Wednesday 29 December 2021

Were European wild horses black or black dun?

Coat colour is the only aspect of the life appearance of the Holocene European wild horse that can be determined with a high degree of certainty thanks to studies examining the coat colour genotype of aDNA samples from wild horses. 

A 2017 study found that the a allele on the Agouti locus, which originated in Iberia in the late Pleistocene and which causes a black phenotype on an E+/E+ genotype in non-dun horses, became increasingly common among European wild horses in the Holocene until it became the prevalent allele in the later Holocene [1]. This alone does not tell us the phenotype of the animals, because they could have either been black or black dun (also called grullo, mouse dun or blue dun). The question now is: was the late Holocene European wild horse black or black dun?

The Dun locus was resolved in 2015 [2]. It was found that there are three alleles on this locus in horses: dun D, which is wildtype and basal for all living Equus, wildtype non-dun d1 which dates back well into the Pleistocene, and the domestic non-dun d2

The black allele was restricted to Europe, it was not found in Siberia so far [1]. The wildtype non-dun allele, on the other hand, was so far found only in Siberia [2]. This could lead us to conclude that there were no horses of a genotype a/a d1/d1, therefore being black. However, it must be considered that the Dun locus was so far only tested for Siberian wild horses, and not for European wild horses. I think it is well possible that there were horses with an a/ad1/d1 genotype, because genetics suggest that during the Pleistocene there was one large panmictic population of wild horses from the Pyrenees to Siberia [3], what makes it possible at least that the d1 allele was also found in European Pleistocene wild horses, which were the predecessors of European Holocene wild horses [4]. Furthermore, some cave paintings might depict non-dun wild horses, such as those at Lascaux, which clearly show black and blackish brown or dark brown horses among a large yellowish-brown horse. This must be viewed with caution, however, because the exact shade of a cave painting is always dependent on the pigments available to the artists and may not necessarily reflect the true colour of the animals. But so far, cave paintings proved to be rather accurate on horse colours. F.e. the leopard spotted horses illustrated at Pech Merle were found to be based on horses having that colour [5], and Ekain also shows black dun among bay dun horses. Therefore, I think it is well-possible that there were black wild horses at least during the Pleistocene. 

This might have depended on another allele as well. Sponenberg & Bellone (2017) state that seal brown, which is a colour found in Exmoor ponies, can be caused by a black phenotype being diluted by the dominant pangare allele Pa+[6]. Pangare is a basal equine allele because all living wild equines have it, so it must have been the ancestral state in European wild horses as well and some cave paintings illustrate it very clearly. If seal brown is indeed caused by a black phenotype being diluted by pangare, the existence of black wild horses would depend on if the non-pangare allele panp was present in wild horses or not. So far, this has not been tested. I used to think that black suppresses pangare, also because black dun horses always have a dark head while they sometimes may have faint countershading on the body (and as foals sometimes have a white muzzle). The genetic background of seal brown should be tested to be sure. 

Another clue could be historic evidence. There are several records describing free-ranging horses in Europe, and until recently it was unclear whether they were truly wild horses, feral horses or hybrids. A recent study confirmed that they were most likely hybrids of feral domestic horses and the native European wild horses. Go here for a summary of those records on these hybrid populations. What is striking is that black dun horses seem to dominate these reports. Black horses are mentioned only very rarely. Of course, the phenotype of hybrids is not very strong evidence, only a hint, as the domestic horse introgression likely has changed the frequency of the colour phenotypes present in the populations. But black dun is not very frequent among domestic horse breeds. Also, dun in general is less frequent than both non-dun alleles in domestic horses, so that it is unlikely that introgression from dun-coloured domestic horses turned an originally black wild population into a black dun hybrid population. Considering that non-dun is more frequent than dun in domestic horses, I do not think that introgression increased the frequency of dun in the wild population and instead assume that the population was originally dun in most of the individuals. If the original wild population was black, this colour probably would be mentioned much more frequently than black dun in those reports on the hybrids. 

 

All in all, I think the evidence for late Holocene European wild horses being mostly black dun outweighs that for them being black. I think it is well possible that there were black wild horses, at least in the Pleistocene, but more research would have to be done. For example, Holocene European wild horse remains could be tested for the dun and non-dun alleles as much as for the pangare and non-pangare alleles, and the genetic background of seal brown should finally be confirmed. 

 

Literature

 

[1] Sandoval-Castellanos et al.: Coat colour adaptation of post-glacial horses to increasing forest vegetation. 2017.

[2] Imsland et al.: Regulatory mutations in TBX3 disrupt asymmetric hair pigmentation that underlies Dun camouflage color in horses. 2015. 

