Successive transposition explosions in Drosophila melanogaster and reverse transpositions of mobile dispersed genetic elements.

Transposition outbursts occur in the destabilized Drosophila melanogaster strain ct carrying a mutation in the locus cut induced by an insertion of mdg4. While the distribution of mobile genetic elements remained unchanged in the great majority of germ cells, in a few cells numerous transpositions had occurred involving mdg (copia-like), fold-back and P-elements. We used in situ hybridization to analyze the distribution of five families of mdg elements in the X-chromosome during several consequent mutational changes in D. melanogaster. Each of them was accompanied by many changes in mdg localization, all of which occurred in one and the same cell. Thus, we could observe the series consisting of up to five successive transposition explosions leading to an almost complete change in the distribution of the mdg elements tested. We also found that in the course of successive transposition explosions, mdg elements often inserted into those sub-sections of the X-chromosome where they had previously been located. This phenomenon, designated as reverse directed transposition, was studied in more detail on insertion into the locus yellow. The rate of reverse transposition of the same mdg element to the corresponding locus was 10-100 times as high as that of primary insertion. In some cases, ;the transposon shuttle' into and out of the locus was observed. The existence of ;transposition memory' partially explains the specificity of mdg localization in closely related strains as well as the co-ordinated behaviour of different mdg elements in independent transposition explosions. The evolutionary significance of transposition explosions and directed reverse transposition (transposon shuttle) is discussed.