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Population Genetics of CAT

There were several CAT population genetic studies done on current people as well as skeletal remains (ancient DNA) dating back to the 15th century BC to the 5th century AD. They mainly focused on the analysis of the non-recombining portion of the Y-chromosome (NRY) and the mitochondrial DNA (mtDNA) with only a few studies investigating autosomal microsatellite loci (1-21). These studies have all demonstrated that genetic diversity in Central Asia is among the highest in Eurasia, and the CAT region could be the source of at least three major waves of migration that led to Europe, India, and the Americas. There is a presence of well-differentiated east Eurasian and west Eurasian genetic lineages in all CAT populations studied.

 

Thus, west and east Eurasian mitochondrial haplogroups were found in CAT populations in various proportions, but frequently ranging between 55% (western) and 41% (eastern) (22-24) (Figure 4). Additionally, there is considerable Y-chromosome haplogroup variation in CAT (25) (Figure 5). Admixture analysis based on autosomal markers showed a good representation of West Eurasian, East Asian, South Asian, and Siberian ancestries in Central Asian populations (26) (Figure 6). The presence of western and eastern mtDNA sequences, found in CAT has suggested two scenarios for the admixture of lineages: firstly, Central Asia was probably a “hybrid zone”, where previously inhabited western populations were gradually replaced by eastern populations to some extent; secondly, Central Asia was probably a “contact zone” between previously differentiated Asian and European populations after their initial expansions (11, 22, 24, 26). CAT populations experienced admixture from various ethnic groups since ancient times, but in the current times, they kept admixing mainly between the local tribes leading to the modern CAT identity.

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mt-DNA haplogroups in CAT (Adapted from Røyrvik,et al, 2018)

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The distribution of Y-chromosome haplogroups in CAT (Adapted from Zerjal et al., 2002)

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Figure 6. Ancestry makeup and variations in admixture proportion. ADMIXTURE results of West China with West Eurasia, South Asia, East Asia, and Central Asia Siberia populations at K¼4 (in order to show a full picture of Eurasian populations, all populations of the four major ancestries (including representative populations) were included.) (Adapted from Feng et al, 2017)

Today, the Central Asian populations are divided into two cultural and linguistic groups: The Indo-Iranian (Tajiks) and the Turkic groups. Turkic groups are traditionally pastoral populations, who tend to practice exogamous marriage (a man chooses a bride in a different lineage or clan-patrilineal populations), whereas the Indo-Iranian groups are farmer populations often practicing endogamous marriages with cousins (6, 9, 25). Albeit there has been a transition in the lifestyle of Uzbeks, Azerbaijani, and Turkmens who used to be pastoral nomads and have later gradually adopted a sedentary way of lifestyle from around the 15th century. Consequently, due to the differences in the social organization and lifestyle of CAT populations, currently, we observe high consanguinity rates among Uzbeks, Azeri, Tajiks, and Turkmens and almost non-existent consanguinity among Kazakhs and Kyrgyz. Studies showed that pastoral populations in Central Asia exhibit a substantial loss of Y chromosome diversity in comparison to farmers but that no such a difference is observed at the mt-DNA level (13).

 

Lower genetic differentiation was found among Turkic-speaking populations despite their wide geographic distribution, which suggests a more recent common origin of these populations as compared with Indo-Iranian-speaking populations (13, 26) . A high level of autosomal differentiation and a low level of Y chromosome differentiation was present among Turkic people.

The available studies suggest that CAT populations are genetically unique and diverse due to the considerable admixture of western and eastern Eurasian lineages. Firstly, this highlights the importance of CAT in elucidating global human genetic variation and ensuing disease diversity (22, 24, 26). Secondly, this genetic diversity provides a valuable resource for the study of rare genetic diseases and can inform the development of new treatments and cures.

References:

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14. Pagani L, Lawson DJ, Jagoda E, et al. Genomic analyses inform on migration events during the peopling of Eurasia. Nature. 2016;538(7624):238-242. doi:10.1038/nature19792

15. Feng Q, Lu Y, Ni X, et al. Genetic History of Xinjiang's Uyghurs Suggests Bronze Age Multiple-Way Contacts in Eurasia. Mol Biol Evol. 2017;34(10):2572-2582. doi:10.1093/molbev/msx177

16. Narasimhan VM, Patterson N, Moorjani P, et al. The formation of human populations in South and Central Asia. Science. 2019;365(6457):eaat7487. doi:10.1126/science.aat7487

17. Jeong C, Balanovsky O, Lukianova E, et al. The genetic history of admixture across inner Eurasia. Nat Ecol Evol. 2019;3(6):966-976. doi:10.1038/s41559-019-0878-2

18. Seidualy M, Blazyte A, Jeon S, et al. Decoding a highly mixed Kazakh genome. Hum Genet. 2020;139(5):557-568. doi:10.1007/s00439-020-02132-8

19. Zhabagin M, Sabitov Z, Tarlykov P, et al. The medieval Mongolian roots of Y-chromosomal lineages from South Kazakhstan. BMC Genet. 2020;21(Suppl 1):87. Published 2020 Oct 22. doi:10.1186/s12863-020-00897-5

20. Kairov U, Molkenov A, Rakhimova S, et al. Whole-genome sequencing data of Kazakh individuals. BMC Res Notes. 2021;14(1):45. Published 2021 Feb 4. doi:10.1186/s13104-021-05464-4

21. Khussainova E, Kisselev I, Iksan O, et al. Genetic Relationship Among the Kazakh People Based on Y-STR Markers Reveals Evidence of Genetic Variation Among Tribes and Zhuz. Front Genet. 2022;12:801295. Published 2022 Jan 7. doi:10.3389/fgene.2021.801295

22. Comas D, Calafell F, Mateu E, et al. Trading genes along the silk road: mtDNA sequences and the origin of central Asian populations. Am J Hum Genet. 1998;63(6):1824-1838. doi:10.1086/302133

23. Quintana-Murci L, Chaix R, Wells RS, et al. Where west meets east: the complex mtDNA landscape of the southwest and Central Asian corridor. Am J Hum Genet. 2004;74(5):827-845. doi:10.1086/383236

24. Ellen C. Røyrvik, Nadira Yuldasheva, Susan Tonks, et al. Genetic patterning in Central Eurasia: population history and pigmentation. bioRxiv 255117; doi: https://doi.org/10.1101/255117

25. Zerjal T, Wells RS, Yuldasheva N, Ruzibakiev R, Tyler-Smith C. A genetic landscape reshaped by recent events: Y-chromosomal insights into central Asia. Am J Hum Genet. 2002;71(3):466-482. doi:10.1086/342096

26. Martínez-Cruz B, Vitalis R, Ségurel L, et al. In the heartland of Eurasia: the multilocus genetic landscape of Central Asian populations. Eur J Hum Genet. 2011;19(2):216-223. doi:10.1038/ejhg.2010.153

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