THE EVOLUTIONARY HISTORY OF DROSOPHILA-BUZZATII - XXXIV - THE DISTRIBUTION OF THE RETROTRANSPOSON OSVALDO IN ORIGINAL AND COLONIZING POPULATIONS

The frequency distribution of the retrotransposon Osvaldo in the haplo id genome of Drosophila buzzatii has been studied in five natural popu lations from the Iberian Peninsula and six natural populations from Ar gentina. In Iberian populations, Osvaldo insertion sites do not follow a Poisson distribution, most probably due to eight euchromatic sites with high occupancy, found in all populations. The estimated alpha and beta parameters, which measure the relative importance of drift and n egative selection in shaping frequency distributions, indicate that dr ift is the main force acting upon the distribution of Osvaldo in natur al populations of D. buzzatii in the Iberian Peninsula. On the other h and, Osvaldo distribution in populations from Argentina is similar to the distribution of elements with low copy numbers, such as those desc ribed for Drosophila melanogaster and Drosophila simulans: there are n o indications for deviation from a Poisson distribution, there is a lo w occupancy per insertion site, and genetic drift has no apparent effe ct on the frequency distribution. We propose that the unusual distribu tion found in the populations from the Iberian Peninsula is a conseque nce of the colonization process. Iberian Peninsula populations suffere d a genomic redistribution of Osvaldo, most probably after a founder e ffect. Consequently, certain copies that arrived at high frequencies a re showing a high occupancies today, and the mean copy number of Osval do is higher in Iberian Peninsula populations than in populations from Argentina. All other copies are the result of recent (after colonizat ion) transposition events.