MtDNA haplogroups U3 and X2 were identified in a new study of Tarim Basin inhabitants in the Taklamakan desert (Chinese Turkestan, aka Xinjiang).
Haplogroup U3 is common all around the Black Sea, which made me think that it was one of the main mt-haplogroups associated with Y-haplogroup R1b1b2. R1b1b2 has the highest genetic diversity in northern Anatolia and the Caucasus. My theory is that R1b migrated to the North Caucasus and the Pontic steppe circa 4500 BCE, and the resulting merger with local R1a steppe people gave birth to Proto-Indo-European culture.
Haplogroup X2 has long been a mystery due to its amazingly widespread distribution over all Europe, Russia, Central Asia, Siberia and as far as North America. Its only major peak of frequency is around the Caucasus, indicating that at least one/some subclade originated in that region (probably not the North American X2a). The close contact between steppe people and the Caucasus would have instilled X2 in Proto-Indo-European speakers.
The discovery of U3 and X2 in the Tarim Basin, where R1b1b and R1a1a make up roughly half of the male lineages, confirm the likelihood of a Indo-European link with these two haplogroups. The famous Tarim mummies proved the presence of faired hair Caucasians in this part of Central Asia at least 3800 years ago, during the Indo-European expansion.
Other studies had already found mt-haplogroups H, T, U2e and U4 among the Iron Age population of Xinjiang. All of them are still frequent in the Pontic-Caspian region nowadays. Haplogroup U2 was found in the 30,000 year-old Kostenki skeleton from the middle Volga. Most U2 individuals are now Indian. U2 is actually the only major "European" mt-haplogroup in the Indian subcontinent, along with W (and only traces of H and T). Both U2 and W are much more common in and around European Russia than the rest of Europe nowadays.
Haplogroup H and T being so common all over Europe and the Middle East today, it is difficult to estimate how much of it is of Indo-European origin. H was indubitably found among Palaeolithic Europeans, Russians and Near Easterners. Only a detailed analysis of subclades can shed more light on the problem. So far I would say that H2a1, H5a, H7 and H8 seem to be the best candidates for an Indo-European origin (Caucasus-Anatolia, so mirroring R1b1b more than R1a1a).
Here is a summary of mtDNA haplogroups that were probably part of the Indo-European migrations during the Bronze Age.
R1a1a correlates best with mt-haplogroups: H1b, H1c, H2a1, H6, H7, K1b1b, K1c, K2b, T1a1a1, T2a1b1, T2b2, T2b4, U2e, U4, U5, and some W subclades (W3, W4, W5, W6). Minor mt-haplogroups also include C4a, C5, H27 and V7a.
R1b1b correlate best withmt-haplogroups: H5a, H8, H15, I1a1, J1b1a, K1a3, K2a6, U5 and some V subclades (like V15). Minor mt-haplogroups also include U3 and X2.
Some H1 and T2 subclades might also be associated with both R1a and R1b.