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15-01-2006, 10:23
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חבר מתאריך: 07.10.02
הודעות: 6,484
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המחקר (שהוא הנושא המרכזי פה לדעתי ולא ה"ידיעה"):
Am. J. Hum. Genet., 73:768-779, 2003
0002-9297/2003/7304-0006$15.00
© 2003 by The American Society of Human Genetics. All rights reserved.
Multiple Origins of Ashkenazi Levites: Y Chromosome Evidence for Both Near Eastern and European Ancestries
Doron M. Behar,1,* Mark G. Thomas,2,* Karl Skorecki,1 Michael F. Hammer,4 Ekaterina Bulygina,2 Dror Rosengarten,1 Abigail L. Jones,2 Karen Held,2 Vivian Moses,2 David Goldstein,3 Neil Bradman,2 and Michael E. Weale2
1Bruce Rappaport Faculty of Medicine and Research Institute, Technion and Rambam Medical Center, Haifa, Israel; 2The Centre for Genetic Anthropology and 3The Centre for Population Genetics and Human Health, Department of Biology, University College London, London; and 4Division of Biotechnology, University of Arizona, Tucson, AZ
Received May 8, 2003; accepted for publication June 18, 2003; electronically published September 17, 2003.
Previous Y chromosome studies have shown that the Cohanim, a paternally inherited Jewish priestly caste, predominantly share a recent common ancestry irrespective of the geographically defined post-Diaspora community to which they belong, a finding consistent with common Jewish origins in the Near East. In contrast, the Levites, another paternally inherited Jewish caste, display evidence for multiple recent origins, with Ashkenazi Levites having a high frequency of a distinctive, non
[התמונה הבאה מגיעה מקישור שלא מתחיל ב https ולכן לא הוטמעה בדף כדי לשמור על https תקין: http://www.journals.uchicago.edu/ucp-entities/ndash.gif]
Near Eastern haplogroup. Here, we show that the Ashkenazi Levite microsatellite haplotypes within this haplogroup are extremely tightly clustered, with an inferred common ancestor within the past 2,000 years. Comparisons with other Jewish and non-Jewish groups suggest that a founding event, probably involving one or very few European men occurring at a time close to the initial formation and settlement of the Ashkenazi community, is the most likely explanation for the presence of this distinctive haplogroup found today in >50% of Ashkenazi Levites.
* These authors contributed equally to this work.
Address for correspondence and reprints: Dr. Karl Skorecki, Department of Nephrology and Molecular Medicine, Technion and Rambam Medical Center, P.O.B. 9602, Haifa 31096, Israel. E-mail: skorecki@techunix.technion.ac.il
Introduction
Jewish identity, since at least Talmudic times ( [התמונה הבאה מגיעה מקישור שלא מתחיל ב https ולכן לא הוטמעה בדף כדי לשמור על https תקין: http://www.journals.uchicago.edu/ucp-entities/sim.gif] 100 B. C. E. [התמונה הבאה מגיעה מקישור שלא מתחיל ב https ולכן לא הוטמעה בדף כדי לשמור על https תקין: http://www.journals.uchicago.edu/ucp-entities/ndash.gif] 500 C. E.), has been acquired either by maternal descent from a Jewish woman or by rabbinically authorized conversion. Only in recent years have some strands of the Jewish religion accepted paternal descent as a qualifying criterion. Within the Jewish community, however, membership in the three male castes (Cohen, Levi, and Israelite) is determined by paternal descent. Cohanim (plural of "Cohen," the Hebrew word for priest) are, in Biblical tradition, the descendants of Aaron the brother of Moses; Levites are, in that tradition, considered to be those male descendants of Levi, the third son of the patriarch Jacob and paternal ancestor of Aaron, who are not Cohanim. The Cohanim have both rights and duties in religious law, as well as being subject to restrictions that do not apply to the other castes. They are, for example, called first to the reading of the Torah in synagogue and forbidden entry into a cemetery. Levites have some rights similar to those of Cohanim (for example, exemption from payment of a special tax on the birth of a first-born male) but are not subject to the particular restrictions placed on the Cohanim. Strict adherence to the qualifying rules would mean that the male descendants of men who were not Jews at birth could be Israelites but not Cohanim or Levites (Encyclopaedia Judaica 1972). It is estimated that Cohanim and Levites each comprise [התמונה הבאה מגיעה מקישור שלא מתחיל ב https ולכן לא הוטמעה בדף כדי לשמור על https תקין: http://www.journals.uchicago.edu/ucp-entities/sim.gif] 4% of the Jewish people (Bradman et al. 1999).
