GEOGRAPHICAL STRUCTURING IN THE MTDNA OF ITALIANS

Geographical patterns of mtDNA variation were studied in 12 Italian sa mples (1072 individuals) by two different spatial autocorrelation meth ods. Separate analyses of the frequencies of 12 restriction morphs sho w North-South dines, differences between Sardinia and the mainland pop ulations, and the effects of isolation by distance. A recently develop ed autocorrelation statistic summarizing molecular similarity at all s ites (AIDA; autocorrelation index for DNA analysis) confirms the prese nce of a clinal pattern; differences between random pairs of haplotype s tend to increase with their geographical distance. The partition of gene diversity, however, reveals that most variability occurs within p opulations, whereas differences between populations are minor (G(ST) = 0.057). When the data from the 12 samples are pooled, two descriptors of genetic variability (number of polymorphic sites and average seque nce difference between pairs of individuals) do not behave as expected under neutrality, The presence of clinal patterns, Tajima's tests, an d a simulation experiment agree in suggesting that population sizes in creased rapidly in Italy and Sicily but not necessarily so in Sardini a, The distribution of pairwise sequence differences in the Italian pe ninsula (excluding Sardinia) permits a tentative location of the demog raphic increase between 8000 and 20,500 years ago. These dates are con sistent with archaeological estimates of two distinct;expansion proces ses, occurring, respectively, in the Neolithic and after the last glac ial maximum in the Paleolithic, Conversely, there is no genetic eviden ce that such processes have had a major impact on the Sardinian popula tion.