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Wednesday, March 21, 2007

mtDNA Haplogroup U3

Because of the tremendous genetic diversity in Italy, our project sometimes accumulates a group of folks that would be hard to find elsewhere.

One example is the proportion of mtDNA haplogroup U3 in our project. At nearly 5% of our mtDNA results, U3 is nearly ten times as heavily represented in our project as in mitosearch, for example. Because U3 is not very well studied, I was asked by one of our members to look into it. Here's some of what I found.

Haplogroup U3 is a subclade of haplogroup U, and can be distinguished by two commonly reported markers: 16343G (in HVR1) and 150T (in HVR2). In addition, there are two coding region markers (14139G and 15454C) which seperate U3 from other subclades of U.

Moreover, there are two common subclades of U3. U3a is defined by seven coding region markers and and by the HVR1 marker 16390A. U3b is defined by four different coding region markers (and the absence of 16390A, of course). U3a almost always has the HVR1 marker 16519C as well.

By convention HVR1 results are sometimes reported without the 16000 prefix, so U3b usually has HVR1 results of simply 343G and U3a usually has 343G, 390A, and 519C.

U3 is found at the highest frequency among populations around the Black Sea (e.g Bulgaria and Georgia), but is found throughout Europe. It most likely spread from the Caucusas as part of Neolithic expansion into Europe along the Danube River basin, as the map on the right suggests (click here for a printable version).

U3 (especially U3b) is also found at very high levels among some European Roma populations, likely due to a particularly strong founder effect. U3 is also found throughout the Near East and in North Africa.

So why is U3 disproportionately common in Italy, especially in Sicily? One possibility is that the Neolithic expansion that brought U3 into Europe was particularly successful in Italy. Another possibility is that U3, especially U3b, came to Italy with the Roma people in historic times. Additionally, U3 was likely present among many other peoples that had contact with Italy over the millenia (Phonecians, Byzantines, etc.). Perhaps, with more research and further testing, a more accurate picture of U3 will be forthcoming.