In addition to classification by caste, Jews, on the basis of their ancestry and religious practice, can be assigned to one or other of a few long-standing, geographically separated Jewish communities, the most numerous of which are the Ashkenazi and Sephardi groupings (Reif 1993). The term "Ashkenaz" describes a relatively compact area of Jewish settlement in northwestern Europe, including northeastern France and northern Germany, where Jewish settlement is documented dating back to at least the 6th century C. E. From the 10th century, Ashkenazi Jews spoke a common language (Yiddish), written with Hebrew characters but borrowing its lexicon mostly from German. By the 16th century, Jews speaking this language and following the Ashkenazi religious rite and cultural tradition populated communities extending from the Loire in the west to the Dnieper in the east and from Rome in the south to the Danish border in the north. During the past 500 years, there has been rapid population growth, culminating in an estimated population size of [התמונה הבאה מגיעה מקישור שלא מתחיל ב https ולכן לא הוטמעה בדף כדי לשמור על https תקין: http://www.journals.uchicago.edu/ucp-entities/sim.gif] 8 million Ashkenazi Jews just prior to the outbreak of World War II. There is uncertainty concerning the relative contributions to Ashkenazi Jewry of, on one hand, western versus eastern immigration of Jews and, on the other hand, internally generated population growth versus conversion to Judaism. In particular, it has been suggested that subjects of the Khazar Empire (located to the northeast of the Black Sea), who had adopted Judaism in the last quarter of the first millennium C. E., were an important constituent of the nascent Ashkenazi community (Encyclopaedia Judaica 1972).
The term "Sephardi" originally described Jews descended from the communities that existed in Spain prior to the expulsion in 1492 C.E. However, current usage applies this designation to all descendants of the communities of North Africa and the Near East who follow the Sephardi rite of worship and cultural traditions. It is thought that, prior to the middle of the 20th century, gene flow between the Ashkenazi and non-Ashkenazi groups was relatively restricted.
The purported different modes of transmission of Levite and Cohen versus Israelite status provide a priori expectations about patterns of genetic variation on the paternally inherited nonrecombining region of the Y chromosome (NRY). In particular, because of recent shared ancestry, Cohanim and Levites would be expected to display lower gene diversity of NRY haplotypes than would Israelites. In addition, the distribution of haplotype frequencies, in the absence of drift, should be similar (a) in Cohanim and (b) in Levites across the Ashkenazi-Sephardi division, given that this division occurred after the founding of these two groups.
In fact, previous studies have indeed shown that the NRYs of Ashkenazi and Sephardi Cohanim are genetically more closely related to each other than they are to the NRYs of Israelites or non-Jews (Skorecki et al. 1997; Thomas et al. 1998). This pattern arises primarily from differences in the frequency of a particular NRY haplotype (the Cohen Modal Haplotype [CMH], defined by six rapidly mutating microsatellites [Thomas et al. 1998]), and a cluster of closely related haplotypes within a single haplogroup (defined by slowly mutating unique event polymorphisms [UEPs]). Chromosomes belonging to this haplotype and its related cluster were found at high frequency among Cohanim but at a much lower frequency among Israelites. Furthermore, the pattern of diversity within the cluster was found to be consistent with descent from a common ancestor who lived between 2,100 and 3,900 years ago. The CMH is also found, at lower frequency, in non-Jewish populations in the Near East, which would be consistent with its origin in this geographic region. However, the same studies found high frequencies of multiple haplogroups in the Levites, indicating that no single recent origin could be inferred for the majority of this group, despite an oral tradition of a patrilineal descent similar to that of the Cohanim. Moreover, a cluster of closely related NRY haplotypes was identified within a distinctive deep-rooting NRY clade that was found at much higher frequency among Ashkenazi Levites than in either Sephardi Levites or any other Jewish group. However, the reasons for this difference in the Ashkenazi Levites were not explored.
Given the importance of the paternally defined Levite caste in Jewish history and tradition, the multiple theories of the ethnogenesis of the Ashkenazi Jewish community, and a suggestion that Yiddish is a relexified Slavic tongue (Wexler 1993), we undertook a detailed investigation of the paternal genetic history of Ashkenazi Levites and compared the results with matching data from neighboring populations among which the Ashkenazi community lived during its formation and subsequent demographic expansion. By analyzing NRY haplotypes, we have revealed ( a) a plausible historical explanation for the multiple paternal histories of the Levite caste, ( b) the probable period during which a European introgression into the caste took place, and ( c) the likely demographic scale of the event.
Subjects and Methods
Sampling
We analyzed NRY variation in a set of 988 unrelated males from Ashkenazi Jews, Sephardi Jews, and four non-Jewish European populations. Populations were categorized using a combination of geographic, religious, and ethnohistorical criteria and individual affiliation within a category was determined according to self-designation by the participant providing the sample. Three of the four non-Jewish groups (88 Germans, 112 Sorbians, and 306 Belarusians) were chosen because of their geographic location relative to the ancestral European communities of the Ashkenazi Jews, and also because of prior knowledge of the geographic areas known to have high frequencies of the NRY haplogroup within which the distinctive Ashkenazi Levite high-frequency cluster was previously reported. This haplogroup is designated "R1a1" according to the Y Chromosome Consortium ( 2002). An additional non-Jewish group consisting of 83 samples previously collected in Norway (Weale et al. 2002) was chosen, to represent a geographic location that excluded Jewish entry until the middle of the 19th century and is known to be outside the area in which Ashkenazi communities originated.
