Geographical structuring in the mtDNA of Italians.

Abstract

Geographical patterns of mtDNA variation were studied in 12 Italian samples (1072 individuals) by two different spatial autocorrelation methods. Separate analyses of the frequencies of 12 restriction morphs show North-South clines, differences between Sardinia and the mainland populations, and the effects of isolation by distance. A recently developed autocorrelation statistic summarizing molecular similarity at all sites (AIDA; autocorrelation index for DNA analysis) confirms the presence of a clinal pattern; differences between random pairs of haplotypes tend to increase with their geographical distance. The partition of gene diversity, however, reveals that most variability occurs within populations, whereas differences between populations are minor (GST = 0.057). When the data from the 12 samples are pooled, two descriptors of genetic variability (number of polymorphic sites and average sequence difference between pairs of individuals) do not behave as expected under neutrality. The presence of clinal patterns, Tajima's tests, and a simulation experiment agree in suggesting that population sizes increased rapidly in Italy and Sicily but not necessarily so in Sardinia. The distribution of pairwise sequence differences in the Italian peninsula (excluding Sardinia) permits a tentative location of the demographic increase between 8000 and 20,500 years ago. These dates are consistent with archaeological estimates of two distinct expansion processes, occurring, respectively, in the Neolithic and after the last glacial maximum in the Paleolithic. Conversely, there is no genetic evidence that such processes have had a major impact on the Sardinian population.Most studies of mtDNA variation in humans have inferred evolutionary processes by reconstructing history-i.e., genealogies of haplotypes (1-4). With one exception (5), geographical information has been digregarded, or it has been used simply to classify populations, as an alternative to the subjective criteria of racial classification (6, 7). There is no doubt, however, that spatial patterns of genetic diversity also contain useful information for evolutionary inferences. One reason for this omission may lie in the relative paucity of statistical tools suited for spatial analysis of molecular data.Genetic variation in space can be summarized by spatial autocorrelation measures (see, e.g., ref. 8). These statistics have also been used to test hypotheses on past demographic processes (9, 10). Here we apply them to a data base of mtDNA restriction fragment length polymorphism data. We reconstruct patterns of genetic variation from molecular information, and we try to draw inferences about the underlying microevolutionary phenomena.In the first part of this paper, we analyze the frequencies of 12 common restriction morphs, treating them as if they were allele frequencies. In the second part, we use a recently developed autocorrelation statistic, AIDA (autocorrelation index for DNA analysis) (11), which compares DNA sequences (and not only their frequencies) ...

Geographic homogeneity and non-equilibrium patterns of mtDNA sequences in Tuscany, Italy

“…In agreement with this, mtDNA restriction fragment length polymorphism (RFLP) variation in Italy is consistent with the effects of a demographic expansion in the late Palaeolithic or in the early Neolithic, i.e. between 8000 and 20 000 years before present (Barbujani et al 1995a). The analysis of the present data set based on different statistical methods and its comparison with other European data agree with this, confirming that the population ancestral to the current Southern Tuscans may have increased substantially in or before early Neolithic times (Francalacci et al 1996).…”

“…However, it does appear that important aspects of the current population structure of Tuscany have been determined during the course of a demographic increase. As we mentioned above, analysis of mitochondrial RFLPs suggests that populations expanded rapidly over much of Italy, with the probable exception of Sardinia, some time between 8000 and 20 000 years ago (Barbujani et al 1995a). We see no reason to believe that this demographic phenomenon should have been any different for Tuscany.…”

The Italian genome reflects the history of Europe and the Mediterranean basin

“…Geographical patterns of Y chromosomal and mtDNA diversity in Italy, mainly determined by the combined action of drift and founder effects, have been described. 8,9 A correlation between genetic and geographic structure in Europe has also been found, 10,11 with a detectable distinction between Southern Italians and other Europeans.…”

Evidence for Gradients of Human Genetic Diversity Within and Among Continents

“…Although this approach has been advocated (e.g., King and Motulsky 2002;Kittles and Weiss 2003), it has not been widely implemented because of logistical difficulties. However, studies of genetic diversity from restricted geographical areas, where large numbers of individuals are sampled and a reasonable geographic coverage of the variation is achieved, generally reveal spatial gradients of allele frequencies (Barbujani et al 1995;Krings et al 1999;Ding et al 2000;Rosser et al 2000;Karafet et al 2001) that are only occasionally disrupted by local discontinuities corresponding to linguistic or geographical barriers Sokal et al 1990). This suggests that isolation by distance (i.e., decreasing gene flow with increasing geographical distances) may be the most appropriate description of human genetic diversity .…”