Fixation of transposable elements in the Drosophila melanogaster genome.

We have investigated at the molecular level four cases in which D. melanogaster middle repetitive DNA probes consistently hybridized to a particular band on chromosomes sampled from a D. melanogaster natural population. Two corresponded to true fixations of a roo and a Stalker element, and the others were artefacts of the in situ hybridization technique caused by the presence of genomic DNA flanking the transposable elements (TEs) in the probes. The two fixed elements are located in the beta-heterochromatin (20A and 80B, respectively) and are embedded in large clusters of other elements, many of which may also be fixed. We also found evidence that this accumulation is an ongoing process. These results support the hypothesis that TEs accumulate in the non-recombining part of the genome. Their implications for the effects of TEs on determining the chromatin structure of the host genomes are discussed in the light of recent evidence for the role of TE-derived small interfering-RNAs as cis -acting determinants of heterochromatin formation.