The Jewish samples comprised 236 Ashkenazi Jews (AJ) and 163 Sephardi Jews, who were further divided into 100 Ashkenazi Israelites (AI), 76 Ashkenazi Cohanim (AC), 60 Ashkenazi Levites (AL), 63 Sephardi Israelites (SI), 69 Sephardi Cohanim (SC), and 31 Sephardi Levites (SL). The 60 AL samples were collected from Ashkenazi males who identified themselves as Levites, with a paternal ancestry from one of the following nine Ashkenazi Jewish communities; Austria-Hungary (10), Belarus (4), France (6), Germany (10), Lithuania (8), Netherlands (5), Poland (7), Romania (4), and Russia (6). Current political borders (including the current borders of Austria and Hungary) were used to define geographic origin. When the Ashkenazi Levite sample was split into western (France, Netherlands, and Germany) versus eastern (all others) communities, no significant differences were found in haplogroup or haplotype frequencies (using the exact test of Raymond and Rousset [ 1995]), but we note that the sample size is too small to have power to detect anything other than very large differences in frequency. Information regarding paternal lineage was collected from each of the participants. Paternal population affiliation was determined by the oldest male-line ancestor that the participants identified, which extended back to at least the grandparental level. The participants were not related through their paternal lineages. The ethics review committees of the participating institutions approved the sampling protocols.
Genotyping
Haplogroups
In all population samples except the Ashkenazi Levites and Germans, we typed a set of 12 biallelic markers including one Alu insertion and 11 SNPs to investigate the data reported herein. The 12 markers were: 92R7, M9, M13, M17, M20, SRY+465, SRY4064, SRY10831, sY81, Tat, YAP, and p12f2. PCR protocols for detection of these polymorphisms have been reported elsewhere (Thomas et al. 1999; Rosser et al. 2000). These 12 markers give rise to the following 10 observed haplogroups, labeled according to the Y Chromosome Consortium ( 2002) nomenclature: Y*(xBR,A3b2), BR*(xDE,JR), E*(xE3a), J, K*(xL,N3,O2b,P), L, N3, P*(xR1a), R1a*, and R1a1. We note that one of these is a very rare haplogroup (R1a*) that has previously been observed only in two Armenian men (Weale et al. 2001). We now report the same haplogroup in one additional Belarusian man. The Ashkenazi Levites were typed for a set of 25 biallelic markers including P9, M216, M217, YAP, P2, M35, P14, M201, P15, M52, P19, 12f2b, M172, M9, M20, LLY22g, Tat, M175, P27, P36, M207, M173, SRY10831, M17, and P25. PCR protocols for detection of these polymorphisms have been reported elsewhere (Hammer et al. 2001; Underhill et al. 2001). This set of biallelic markers allowed the designation of the Ashkenazi Levite chromosomes into the same 10 observed haplogroups. The exact labeling of the Ashkenazi Levite haplogroups, by use of all 25 biallelic markers, is reported in table A (online only). The German data are reported elsewhere (Capelli et al. 2003) and were not typed for M13, M20, SRY10831, and SRY+465 as these markers distinguish haplogroups (taking other typed markers into account) that are extremely rare in Europe and the Near East (Underhill et al. 2000). In place of YAP, SRY4064, and sY81, the German sample was typed for M35, as intermediate haplogroups (derived for YAP, ancestral for M35) are, again, very rare in this part of the world.
Haplotypes
We genotyped all samples for six STRs, DYS19 (equivalent to DYS394), DYS388, DYS390, DYS391, DYS392, and DYS393, according to protocols previously reported by Thomas et al. ( 1999). In the case of the Ashkenazi Levite samples, we also typed six additional STRs (DYS385a, DYS385b, DYS389I, DYS389II, DYS426, and DYS439), to allow more accurate estimation of the time to most recent common ancestor (TMRCA). PCR primers and conditions for the typing of the additional six STRs are as described by Redd et al. ( 2002), multiplexes I and II. PCR products were analyzed on ABI310 and ABI3100 DNA Analyzers, and fragment lengths were converted to repeat numbers by the use of allelic ladders. We define DYS389CD as equivalent to DYS389I, and we define DYS389AB as equivalent to DYS389II minus DYS389I (Rolf et al. 1998).
