Selection against transposable elements in D. simulans and D. melanogaster.

The insertion site numbers of the transposable elements (TEs) copia, mdg1, 412 and gypsy were determined in various natural populations of Drosophila melanogaster and D. simulans by in situ hybridization. We showed that, while all elements except gypsy had many insertion sites scattered over the chromosomes in D. melanogaster, only the 412 element in D. simulans presented a high number of insertions, and this number was lower than in D. melanogaster. This low 412 site number per genome in D. simulans was associated with a lower proportion of insertions on the X chromosome in comparison with D. melanogaster, as determined in diploid genomes (0.090 for D. simulans against 0.137 for D. melanogaster) and in haploid genomes (0.102 against 0.146), each value being, moreover, lower than the value of 0.20 expected on the hypothesis of no selection against insertional mutations. These results suggest that selection is a major mechanism explaining 412 copy number regulation in Drosophila, and is stronger in D. simulans than in D. melanogaster.