THE ORIGINS OF TODAY'S : PHYLOGENETIC NETWORK BASED ON mtDNA ANALYSES PROVIDES ANSWERS

A phylogenetic network, constructed by a German/British Team of researchers (JANSEN ET AL., 2002), and based on the largest currently available data bank, provides answers to questions such as:

Are domestic horses descendants of one or more postglacial primeval horses? Has there been one or more domestication events? Are genotypes geographically linked? Is the Sorraia the ancestor to modern Andalusians and ? Do the pony breeds of northern and western Europe have common ancestors? Are Arabian and Barb horses related? Is the Mongolian ancestral to domestic breeds?

Mitochondrial D-loop sequencing is a reliable state-of-the-art method to determine relatedness between populations and breeds, and to establish phylogenetic facts. 318 horses from 25 oriental and European breeds, American , and Mongolian wild horses were included in this research. Together with previously published data, including such from prehistoric permafrost horses, this amounted to 652 horses, the largest data base available. The phylogenetic network constructed on the basis of these sequences showed 93 different mtDNA types, which grouped into 17 distict phylogenetic clusters (genotypes). The network revealed also that several genotypes correspond to geographic areas, and/or breeds, indicating geographically distant domestication events.

The sheer number of different mtDNA types found indicates the existance of different postglacial primeval horses, which, according to zoological systematics, should be referred to as subspecies, and which evidently were sources for the domestication process. Considering the horse’s mtDNA mutation rate and the archeological timeframe, the results of this study would require a minimum estimate of 77 to have been recruited from the wild for the domestication process, and which must have successfully reproduced in captivity/domestication. This alone is unrealistic as an achievement of a single prehistoric community, but there is the geographical linkeage, which makes it simply impossible: Each of these 77 mares would also have been of different genotype, which means, according to the results of this study, that at least part of them stemmed from geographically distant areas! This presents a feat which to accomplish would have been absolutely unrealistic for any prehistoric community.

The conclusion can only be that the extensive genetic diversity of this (minimum) of 77 ancestral mares means that a number of distinct horse populations were involved in the domestication of the horse.

Computer-generated phylogenetic network based on mtDNA analyses © Thomas Jansen A legitimate question would be whether it wouldn't be normal for wild horse populations to include different genotypes, in order to explain the number of genotypes found in today's horses. However, the Mongolian wild horse, for instance, comes only in one genotype. One could argue, of course, that certain subspecies, like the Mongolian wild horse (Przewalski's horse), might be of diminished genetic diversity today due to bottleneck effects, but comparisons with other horse populations indicate rather uniform genotypes for populations of subspecies:

The answer to the question if prehistoric wild horse populations really had such great genetic diversity that different genotypes found today may derive from the domestication of horses from just one population is clearly a no - analyses of Alaskan permafrost horses spanning a time period of 16,000 years show six of the eight ancient mtDNA samples to cluster monophyletically (i. e. represent one genotype)!

That the single genotype found in the Mongolian, or Asiatic, wild horse is a relevant, and significant, piece of evidence is born out by the fact that the foundation horses of today’s population of Mongolian wild horses were captured in at least two different expeditions, which took place several decades apart, and in at least three different geographic regions – and yet, only one genotype is present! This, together with the findings in prehistoric permafrost horses, indicates that wild populations are/were relatively uniform in genotype.

This simplified diagram is based on the network established by Jansen et al. and shows only the genotypes which are of interest in this context, and which are geographically linked © Hardy Oelke Of the genotypes which correspond with geographic areas, cluster C1 is the most striking one: It is geographically restricted to central Europe, the British Isles, and Scandinavia, including Iceland. 17 of 19 documented horses of the C1 type are North European ponies: Exmoor, Norwegian Fjord, Icelandic pony, and Scottish Highland. Furthermore, 14 of 27 horses with insufficiently documented pedigrees of C1 type were ponies, including Connemaras. Two ancient Viking horses were also found to have the C1 type. Another mtDNA type, cluster E, consisted entirely of Icelandic, Shetland, and Fjord ponies.

Another geographically striking cluster is D1. Its widespread distribution is no surprise, given the strong influence Iberian horses have had on most domestic breeds, but there is a clear frequency maximum in Iberian breeds, as well as in North African horses (Barbs). Due to the historical Spanish presence in the Americas, a high percentage of American mustangs could be expected to be also of this genotype, which mtDNA sequencing did indeed confirm (31 %). The results show clearly that Andalusians, Lusitanos, and Barbs stem from the same primeval population, but the almost complete absence of this genotype in Arabian horses (only 5 %) proves that Arab and Barb horses are not related and of different genetic origins.

Sorraia horses originated from a small group of horses preserved by the late Ruy d'Andrade, after he had seen phenotypically identical wild horses in 1920 near Coruche, . Although d'Andrade considered this horse to be the ancestor of today's Andalusians and Lusitanos, the mtDNA research shows the Sorraia to have a quite different origin. All 18 Sorraias sampled for this study had either of two mtDNA patterns, both on the same branch of the phylogenetic network. As there are only two direct maternal lines that survived in the Sorraia, there could be no more than two mtDNA patterns still in existence. The genotype found in Sorraias is totally unrelated to the D1 type predominant in other Iberian horses. This underlines the singular status of the Sorraia horse. Some American mustangs were found to also have the Sorraia genotype.

A genotype found in Lusitanos (A3) is rather closely related to the one found in Sorraias, but it has yet to be established if these are all variants within one and the same genotype.

The Mongolian wild horse provides another geographically linked genotype (central Asia). Two mtDNA patterns were found for the Mongolian wild horse in this study. ISHIDA ET AL. had published a third mtDNA type for the Mongolian wild horse, but all three are closely related. It was found by Jansen et al. that the Mongolian wild horse had no ancestral role in regard to domestic horse breeds - the mtDNA patterns of the Mongolian wild horse were not found in any other breed or race. This is in agreement with ISHIDA ET AL., who had sampled only a few horses, while the new results were based on a much greater number of individuals.

Reference: T. Jansen, P. Forster, M. A. Levine, H. Oelke, M. Hurles, C: Renfrew, J. Weber, K. Olek:

MITOCHONDRIAL DNA AND THE ORIGINS OF THE DOMESTIC HORSE Proc. Natl. Acad. Sci. USA 99 (16), pp. 10905-10910 (2002)