Statistical Analysis
Estimation of Time to Most Recent Common Ancestor
If the root haplotype is known, central estimates can be obtained of the TMRCA that are independent of population demography and shape of the genealogy and are dependent only on the mutation model assumed (Stumpf and Goldstein 2001). We obtained central estimates of the TMRCA under two models of microsatellite mutation. Under the simple stepwise mutation model (S-SMM), mutation rate is independent of microsatellite repeat number and mutations are equally likely to result in an increase or decrease in length. It can be shown that [התמונה הבאה מגיעה מקישור שלא מתחיל ב https ולכן לא הוטמעה בדף כדי לשמור על https תקין: http://www.journals.uchicago.edu/AJHG/journal/issues/v73n4/40097/img1.gif] = [התמונה הבאה מגיעה מקישור שלא מתחיל ב https ולכן לא הוטמעה בדף כדי לשמור על https תקין: http://www.journals.uchicago.edu/AJHG/journal/issues/v73n4/40097/img2.gif] / [התמונה הבאה מגיעה מקישור שלא מתחיל ב https ולכן לא הוטמעה בדף כדי לשמור על https תקין: http://www.journals.uchicago.edu/AJHG/journal/issues/v73n4/40097/img3.gif] is an unbiased estimator of the TMRCA (in generations), where [התמונה הבאה מגיעה מקישור שלא מתחיל ב https ולכן לא הוטמעה בדף כדי לשמור על https תקין: http://www.journals.uchicago.edu/AJHG/journal/issues/v73n4/40097/img2.gif] is the average squared difference in repeat length between each sampled NRY and the root haplotype, averaged over loci, and [התמונה הבאה מגיעה מקישור שלא מתחיל ב https ולכן לא הוטמעה בדף כדי לשמור על https תקין: http://www.journals.uchicago.edu/AJHG/journal/issues/v73n4/40097/img3.gif] is the S-SMM mutation rate (per generation) averaged over loci (Goldstein et al. 1995a, 1995b; Slatkin 1995). Locus-specific estimates of mutation rates are currently hampered by lack of data, so we chose to estimate a common mutation rate by combining data on all tri- and tetra-nucleotide NRY microsatellites for which direct data (including mean repeat sizes) are available. Combining the pedigree-based data of Bianchi et al. ( 1998), Forster et al. ( 1998), Heyer et al. ( 1997), and Kayser et al. ( 2000) leads to an estimate of [התמונה הבאה מגיעה מקישור שלא מתחיל ב https ולכן לא הוטמעה בדף כדי לשמור על https תקין: http://www.journals.uchicago.edu/AJHG/journal/issues/v73n4/40097/img3.gif] = 18/9347 = 0.00193. A more recent pedigree-based study estimated a higher mutation rate of 0.0042 for the same 12 STRs as were used for the Ashkenazi Levites (Bonne-Tamir et al. 2003), as did a study based on mutational analysis of single sperm cells (Holtkemper et al. 2001). In contrast, some other studies based on dating deep-rooting Y chromosome clades have suggested lower "evolutionary" mutation rates (Caglia et al. 1997; Forster et al. 2000). These considerations introduce additional uncertainty in the true value of [התמונה הבאה מגיעה מקישור שלא מתחיל ב https ולכן לא הוטמעה בדף כדי לשמור על https תקין: http://www.journals.uchicago.edu/AJHG/journal/issues/v73n4/40097/img3.gif] for our loci (Zhivotovsky et al. 2001). We allowed for some uncertainty in [התמונה הבאה מגיעה מקישור שלא מתחיל ב https ולכן לא הוטמעה בדף כדי לשמור על https תקין: http://www.journals.uchicago.edu/AJHG/journal/issues/v73n4/40097/img3.gif] when calculating confidence limits for the TMRCA, and we note that the length-dependent model described below may also accommodate some locus-specific variation in mutation rate (Carvalho-Silva et al. 1999). Two microsatellite loci typed in our study, DYS385a and DYS385b, are homologous, so that reported repeat lengths cannot be assigned unambiguously to this locus. This problem is overcome by adding the two repeat lengths together and treating this as a single "locus" with a mutation rate of 2 [התמונה הבאה מגיעה מקישור שלא מתחיל ב https ולכן לא הוטמעה בדף כדי לשמור על https תקין: http://www.journals.uchicago.edu/ucp-entities/mu.gif] .
The second microsatellite mutation model we used was a linear length dependent stepwise mutation model (LLD-SMM). Under this model, motivated by a proportional slippage mutation mechanism, the mutation rate is determined by the linear equation [התמונה הבאה מגיעה מקישור שלא מתחיל ב https ולכן לא הוטמעה בדף כדי לשמור על https תקין: http://www.journals.uchicago.edu/ucp-entities/mu.gif] = a + bL, where L is the current repeat size. Length-dependency has been proposed as one possible explanation for large inter-locus variability in Y chromosome microsatellites (Forster et al. 2000). It can be shown that [התמונה הבאה מגיעה מקישור שלא מתחיל ב https ולכן לא הוטמעה בדף כדי לשמור על https תקין: http://www.journals.uchicago.edu/AJHG/journal/issues/v73n4/40097/img1.gif] = [התמונה הבאה מגיעה מקישור שלא מתחיל ב https ולכן לא הוטמעה בדף כדי לשמור על https תקין: http://www.journals.uchicago.edu/AJHG/journal/issues/v73n4/40097/img2.gif] / [התמונה הבאה מגיעה מקישור שלא מתחיל ב https ולכן לא הוטמעה בדף כדי לשמור על https תקין: http://www.journals.uchicago.edu/AJHG/journal/issues/v73n4/40097/img3.gif] 0 is an unbiased estimator of the TMRCA, where [התמונה הבאה מגיעה מקישור שלא מתחיל ב https ולכן לא הוטמעה בדף כדי לשמור על https תקין: http://www.journals.uchicago.edu/AJHG/journal/issues/v73n4/40097/img2.gif] is as defined above and [התמונה הבאה מגיעה מקישור שלא מתחיל ב https ולכן לא הוטמעה בדף כדי לשמור על https תקין: http://www.journals.uchicago.edu/AJHG/journal/issues/v73n4/40097/img3.gif] 0 is the mutation rate for the root haplotype, averaged over loci (Calabrese et al. 2001). Again, due to limited data we assume common values for a and b over all loci, and use the estimates reported by Stumpf and Goldstein ( 2001), anchored by assuming [התמונה הבאה מגיעה מקישור שלא מתחיל ב https ולכן לא הוטמעה בדף כדי לשמור על https תקין: http://www.journals.uchicago.edu/ucp-entities/mu.gif] = 0.00193 when L = 15.77 (from the direct data, described above, of Bianchi et al. [1998], Forster et al. [1998], Heyer et al. [1997], and Kayser et al. [2000]). This results in estimates of a = -4.03 × 10-3 and b = 3.78 × 10-4. The two homologous loci DYS385a and DYS385b were accommodated by adding their repeat lengths together into a single value of L and treating them as a single "locus" with [התמונה הבאה מגיעה מקישור שלא מתחיל ב https ולכן לא הוטמעה בדף כדי לשמור על https תקין: http://www.journals.uchicago.edu/ucp-entities/mu.gif] = 2 a + bL. If negative values of [התמונה הבאה מגיעה מקישור שלא מתחיל ב https ולכן לא הוטמעה בדף כדי לשמור על https תקין: http://www.journals.uchicago.edu/ucp-entities/mu.gif] were produced, these were treated as zero.
