Genetic Roots of the Ashkenazi Jews Jan 6, 2019 22:13:49 GMT
Post by Admin on Jan 6, 2019 22:13:49 GMT
The origins of Ashkenazi Jews—the great majority of living Jews—remain highly contested and enigmatic to this day1,2,3,4,5,6,7,8,9,10,11. The Ashkenazim are Jews with a recent ancestry in central and Eastern Europe, in contrast to Sephardim (with an ancestry in Iberia, followed by exile after 1492), Mizrahim (who have always resided in the Near East) and North African Jews (comprising both Sephardim and Mizrahim). There is consensus that all Jewish Diaspora groups, including the Ashkenazim, trace their ancestry, at least in part, to the Levant, ~2,000–3,000 years ago5,12,13,14. There were Diaspora communities throughout Mediterranean Europe and the Near East for several centuries prior to the destruction of the Second Temple in Jerusalem in 70 CE (Common Era), and some scholars suggest that their scale implies proselytism and wide-scale conversion, although this view is very controversial9,15.
The Ashkenazim are thought to have emerged from dispersals north into the Rhineland of Mediterranean Jews in the early Middle Ages, although there is little evidence before the twelfth century5,15. After expulsions from Western Europe between the thirteenth and fifteenth centuries, the communities are thought to have expanded eastwards, especially in Poland, Lithuania and then Russia. The implied scale of this expansion has led some to argue, again very controversially, for mass conversions in the Khazar kingdom, in the North Caucasus region to the north and east of the Black Sea, following the Khazar leadership’s adoption of Judaism between the ninth and tenth centuries CE8,9.
We are then faced with several competing models for Ashkenazi origins: a Levantine ancestry; a Mediterranean/west European ancestry; a North Caucasian ancestry; or, of course, a blend of these. This seems an ideal problem to tackle with genetic analysis, but after decades of intensive study a definitive answer remains elusive. Although we might imagine that such an apparently straightforward admixture question might be readily addressed using genome-wide autosomal markers, recent studies have proposed contradictory conclusions. Several suggest a primarily Levantine ancestry with south/west European admixture3,4, but another concludes that the ancestry is largely Caucasian16, implying a major source from converts in the Khazar kingdom17. An important reason for disagreement is that the Ashkenazim have undergone severe founder effects during their history, drastically altering the frequencies of genetic markers and distorting the relationship with their ancestral populations.
This problem can be resolved by reconstructing the relationships genealogically, rather than relying on allele frequencies, using the non-recombining marker systems: the paternally inherited male-specific part of the Y chromosome (MSY) and the maternally inherited mitochondrial DNA (mtDNA). This kind of analysis can be very powerful, because nesting of particular lineages within clusters from a particular geographical region allows us to pinpoint the source for those lineages, by applying the parsimony principle. This has indeed been attempted, with the MSY results interpreted plausibly to suggest an overwhelming majority of Near Eastern ancestry on the Ashkenazi male line of descent11,18,19,20,21, albeit with much higher levels (>50%) of European (potentially east European) lineages in Ashkenazi Levites22, suggesting a possible Khazar source in that particular case.
The maternal line has also been studied, and indeed Ashkenazi mtDNAs are highly distinctive, but they have proved difficult to assign to a source population1,2,11. Some progress has been made by targeting whole-mtDNA genomes or mitogenomes, which provide much higher genealogical (and therefore geographical) and chronological resolution than the control-region sequences used previously—although the far larger control-region database remains an invaluable guide to their geographic distribution. Using this approach, Behar et al.2 identified four major founder clusters, three within haplogroup K—amounting to 32% of sampled Ashkenazi lineages—and one within haplogroup N1b, amounting to another 9%. These lineages are extremely infrequent across the Near East and Europe, making the identification of potential source populations very challenging. Nevertheless, they concluded that all four most likely arose in the Near East and were markers of a migration to Europe of people ancestral to the Ashkenazim only ~2,000 years ago1,2. The remaining ~60% of mtDNA lineages in the Ashkenazim remained unassigned to any source, with the exception of the minor haplogroup U5 and V lineages (~6% in total), which implied European ancestry1,23.
Here we focus on both major and minor founders, with a much larger database from potential source populations. We first analyse 956 (72 newly generated) mitogenomes from haplogroup U8 (including 909 from haplogroup K, U8’s major subclade): 477 of these are from Europe and 106 from the Near East/Caucasus. We show that European and Near Eastern lineages largely fall into discrete, ancient clusters, with minor episodes of gene flow, suggesting that haplogroup K diversified separately in Europe and the Near East during the last glacial period. Of the three Ashkenazi founders, K1a1b1a and K1a9 were most likely assimilated in west (perhaps Mediterranean) Europe and K2a2a1 in west/central Europe. Most surprisingly, by analysing two new N1b2 sequences selected from a database of 278 N1b HVS-I sequences, in the context of 44 published N1b sequences24, we show that the highly distinctive N1b2 subclade, making up another 9% of Ashkenazi lineages, was likely assimilated in Mediterranean Europe, rather than in the Near East as previously proposed2. Moreover, from a survey of another >2,500 complete mtDNA genomes and >28,000 control-region sequences from Europe, the Near East and the Caucasus, in comparison with the available database of 836 Ashkenazi control-region sequences and a handful of published mitogenomes, we also evaluate the minor founders. Overall, we estimate that most (>80%) Ashkenazi mtDNAs were assimilated within Europe. Few derive from a Near Eastern source, and despite the recent revival of the ‘Khazar hypothesis’16, virtually none are likely to have ancestry in the North Caucasus. Therefore, whereas on the male side there may have been a significant Near Eastern (and possibly east European/Caucasian) component in Ashkenazi ancestry, the maternal lineages mainly trace back to prehistoric Western Europe. These results emphasize the importance of recruitment of local women and conversion in the formation of Ashkenazi communities, and represent a significant step in the detailed reconstruction of Ashkenazi genealogical history.