Interspecific hybridization increases transposition rates of Osvaldo

Several authors have postulated that genetic divergence between populations could result in genomic incompatibilities that would cause an increase in transposition in their hybrids, producing secondary effects such as sterili ty and therefore starting a speciation process. It has been demonstrated th at transposition largely depends on intraspecific hybridization for P, hobo , and I elements in Drosophila melanogaster and for several elements, inclu ding long terminal repeat (LTR) and non-LTR retrotransposons, in D. virilis . However, in order to demonstrate the putative effect of transposable elem ents on speciation, high levels of transposition should also be induced in hybrids between species that could have been originated by this process and that are still able to interbreed. To test this hypothesis, we studied the transposition of the LTR retrotransposon Osvaldo in Drosophila buzzatii-Dr osophila koepferae hybrids. We used a simple and robust experimental design , analyzing large samples of single-pair mate offspring, which allowed us t o detect new insertions by in situ hybridization to polytene chromosomes. I n order to compare transposition rates, we also used a stock recently obtai ned from the field and a highly inbred D. buzzatii strain. Our results show that the transposition rate of Osvaldo is 10(-3) transpositions per elemen t per generation in all nonhybrid samples, very high when compared with tho se of other transposable elements. In hybrids, the transposition rate was a lways 10(-2), significantly higher than in nonhybrids. We show that inbreed ing has no effect on transposition in the strains used, concluding that hyb ridization significantly increases the Osvaldo transposition rate.