COLONIZATION OF AMERICA BY DROSOPHILA-SUBOBSCURA - THE FOUNDER EVENT AND A RAPID EXPANSION

The recent colonization of the American continent by Drosophila subobs cura offers a unique opportunity to analyze the response of different genomic entities to a new environment. Here, results from a study of 1 5 allozyme loci in six North American and seven South American populat ions are compared with existing data from Palearctic populations. In g eneral, only alleles with a frequency higher than 0.1 in European popu lations are present in America. The observed alteration in allele freq uencies can be explained by a founder event. Although some significant latitudinal dines for allozyme frequencies have been detected, the re sults obtained are not as clear-cut as are those for chromosomal inver sions. Overall heterozygosity is similar between North and South Ameri can populations and does not differ from that of Palearctic population s, although the mean number of alleles is clearly lower in the coloniz ing populations. This observation experimentally corroborates the resu lts 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 Ame rican populations is clearly manifested by a geometrical representatio n using Bhattacharyya's distance and a multidimensional scaling techni que. Furthermore, these American populations are clearly differentiate d from the Palearctic populations.