Unlike the central estimates, CIs on the TMRCA are dependent on population demography, regardless of the mutation model used. These were found by Monte Carlo simulation, using a coalescent-with-exponential-growth model scaled in units of the TMRCA, incorporating uncertainty in mutation rate, and implemented in the program Ytime.
Estimation of genetic diversity and distance
Genetic diversity was measured using Nei's unbiased h statistic (Nei 1987). Tests for population differentiation were performed using the exact test of Raymond and Rousset ( 1995). Two common measures of genetic distance are FST (Reynolds et al. 1983) and 1- I, where I is Nei's Genetic Identity. We found a poor correlation between FST and 1- I for the population samples analyzed in this study ( r = 0.383 among the 10 population data sets using haplotype frequencies and using all 45 pairwise combinations). We propose that 1- I is a better measure of genetic distance (in the sense of a metric summarizing useful aspects of interpopulation differences in gene frequency), for comparisons among populations where recent admixture is a likely demographic model. This is because if two different populations A and B admix to form a population C, the expected immediate I value between C and either A or B is dependent only on the admixture proportion between A and B, the I value between A and B, and the relative gene diversities (via the ratio (1 - hA)/(1 - hB)), whereas the expected FST value is dependent not only on these factors but also on the absolute diversities of A and B. This results in an unwanted dependency of FST on the absolute gene diversities of A and B ( FST is larger when the gene diversities are smaller, because FST is a measure of between-population diversity divided by the sum of between- and within-population diversities). This unwanted dependency is avoided by the use of 1- I. We note that the same arguments apply to Nei's Genetic Distance, D = - log( I), as to 1- I, since there is a monotonic relationship between the two. We found the same conclusions when we analyzed our data using D as we did using 1- I.
Visualization of genetic-distance patterns
We visualized patterns of genetic differentiation using Principal Coordinates Analysis, performed on I to form the similarity matrix. Values along the main diagonal of the similarity matrix, representing the similarity of each population sample to itself, were calculated from the estimated genetic distance between two copies of the same population sample (for I values, the resulting self-similarity values simplify to n/(n - 1), where n is the sample size).
Results
The haplogroup frequencies found in the six Jewish and four non-Jewish data sets are shown in table 1. The table confirms the presence of R1a1 as the modal haplogroup in the Ashkenazi Levite Jews (52% of chromosomes). This haplogroup is found at similarly high frequencies in the two Slavonic-speaking populations (Sorbians and Belarusians), but at a maximum frequency of only 5.8% among the other five Jewish data sets (mean frequency 3.2%). Pairwise genetic similarities (Nei's genetic identity I) and P values for population differentiation at the haplogroup level are shown in table 2, and figure 1 displays the principal coordinates plot derived from the genetic similarity values. Figure 1 reflects the unusual distribution of Ashkenazi Levite haplogroup frequencies. The other Jewish data sets separate according to whether they are Cohanim or non-Cohanim, and, within these two groups, there are no significant differences between Ashkenazi and Sephardi data sets (all tests for population differentiation performed using the exact test of Raymond and Rousset ( 1995). The genetic similarities among these five Jewish data sets range from 0.79 to 1.0. In contrast, the Ashkenazi Levites cluster more with the Slavonic data sets than they do with the other Jewish data sets. The genetic similarities with the other Jewish data sets range from 0.22 to 0.47, whereas the I values with the Sorbian and Belarusian data sets are 0.95 and 0.88, respectively. When a bootstrap test is used, the I value for Ashkenazi Levites with Sorbians and Belarusians is, in both cases, significantly higher than the I value for Ashkenazi Levites with Sephardi Israelites, the most similar Jewish data set to the Ashkenazi Levites ( P = .004 for Sorbians and .008 for Belarusians).
At the haplotype level (combining UEP information with the six microsatellite loci), significant differences in haplotype frequency emerge between all data sets ( table 3), but the same underlying patterns of similarity remain ( fig. 2). Full haplotype information for all data sets is available in supplemental tables A and B (online only). The genetic similarities between the Ashkenazi Levites and the other Jewish data sets range from 0.04 to 0.18, whereas the I values with the Sorbian and Belarusian data sets are 0.52 and 0.49 respectively. Using a bootstrap test, the I value for Ashkenazi Levites with Sorbians and Belarusians is again significantly higher in both cases than the corresponding I value for Ashkenazi Levites with their most similar Jewish group, the Ashkenazi Israelites ( P = .006 for Sorbians and .004 for Belarusians).
