Friday, January 11, 2013

Galilean Druze X2 mtDNA

Lineage analysis within the mtDNA X-haplogroup of the Galilee Region is particularly enlightening. It should be noted that the estimated coalescence times for the major mtDNA X subhaplogroups X1 and X2 are 42,900±18,100 and 17,900±2,900 respectively. It is striking that those different lineages (from the same parental haplogroup) whose genetic divergence date back more than ten thousand years would remain so concentrated within such a small geographic region. Mutation rates for the mtDNA coding region, are not consistent with the possibility that this number of different coding region defined lineages within haplogroup X could have resulted from the recent expansion of a monophyletic clade within the past 1000 years. Rather this combination among the Druze, of a large number of lineages, together with a high frequency of the haplogroup in which these lineages are found, suggests descent from an ancestral population, in which the X haplogroup was more abundant than it is in the contemporary Near East, and which reflects the prevailing Near East genetic landscape at that time, antedating the establishment of the Druze religion in 1017 A.C.E. This supports the notion that the Druze represent a refugium of the population genetic structure from the time period prior to the “Dawa”, and also confirms the hypothesis of high endogamy among the Druze. The refugium hypothesis based on mtDNA haplogroup X analysis was corroborated by the finding of high diversity for the Druze mtDNA haplogroups H and K, with the added finding of novel lineages not shared with nearby populations.

Furthermore, the formal rejection of the alternate hypothesis relating to immigration to the region of individuals sharing the same mtDNA haplogroup but with lineages that diverged in antiquity, further strengthens the Druze refugium model. Although, we cannot exclude the possibility that some ratio between non-random migration and colonization did occur during and following the “Dawa” period and generated the differences in haplogroup frequencies among the current Druze subregions, and between the Druze and other populations, this explanation is highly unlikely in face of the demographic modeling results. The overall low migration rate between the Druze and all other nearby populations cannot explain the high diversity and high frequency of X haplogroup lineages in the Galilee region. Low migration rates were also evident between the Galilee Druze and Druze from other subregions. The finding of the enrichment of the NRY haplogroup K among the Galilee Druze with no detection in samples from other subregions, further supports the relative isolation of this region, even among the Druze. Taken together these findings support the hypothesis that the Galilee Druze are a further more isolated subpopulation of the Druze, who in turn represents a refugium of the population genetic architecture of the Near East in antiquity.

Demographic modeling can also provide estimates of divergence times for populations with shared ancestries. The demographic modeling in the current study indicates most recent divergence of the Druze from an ancestral population shared with Egyptians, Ashkenazi Jews, Adygeis and Greeks. The Egyptian shared ancestry is also consistent with Druze oral tradition. The migration rates of the Druze with these populations are exceedingly low, and this can be attributed to endogamy and geographic isolation following divergence. It should be kept in mind however, that the computational algorithms used for the demographic modeling are designed for models involving an ancestral population which split and maintained constant migration rate among the two daughter populations. Such a simplified model does not take into account the effect of shared party migration, and therefore would tend to overestimate the migration rate, and underestimate the divergence time. Therefore the Druze would seem to have an even greater degree of genetic isolation, than indicated by these results of the demographic modeling.

The historical events and time frame for the loss or dilution of haplogroup X individuals is consistent with the population upheavals and patterns of migration that have characterized the Near East during the past two millennia at least. The preservation of this refugium of mtDNA lineage diversity among the Druze, mainly due to genetic isolation may be the result of their location in relatively more defensible mountainous regions, and the practice of conciliation with governing authorities and dissimulation called the “Taqiyya”, or due to other factors which facilitated preservation of societal integrity during periods of demographic and political change in the region.

Conclusions : The findings suggest that the Near East maternal genetic landscape differed substantially in the past from its current structure, and was enriched in diverse lineages of the mtDNA X haplogroup. These findings have been uncovered due to the unique demographic features of the Druze population, and the adjusted sampling method employed in the current study. The combination of a high frequency and diversity of the Druze mtDNA haplogroup X lineages, in a confined geographic region, and the low migration rate with nearby populations make it unlikely that this diversity was imported. It is thus likely that the global diversity of this haplogroup evolved in the Near East and adjacent regions of western Eurasia, during a long incubation period coinciding with and following the most recent out of Africa expansion as dated by mtDNA coalescence simulations[44]. The Druze population of the Galilee represents a contemporary refugium of this past genetic landscape. More ...

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