Microsatellite variation in colonizing and palearctic populations of Drosophila subobscura

The recent colonization of North America by Drosophila subobscura has provi ded a great opportunity to analyze a colonization process from the beginnin g. A comparative study using 10 microsatellite loci was conducted for five European and two North American populations. No genetic differentiation bet ween European populations was detected, indicating that gene flow is high a mong them and that the microsatellites used in the present work represent n eutral markers not subject to differentiation due to selection. Extensive r eduction in the number of alleles and a significant decrease in heterozygos ity in colonizing populations were detected that could be explained by the founder effect and a subsequent quick but not infinite expansion. Assuming that all alleles present in the colonized area were carried by the sample o f colonizers, we estimated that most probably 4-11 individuals expanded in the new area. F-ST and the chord distance measures reflect the colonization process more accurately, since drift has been the major force in different iating the Old and New World populations, and thus other measures consideri ng allele size differences, such as Rho(ST) and delta mu (2), are less reli able for studying nonequilibrium populations. Finally, our results were con sistent with the two-phase microsatellite mutational model, indicating that most alleles are generated by gain or loss of a repeat unit, while some al leles originate by more complex mutations.