The Geographic Origin of Dogs

An indication of the geographic origin of the three dog clades, and thereby the origin of the dog, can be obtained from a comparison of the genetic variation between geographical regions. If an ancestral population and a derived population (formed from a subset of the genetic types of the ancestral population) are compared, the number of mtDNA types and the nucleotide diversity are expected to be higher in the ancestral population. Comparing Europe, Southwest Asia and East Asia, the three regions that have been the strongest candidates as geographical origin of the domestic dog, East Asia had a larger genetic variation for clades A and B as measured by a number of methods, while for clade C there were no significant differences (Savolainen et al., 2002). Thus, for clade A, the mean pairwise sequence distance, which gives a rough measure of the genetic variation, was 3.39 (SD=0.13) substitutions in East Asia, 2.28 (SD=0.23) in Southwest Asia and 2.97 (SD=0.08) in Europe. Furthermore, the number of mtDNA types, and the number of types unique to the region, were much larger in the East Asian sample than in the European and the Southwest Asian ones (Table 2.1). The difference between the East Asian and the European samples is striking, and, when corrected for sample size by resampling to correct for different sample size, there were significantly more mtDNA types in clade A among 51 East Asian dogs, than among the 51 Southwest Asian dogs, 20.2 compared to 16. Also compared to the other regions there were more mtDNA types in East Asia when corrected for sample size, but not reaching a significant level for the small samples of Africa, Siberia and India. It is notable that out of the 44 mtDNA types found in East Asia, 30 were unique to this region. Thus, the number of mtDNA types unique to East Asia was larger than the total number of types in Europe (Table 2.1). For clade B, the mean pairwise sequence distance was larger for East Asia (0.93 substitutions, SD=0.17) than for Europe (0.45, SD=0.14) and Southwest Asia (0.36, SD=0.11), the East Asian sample had significantly more mtDNA types than those of Europe and Southwest Asia, and a majority of all mtDNA types were unique to East Asia. The difference in number and distribution of mtDNA types in East Asia, Europe and Southwest Asia can be studied in detail in minimum-spanning networks of the three clades . For clade A it is notable that East Asian mtDNA types were distributed throughout the network, while for Europe and Southwest Asia, parts of the network, largely the same in the two populations, were empty.

To conclude, in a comparison of the dog populations of East Asia, Europe and Southwest Asia, there is a much higher genetic variation in East Asia for the two major dog mtDNA-clades, clades A and B, which comprise >90% of all domestic dog mtDNA sequences. This suggests an origin somewhere in East Asia for the mtDNA types of clades A and B, and a subsequent spread of only a subset of these mtDNA types to the rest of the world. For clade C there is no significant difference between the regions, but the similar frequency of clade C among dog populations of the Old World suggests a common origin of clade C together with clades A and B. Thus, the available mtDNA data indicate that the domestic dog has a single geographical origin, somewhere in East Asia.