Diversity of mitochondrial D-loop haplotypes from ancient Thracian horses in Bulgaria

The domestication of the horse began possibly more than 5000 years ago in the western part of the Eurasian steppe, and according to the leading hypothesis, horses first spread from the Steppe toward the region of the Thracian culture, starting in the second half of the 2nd millennium BCE and flourished from the fifth to first centuries BCE, mainly located in present-day Bulgaria. We analyzed 17 horse bone remains excavated from Thracian archaeological sites (fourth to first centuries BCE) in Bulgaria and successfully identified 17 sequences representing 14 different haplotypes of the mitochondrial D-loop. Compared with the mtDNA haplotypes of modern horses around the world, ancient Thracian horses in Bulgaria are thought to be more closely related to modern horses of Southern Europe and less related to those of Central Asia. In addition, the haplotypes we obtained represented 11 previously reported modern horse mtDNA haplogroups: A, B, D, E, G, H, I, L, N, P, and Q. All the haplogroups contain modern and regionally predominant haplotypes occurring in Europe, the Middle East, and Central Asia. Our results indicate that Thracian horses in Bulgaria have had relatively high genetic diversity and are closely related to modern horse breeds.