Mobile genetic elements are found in the genomes of many organisms, and because of their effects on genes and their ability to induce chromosomal rearrangements they are an important source of genetic variability. Transposition rates are usually found to be low, estimated at around 10(-3) per generation. Higher rates of transposition are observed, however, in crosses between certain strains of Drosophila melanogaster ('hybrid dysgenesis'), which can lead to a dramatic rearrangement of many mobile elements ('transposition bursts'). We have studied the chromosomal distribution of mdg-1 and copia mobile elements in 17 highly inbred lines of D. melanogaster, after 69 generations of sib-mating. Most lines show no changes, but one showed a complete reshuffling of the copia element. We conclude that the transpositions of the copia element in this line occurred rapidly in a few generations. This phenomenon, distinct from 'transposition bursts' in that only copia elements are involved, may account for the instability sometimes observed in inbred lines and may be important in creating genetic variability in highly homozygous populations.
The chromosomal location of the mobile element mdg-1 was studied in 17 highly-inbred lines of Drosophila melanogaster. Although some lines were stable for their pattern of insertion sites from the 15th to the 27th and 35th brother-sister generations, others showed a high rate of gain of new insertion sites or a high rate of excision (loss of elements). In one line, 8 new insertion sites on the third chromosome were associated with an inversion on this chromosome; there is evidence that the rearrangement occurred premeiotically, thus suggesting a "transposition burst" in this line. There was no correlation between the rate of gain of new insertions and the rate of excision, this latter rate being higher. We have strong evidence that transposition is replicative. Excisions and, to a lesser extent, gains of insertions have been found to be copy-number dependent, suggesting that this is a main mechanism for copy-number regulation. Most inbred lines tended toward an optimal mdg-1 copy number for which they no longer showed transposition or excision (they became stable). These results may explain the instability sometimes reported in inbred lines.
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