Within haplogroup R1a1, the microsatellite haplotypes found in the AL data set are tightly clustered around a modal haplotype (16-12-25-10-11-13) that comprises 74% of Ashkenazi Levites within this haplogroup, and 38% of Ashkenazi Levites overall ( table 4). This modal haplotype is evenly distributed across the geographically defined communities from which the Ashkenazi Levite sample was taken (see the " Subjects and Methods" section and table A), so that clues to its origin could not be found from these data. The very high frequency of this modal haplotype makes the genetic diversity of Ashkenazi Levite NRYs within the R1a1 haplogroup much lower than in the non-Jewish comparative data sets in which R1a1 is found at high frequency ( table 5). Bootstrap tests indicate that, within R1a1, h is significantly lower in AL than in the Sorbian, Belarusian, and Norwegian data sets ( P < .001 in each case). The P value for the comparison with the German data set is of borderline significance ( P = .064), but the number of R1a1 chromosomes within this data set is small ( n = 11), resulting in a larger sampling variance. The additional six microsatellites genotyped for Ashkenazi Levites only reduce the modal haplotype frequency within the R1a1 haplogroup to 58%, also confirming the high degree of haplotype homogeneity ( table 6). As is explained in the " Discussion" section, this arrangement of R1a1 haplotypes within the Ashkenazi Levite data set is consistent with common descent from a recent ancestor and is unlikely to have resulted from a large number of founding lineages. By assuming that the most recent common ancestor (MRCA) possessed the modal haplotype and by using a male intergeneration time of 25 years, we estimated a mean TMRCA of 663 years before present under the Simple Stepwise Mutation Model and a mean time of 1,000 years before present under the Linear Length-Dependent Stepwise Mutation Model (see the " Subjects and Methods" section). These dates coincide with the historically estimated time when compact settlements of Jews in northwest Europe began and the important religious communities of Mainz and Worms were established (Encyclopaedia Judaica 1972), but they also come with very wide CIs. If we assume a genealogy generated under no population growth, the 95% CIs are 0 [התמונה הבאה מגיעה מקישור שלא מתחיל ב https ולכן לא הוטמעה בדף כדי לשמור על https תקין: http://www.journals.uchicago.edu/ucp-entities/ndash.gif] 2,425 years before present under the simple stepwise mutation model, and 0 [התמונה הבאה מגיעה מקישור שלא מתחיל ב https ולכן לא הוטמעה בדף כדי לשמור על https תקין: http://www.journals.uchicago.edu/ucp-entities/ndash.gif] 3,672 years before present under the linear length-dependent stepwise mutation model. However, the Ashkenazi Jewish population has undergone rapid population growth in the past 1,000 years, estimated at a 17.5% growth rate per generation in the Ashkenazi Jewish population over this period (DellaPergola 2001). The starlike pattern for the R1a1 haplotypes for AL is consistent with this pattern of population expansion for the Ashkenazi Jewish population in general. By use of this estimate of 17.5% for growth and under the assumption of 4.5 million male Ashkenazi Jews in 1939, a 1 : 5 ratio of effective population size to census population size due to reproductive variance and the fact that Ashkenazi Levites make up 4% of Ashkenazi Jews, the 95% CI is reduced substantially to 244 [התמונה הבאה מגיעה מקישור שלא מתחיל ב https ולכן לא הוטמעה בדף כדי לשמור על https תקין: http://www.journals.uchicago.edu/ucp-entities/ndash.gif] 1,570 years before present (BP) under the simple stepwise mutation model, and 375 [התמונה הבאה מגיעה מקישור שלא מתחיל ב https ולכן לא הוטמעה בדף כדי לשמור על https תקין: http://www.journals.uchicago.edu/ucp-entities/ndash.gif] 2,248 years BP under the linear length-dependent stepwise mutation model. It should be noted that our estimations of the TMRCA rely on pedigree mutation rates. It has been argued that "evolutionary" mutational rates may occur at a lower rate (Caglia et al. 1997; Forster et al. 2000). If so, the TMRCA is underestimated by our method. On the other hand, if our assumption of a single founder is false, then the TMRCA of this clade would overestimate the founding event. An alternative method for dating the founding event, again under the assumption of a single founder, derives from consideration of the fact that it is easier to affect a large frequency change in the Ashkenazi Levite male gene pool when it is small than when it is large. We allowed for a possible initial reproductive advantage to the founder [התמונה הבאה מגיעה מקישור שלא מתחיל ב https ולכן לא הוטמעה בדף כדי לשמור על https תקין: http://www.journals.uchicago.edu/ucp-entities/mdash.gif] for example, due to elevated wealth or status [התמונה הבאה מגיעה מקישור שלא מתחיל ב https ולכן לא הוטמעה בדף כדי לשמור על https תקין: http://www.journals.uchicago.edu/ucp-entities/mdash.gif] such that he contributed, at most, 100 grandsons to the Ashkenazi Levite population (i.e., 20 grandsons to the effective population size, under the assumption of a 1 : 5 ratio, as above), after which time the differential reproductive fitness was lost. As above, we assumed a general Ashkenazi Levite growth rate of 17.5% per generation and an Ashkenazi Levite effective population size of 36,000 in 1939. Performing coalescent simulations with exponential growth, we found that the 20 grandsons needed to be added at least 35 generations ago to provide a chance of >5% that 31 of 60 modern Ashkenazi Levites sampled would descend from this founding group. Under the assumption of a male intergeneration time of 25 years, this implies an original founding event that occurred at least 990 years BP (925 years before 1939). Taken together, these separate time estimates provide a timeframe that coincides with the initial formation and early settlement of the Ashkenazi Jewish population. Finally, we used the same growth model to investigate how many males could have moved into the Ashkenazi Levite population at the same time as the founder and yet have at least a 5% chance of leaving no descendents in a sample of 60 Ashkenazi Levites today. Because we allow for a potentially very small effective population size, it is possible for an introgression to comprise a sizeable proportion of the Ashkenazi Levite NRY gene pool at the time and yet correspond to only a relatively small number of individuals. If the event took place 37 generations before 1939 (990 years BP), such "silent" introgressors could have made up as much as 19% of the Ashkenazi Levite gene pool, equivalent to an addition of 50 men to the existing Ashkenazi Levite population. The corresponding figures for 45 generations before 1939 (1,190 years BP) and 55 generations before 1939 (1,440 years BP) are 53% (33 men) and 94% (10 men).
