Saturday 4 December 2021

Wild horses: bay, wildtype bay, seal brown, brown?

I repeatedly wrote on my blog that western wild horses had, according to genetic studies, several colours: black, black dun, bay, bay dun and leopard spotted. 

The basis for the presence of bay in wild horses is that genetic tests in predomestic DNA samples have identified the presence of both the allele a and A on the Agouti locus in wild horses [1]. But it is, unfortunately, not as simple as it seems at first. 

A bay European wild horse. All rights reserved

There are two types of bay colour: “normal” bay and wildtype bay (or wild bay). The wildtype bay is similar to bay but lighter in colour, and only the toes are dark-coloured while in bays the whole legs are dark. Wildtype bay is hypothesized to be caused by the hypothetical allele A+ [2]. Personally, I do not believe that wildtype bay is actually the wildtype version of bay. The dark areas on the legs in Przewalski’s horses cover almost the entire leg and not only the toes, as in horses of a normal bay colour, what suggests that they share the allele which would then very likely be the wildtype allele. So, we have bay and a form of bay which is called wildtype but is not necessarily wildtype. 

And then there is seal brown, a colour found for example in Exmoor ponies. The genetic background of seal brown seems to be unclear. It is speculated that it is caused by the hypothetical allele At on the Agouti locus. There was a genetic test for this allele, which is now considered inadequate, and the sequence of the allele has not been published [2]. 

Druml et al have a different hypothesis on the genetic background of seal brown. They postulate that this phenotype is caused by the genotype A/a, therefore being the result of heterozygosity of the allele causing black in homozygous individuals (a) and the allele causing bay in homozygous individuals (A). What is striking is that they consider bay itself to be the result of a heterozygous genotype, namely A/A E/e [3]. The e allele is the domestic allele on the Extensionlocus that causes a chestnut colour in homozygous individuals. Therefore, bay would be a domestic colour, and the wildtype E/E A/A colour would be brown, which, according to them, is caused by that genotype. I have to say that I doubt the purported genotypes for these colours, for once because they differ from what is usually considered to be the genotype for those colours, and because 1) if seal brown was the result of A/a, there would be not only bay and seal brown Exmoor ponies but also black Exmoor ponies, which is not the case in the current population. Also, if bay was the result of the genotype E/e, there would be chestnut (e/e) Exmoor ponies, which is not the case and 2) if bay was the result of a heterozygous state, it would be impossible to breed a breed that is exclusively bay. However, there is at least one such a breed, the Cleveland Bay horse [2]. Therefore, bay cannot be the result of a heterozygous state. Brown, is, as defined by Sponenberg and Bellone (2017), sooty + bay. The genetic background of sooty is unclear [2].

One could assume that since, according to the literature, both the alleles A and a but not A+ and At have been found in European wild horses the case is clear that seal brown and wildtype bay are not wildtype colours. But the problem is that the common genetic test for the alleles A and a can only confirm the presence of a, and the presence of A is deduced by the absence of a [2]. Therefore, the test cannot discriminate between AA+ and A(if the latter two exist at all). If the authors of the papers proposing the presence of in wild horses used that test, the presence of the hypothetical alleles A+ and At in wild horses cannot be ruled out currently. 

To further complicate the subject, Sponenberg and Bellone (2017) state that seal brown can also be caused by the pangare allele Padiluting a black phenotype [2]. This would have consequences for the phenotype of wild horses. If that is correct, it would mean that this form of seal brown is a wildtype colour and that the non-pangare allele Panpwould have to have been present in wild horses besides the pangare allele, otherwise black and other non-pangare phenotypes would be domestic colours. The pangare allele has been identified [2], therefore I hope that it will be tested for extinct wild horse samples one day. 

Another factor that plays in the game is the Dun locus. Since 2015 we know that there are three alleles on this locus: dun (a wildtype colour identified in all living wild equines and Pleistocene wild horse samples), non-dun 1 d1 which is wildtype as well, and non-dun 2 d2 which is domestic [2,4]. Wildtype non-dun and domestic non-dun look different; the former have a clearly visible dorsal stripe and the surrounding colour is lighter than in the latter, the latter are darker all over the body [4]. Wildtype non-dun bay horses, such as some Gotland ponies, almost look like bay dun horses, only slightly darker in shade. 