[3] Cieslak et al.: Origin and history of mitochondrial DNA lineages in domestic horses. 2011. 

[4] Fages et al.: Tracking five millennia of horse management with extensive ancient genome time series. 2019.

[5] Pruvost et al.: Genotypes of predomestic horses match phenotypes painted in Paleolithic works of cave art. 2011. 

[6] Sponenberg & Bellone: Equine color genetics. 2017. 

 

 

 

 

18 comments:

  1. Do we have any idea what black dun horses with pangare look like? To my knowledge we also don't know if there were any non-pangare horses amongst true European wild horses.

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    1. Some black dun foals have in fact a faint mealy muzzle, so I suspect that some black dun horses may have the pangare allele but it is completely masked by the a allele. There are even some black dun horses that have a faint countershading, such as one of the Heck horses on Wikipedia (the one from Haseldüne). Regarding non-pangare in wild horses, some cave paintings show horses without pangare. It would have to be tested in aDNA samples to be sure.

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    2. Yes, my concern with the cave paintings would be that they might just not be detailed enough to show the trait, even if it were actually present

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    3. Some cave paintings, for example, show bay or bay dun horses with light bellies but without white muzzles or eyes, which makes me think its an issue of resolution

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  2. Do you know if it is possible to breed a black dun Przewalski type horse? If not do you know why? Red (fox) dun exists which is thought to be caused by the influence of domestic horses and was bred out.

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    1. It certainly is possible to breed a black und Przewalski's horse, but I wonder what would be the point of that. Przewalski's horses are bay dun, which is the basal colouration of the species.

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    2. Just wondered if the Heck horse and Konik breeding programmes were, unknowingly at the time, recreating a hybrid rather than a true wild horse and if a "mousy" dun is the desired requirement for rewilding in Europe whether a black dun przewalski type would be a better option, thus separating it from the bay dun type subspecies in Asia.

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  3. Siberian horses are mentioned here a few times, are these ancient horse remains that are related to European wild horses? Was there any relationship between these Siberian horses and Przewalski horses?

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    1. Good question, the paper was not specific on the phylogenetic position of those Siberian horses evaluated. The Pleistocene horse might well have been a member of E. c. lenensis, which is a sister clade to the Przewalski+Domestic horse clade according to genetic information.

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  4. This comment has been removed by the author.

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  5. I remember a discussion in the German "cameroon sheep breeders club" a few years ago, which had some similarities with the seal brown in horses. We speculated in many occasions about the background of one rare phenotype called "wechselfarbig". Some said it is a dilutiongene on rezessiv black, because these animals are nearly black in winter, others (me too) said it is a separate mutation on the agoutilocus like the most other colortypes in this breed. I hold the mind that if this phenotype is based on the rezessiv black (eumelanin), the animal is not able show another color than black and there is no gene that could change the melaninstructure into pheomelanin. When i am wrong, please correct me. But the light areas in the face of this sheep and the lighter color during summer shows me that this sheep produce eumelanin and pheomelanin.
    https://kamerun-schafe.de/farbschlaege/ (its the color called wechselfarbig)

    In my mind, this case is comparable with the seal brown. So I don´t think that seal brown horses are black horses with the pangaregene. I wish there would be more genetic studies about the origin of such coatcolors.

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    1. But we do know that recessive black can be diluted in horses, as that's exactly what has happened in black-duns

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    2. The Wechsel colour change could be explained quite simply by guard hairs being black and undercoat red. Winter coat would have longer guard hairs, almost fully covering the (also longer than in summer) undercoat; summer coat would be short all over so the predominant red undercoat would show. This is similar to how roan horses' pattern is clearest in spring and autumn and less pronounced or almost invisible in summer and winter.

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  6. The DNA of three aurochs found next to the Elba shepherdess opens up a new enigma for palaeontology:
    https://www.eurekalert.org/news-releases/623063
    https://www.eurekalert.org/multimedia/617691

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    1. Interesting on the genetic front. But I don't know why the assumption is that shes a shepherd.
      She and the 3 aurochs might have fallen in at different times. That wouldn't show up with carbon dating if it's within a few hundred years of each other.
      Or, as it's a pit perhaps she was trying to kill the animals for meat and ended up being killed in the process.
      There's evidence of humans using pits to hunt animals. Perhaps this was a hunt gone wrong and she got hurt, dying of her injuries shortly after she killed the wild Aurochs.
      Perhaps she had to hunt alone because the men or her tribe were killed in battle or some other misadventure. Hunting alone meant she couldn't effectively use the hunting pit by herself and did so at great risk, paying the ultimate price.

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  7. Ancient chronicles from Iberia described wild horses as rat color or cinderella color. We can't know if they were feral,wild or hibrids. Then, they were blak dun.

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