Discussion
Our study confirms the previously reported finding of a caste-specific high-frequency haplogroup within the Ashkenazi Levites (Thomas et al. 1998). The presence of this haplogroup, R1a1, within Ashkenazi Levites is striking for several reasons. Firstly, it is found at high frequency in the Ashkenazi Levites but not in Sephardi Levites or any other Jewish grouping examined so far. This means that in paternal ancestry Ashkenazi and Sephardi Levites are genetically dissimilar, unlike Ashkenazi and Sephardi Cohanim ( fig. 1). The Ashkenazi and Sephardi Israelites are also relatively similar to each other, which is consistent with the previous reports of shared overall paternal Near Eastern ancestries for these populations (Hammer et al. 2000; Nebel et al. 2000). Secondly, the microsatellite haplotypes within this haplogroup form a tight cluster within the Ashkenazi Levites, indicative of very recent origin from a single common ancestor. Thirdly, the haplogroup is extremely rare in other Jewish groups and in non-Jewish groups of Near Eastern origin, but is found at high frequency in populations of eastern European origin. This contrasts with the Cohen Modal Haplotype, for example, which belongs to a haplogroup that is more likely to be of Near Eastern origin. Finally, the haplogroup represents [התמונה הבאה מגיעה מקישור שלא מתחיל ב https ולכן לא הוטמעה בדף כדי לשמור על https תקין: http://www.journals.uchicago.edu/ucp-entities/sim.gif] 50% of all Ashkenazi Levite NRYs, which indicates that, as a group, the Ashkenazi Levites have heterogeneous origins in comparison with the Cohanim, who are dominated by a single haplogroup and whose origins are consistent with an event antedating the Diaspora.
The greatly elevated frequency of haplogroup R1a1 only within the Ashkenazi Levites suggests an event specific to the Ashkenazi Levites. This is borne out by the fact that the haplogroup is virtually absent from the Sephardi Levites, which would indicate an event that occurred after the separate formation of the Ashkenazi and Sephardi groupings, and also by the very recent inferred date for the common ancestor of the NRYs within this haplogroup in the Ashkenazi Levites, which also supports an event occurring after the Ashkenazi split from other Jewish populations.
The pattern of microsatellite haplotype diversity within haplogroup R1a1 in the Ashkenazi Levites suggests that any founding event is unlikely to have involved a large number of founding lineages contributing to today's Ashkenazi Levite NRY gene pool. If large numbers were involved, then the lack of microsatellite diversity would imply that all the founding lineages were very closely related to each other. Furthermore, under this scenario the microsatellite diversity seen today would partly result from diversity among the founding lineages, in which case the evidence would suggest a founding event even more recent than that estimated above, under the assumption of a single founding individual. Although a more recent event is not impossible, the current range of estimated dates fits very nicely with the known origins of the Ashkenazi, a time when the estimated population size was small and which would therefore make it easier for the descendents of a single founder to expand to high frequency within the population, as our simulations demonstrate. Furthermore, if there had been a large influx of founder lineages, the haplotype frequency distribution of the source population would be expected to match the haplotype frequency distribution found in Ashkenazi Levites today. Such a matching source population has yet to be identified. In the case of the two non-Jewish populations of Eastern European origin examined in this study, neither the Sorbians nor the Belarusians would be suitable candidates. This is because, in both cases, the modal haplotype within haplogroup R1a1 for the Sorbians and the Belarusians is either completely absent or found only as a singleton within the Ashkenazi Levite sample ( table 4).
Although the number of founding lineages contributing to the current Ashkenazi Levite NRY gene pool appears to be small and could involve only a single founding male, it is possible that the founding event also coincided with a larger introgression event but subsequent drift led to the loss of these other lineages from the NRY gene pool. Our simulations suggest that up to 50 men could have been involved in such an event. This result assumes a 5 : 1 ratio of real : effective population size, and there are processes that could make this ratio even more pronounced (see, for example, the work of Gagnon and Heyer [ 2001]). However, this would result in such small effective population sizes and so much resultant drift that one would not expect to find the close NRY genetic similarity observed between Ashkenazi and Sephardi Cohanim, who make up a percentage of the Jewish population very similar to the Levites'. Two further arguments against a recent large introgression of non-Jews into the Ashkenazi Levite caste are: ( a) it would have to breach a well-regulated rabbinically controlled barrier, and ( b) it would most likely leave some prominent trace in the historical record [התמונה הבאה מגיעה מקישור שלא מתחיל ב https ולכן לא הוטמעה בדף כדי לשמור על https תקין: http://www.journals.uchicago.edu/ucp-entities/mdash.gif] which it has not.
