60637-1573, USA
The total genomic copy numbers of ten families of transposable elements of Drosophila melanogaster in a set of ten isogenic lines derived from a natural population were estimated by slot-blotting. The numbers of euchromatic copies of members of each family were determined for each line by in situ hybridization of element probes to polytene chromosomes. Heterochromatic numbers were estimated by subtraction of the euchromatic counts from the total numbers. There was considerable variation between element families and lines in heterochromatic abundances, and the variance between lines for many elements was much greater for the heterochromatin than for the euchromatin. The data are consistent with the view that much of the β-heterochromatin consists of sequences derived from transposable elements. They are also consistent with the hypothesis that similar evolutionary forces control element abundances in both the euchromatin and heterochromatin, although amplification of inert sequences derived from transposable elements may be in part responsible for their accumulation in heterochromatin.
c1 Corresponding author.
p1 Present address: Généetique & Environment, Institut des Sciences de L'Évolution, Université Montpellier II, Place E. Bataillon, 34095 Montpellier Cedex 5, France.