 

So, which colour did the wild horses have that were neither leopard spotted, black, black dun or any other form of dun? I think that this cannot be said with a 100% certainty with the current knowledge on horse colour genetics and the current genetic tests we have. Very likely the horses had the A allele since it is the basal allele of wild equines, but the presence of other alleles on the Agouti locus cannot be ruled out yet. The genetic background of sooty and seal brown would have to be identified and tested for wild horses. Also, it would be interesting to know if wild horses had the non-pangare allele. The colour of Exmoor ponies, however, is domestic in any case because so far this breed has been found to exclusively carry the domestic non-dun 2 allele d2. It would be interesting to know what seal brown combined with a d1/d1 phenotype would look like. 

 

Literature

 

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

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

[3] Druml et al.: Discriminant analysis of colour measurements reveal allele dosage effect of ASIP/MC1R in bay horses. 2018. 

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

19 comments:

  1. Would it be possible that Exmoors are simply Bay horses with both the Pangare and Sooty traits? This would mean that the Bay parts that were not affected by the light countershading from Pangare would instead be given dark countershading.

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    1. I think that some Exmoor ponies are certainly bay + sooty + pangare, some are only bay + pangare, and some might be seal brown + pangare based on their looks. Some Exmoor ponies are clearly bay while others are much darker, but they always have a well-expressed pangare because they were selected for that trait. I think the big question is if it is true that seal brown is (or can be) black + pangare.

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    2. Pangaré doesn't show on black. The idea that seal brown is caused by black+pangaré has been invalidated by genetic testing. Wild horses that were black or grullo wouldn't have shown pangaré wether they had the pangaré gene or not.

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    3. Interesting, do you maybe have some sort of reference for that genetic testing of seal brown? That would be very helpful, thanks

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    4. There exists no genetic testing for seal bay. It is a visual colour attriubution only. Seal bays always test for positive for bay, but the gene(s) involved for creating seal bay are not determined yet.

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  2. I used to think that pangare simply had a different effect on black horses, causing light countershading on the stomach but not the muzzle. Now I'm not so sure. Certainly Koniks do not seem to have the Pangare trait, as the few sorrel individuals defintely do not have light muzzles either.

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  3. https://en.wikipedia.org/wiki/Gotland_pony

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  4. I know prehistoric cave paintings are open to interpretation but many show dun horses with a pangare type colouration. There are also black, and brown horses with dark points, as well as the spotted horses. However, some paintings have colourations, particularly those with prominent shoulder striping, which I am unaware of in living horse breeds. Do we rely too much on modern horse breed genetics in describing wild horse colours?

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    1. Which cave paintings are you referring to that show colours not found in domestic horses? The striping in Pleistocene wild horses was rather intense in some individuals at least, more so than in living domestic horses or Przewalski's horses, but the basic colours (bay dun, black dun) are still found in domestic horses.

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    2. Could those shoulder markings even be bider markings?

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    3. There is even a pre-domestication, >15.000 ybp painting at the Diverticule Axial in Lascaux, called Frieze of the Small Horses, seemingly showing a piebald coloured horse.

      https://www.donsmaps.com/images36/p64tenlascauxdoubleimage600dpidescreensm.jpg
      https://pixels.com/featured/lascaux-horses-weston-westmoreland.html

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  5. I was referring to the amount of black and striping over the basic dun which is not seen in modern breeds. The original description of the sorraia was said to have stripes along the back but this has been lost in the derived breed. The Lascaux Cave has a black dun horse with heavy striping and dark head and rump. In the original frieze it follows a Przewalski type dun horse: https://www.sciencephoto.com/media/467944/view/lascaux-ii-cave-painting-replica
    Many horses are depicted as having a very dark head and neck. Some koniks have less intense dark heads but never to the same degree:
    https://fosterartprogramslc.files.wordpress.com/2012/11/bulls-and-horses-lascaux-caves.jpg
    The Ekain Caves in Spain have horses with dark heads and torsos with white rumps and well striped front legs: https://www.ekainberri.eus/en/discover-ekain/
    Probably the most obscure is at Lascaux with a frieze with one horse which appears to be piebald, followed by a chestnut, together with dun horses. However, I cannot find any other information on this - any thoughts?:
    https://paleolithic-art.tumblr.com/post/657460560992124929/frieze-of-the-small-horses-axial-gallery-lascaux