For the reasons stated above, it is likely that the event leading to a high frequency of R1a1 NRYs within the Ashkenazi Levites involved very few, and possibly only one, founding father. A question, then, arises regarding the possible origins of the founder(s). Haplogroup R1a1 is found at very low frequency in other Jewish groups ( table 1). It is possible, therefore, that this haplogroup was also present at very low frequency in the Ashkenazi Levites, before some event specific to the Ashkenazi Levites occurred that led to a founder who, by chance, had this very rare haplogroup and whose descendents became very numerous within the Ashkenazi Levites. Likewise, the haplogroup is also found at very low frequency within some non-Jewish populations of Near Eastern origin (data not shown). It is therefore also possible that a conversion event prior to the formation of the Ashkenazi grouping led to the founding of this haplogroup and its emergence at high frequency within the Ashkenazi Levites. Although it is not possible to formally refute either of these two possible explanations, it would be a remarkable coincidence that the geographic origins and demographic expansion of the Ashkenazi are within Northern and Eastern Europe and that this haplogroup is found at very high frequency within neighboring non-Jewish populations of European origin but not at high frequency elsewhere. An alternative explanation, therefore, would postulate a founder(s) of non-Jewish European ancestry, whose descendents were able to assume Levite status.
If a European origin for the Ashkenazi Levite haplogroup R1a1 component is accepted as a reasonable possibility, it is of interest to speculate further on the possible timing, location, and mechanism of this event. Because the modal haplotype of haplogroup R1a1 found in the Ashkenazi Levites is found at reasonably high frequency throughout the eastern European region, it is not possible to use genetic information to pinpoint the exact origin of any putative founder from the currently available data sets. Intriguingly, the Sorbian tongue, relexified with a German vocabulary, has been proposed as the origin of Yiddish, the language of the Ashkenazim, but there has been no suggestion of an association between Ashkenazi Levites in particular and the Sorbian language. One attractive source would be the Khazarian Kingdom, whose ruling class is thought to have converted to Judaism in the 8th or 9th century (Dunlop 1967). This kingdom flourished between the years 700 C. E. and 1016 C. E. It extended from northern Georgia in the south to Bulgar on the Volga River in the north and from the Aral Sea in the east to the Dnieper River in the west [התמונה הבאה מגיעה מקישור שלא מתחיל ב https ולכן לא הוטמעה בדף כדי לשמור על https תקין: http://www.journals.uchicago.edu/ucp-entities/mdash.gif] an area that falls within a region in which haplogroup R1a1 NRYs are found at high frequency (Rosser et al. 2000). Archival material also records migration of Khazars into the Hungarian Duchy of Taskony in the 10th century. The break-up of the Khazar Empire following their defeat by invading Rus led to the flight of some Khazars to central and northern Europe. Although neither the NRY haplogroup composition of the majority of Ashkenazi Jews nor the microsatellite haplotype composition of the R1a1 haplogroup within Ashkenazi Levites is consistent with a major Khazar or other European origin, as has been speculated by some authors (Baron 1957; Dunlop 1967; Ben-Sasson 1976; Keys 1999), one cannot rule out the important contribution of a single or a few founders among contemporary Ashkenazi Levites.
Finally, it is interesting to speculate on the possible mechanism by which the descendant of a non-Jew or convert could have acquired Levite status. The fact that Ashkenazi Cohanim NRYs show no evidence for an introgression of this nature suggests a lesser degree of stringency for the assumption of Levite status than for the assumption of Cohen status. This may be because there are more rights and duties associated with the Cohen status than with that of the Levite, leading to more rigorous protection of the former. Cohanim, for example, are called upon, on special occasions, to bless the assembled congregation and are prohibited from marrying divorcees and converts, religious laws that do not apply to Levites. Indeed, Talmudic sources may possibly be interpreted to support the notion of differences in the social, religious, and legal barriers that relate to the assumption of Cohen and Levite status. These include descriptions of the possible assumption of Levite status other than through patrilineal descent, in a Talmudic passage describing a debate regarding the potential assignment of Levite status to a man (and his descendants) whose father was a non-Jew and whose mother was the daughter of a Levite. Such differences could have provided the backdrop for the sanctioned acceptance of Levite status other than through patrilineal descent.
The comparative study of patterns of NRY variation among Ashkenazi Jews and other populations has revealed evidence for an unexpected and unusual historical event, which was not appreciated using other, more conventional historical approaches. This finding may motivate historians and social scientists to seek further information regarding the possibility of such an event and, more generally, to include information gleaned from studies of DNA variation in the repertoire of tools used to uncover historical events of interest.
Acknowledgments
We wish to express our gratitude to the individuals who very kindly provided samples for this study. We thank M. A. Strøksnes, for help in sampling; C. Capelli, for generously providing data prior to publication; and Y. Weiner, for helpful input.
Electronic-Database Information
Accession numbers and URLs for data presented herein are as follows:
References
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