Colonization of America by Drosophila subobscura: the founder event and a rapid expansion.

The recent colonization of the American continent by Drosophila subobscura offers a unique opportunity to analyze the response of different genomic entities to a new environment. Here, results from a study of 15 allozyme loci in six North American and seven South American populations are compared with existing data from Palearctic populations. In general, only alleles with a frequency higher than 0.1 in European populations are present in America. The observed alteration in allele frequencies can be explained by a founder event. Although some significant latitudinal clines for allozyme frequencies have been detected, the results obtained are not as clear-cut as are those for chromosomal inversions. Overall heterozygosity is similar between North and South American populations and does not differ from that of Palearctic populations. This observation experimentally corroborates the results provided by theoretical models in which the average heterozygosity per locus depends not only on the size of bottleneck but also on the rate of population growth. The resemblance between North and South American populations is clearly manifested by a geometrical representation using Bhattacharyya's distance and a multidimensional scaling technique. Furthermore, these American populations are clearly differentiated from the Palearctic populations.