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    1. Does this help:
      https://www.jstage.jst.go.jp/article/jes/18/4/18_4_145/_article/-char/en?
      I haven't read yet, just found it few days ago. And I see the year of publishing so it may be outdated in fast genetic science developing, but it has photos that resembles those you're talking about...

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    2. The "chestnut" horse could well have been just non-dun 1 bay, which is known to exist in wild horses as a wild type.

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  6. There's an interesting video on the Rewilding Europe instagram account, posted yesterday. https://www.instagram.com/rewildingeurope/tv/CXRLRQNJQPe/?utm_medium=share_sheet

    In it there are some horses with both erect and falling manes that are free to roam and graze parks.
    One individual had a bunch of Burdock Burrs caught in its mane (seen in the video at the 2:58 and 5:01 mark). During its second appearance in the vid it can be seen that the horse was visibly irritated by all the burrs. This made me think:
    Falling manes could be a biological disadvantage because they get loads of stuff caught in them and irritate a horse. This makes them preoccupied with the burrs and less aware of predators. Beside irritation, their vision might physically become obscured if they have particularly long manes in the front.
    In extreme cases the burrs might scratch the horse's eyeballs, cause infection and reduce vision for a longer time. A really bad infection could even cause a permanent loss of the eye. I could also see that a horse's ears might not be free or uncomfortable to turn in every direction and also present a missed opportunity to hear a predator approaching in time to flee. Any delay in detection is a disadvantage when facing predators.
    With shorter, upright manes it's less likely to get plants tangled in the manes because they usually remain out of the way when the animal is grazing or browsing and the ears are free to rotate in any direction. Even if it does happen that something gets stuck on an erect mane it's less likely to hang in the horse's face or irritate/itch the side of its neck. None of the horses with erect manes in the video had anything in their caught in their manes. I actually can't recall ever seeing any horse, ass or zebra with erect manes having a lot of stuff caught in their manes.
    So this just adds another reason why erect manes on a European Wild Horse would make more sense.

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    1. This comment has been removed by the author.

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    2. Anonymus,
      nice observation, but there is one incosistency: the problematic plant is (if I see well) 'xanthium strumarium' which is (most likely) of American origin, known as a major invasive weed... Free ranging horses of my area (Posavina, Croatia) have big issues with it in last years, but it wasn't like that before (as I've heard).

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    3. No, that's not Burdock. Xanthium species are called Cocklebur in English, not Burdock.

      Burdock are species of the Arctium plant family.
      The whole arctium family is native to the Eurasian area. There are several Burdock species and hybrids native to Europe.
      https://en.wikipedia.org/wiki/Arctium

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    4. I know the two are not the same and that is why I've already replied to you. I'm pretty sure it is clearaly visible that there is coclebur on the footage. I am also personaly familiar with both. I live by the river and in warm part of year I go to chil in it by bare feet. Last summer I step many times on dry coclebur fruits. It is so hard/firm that its little needles easily brake trough my soles' skin. And that causes a lot of pain. On the other hand other/native burs wich are commonly caught in all kinds of animal furs (and human clothes) are not so hindering. They are much softer and more prone to disintegrating.

      I'll take this ocassion to mention a third bur genus here, my favorite (but from totally different reasons)! It is the Bidens (in Latin; I don't know which English name to use, it seems there are lots of them). The reason is its name in my dialect: 'turice' (in official Croatian that name is given to another plant genus, but that is wrong, you'll see why!). 'Turice' is plural form whose singular would be 'turica'. '-ica' is common suffix for making dimminutives and/or feminine gramatical gender (most ften that means semantical too). 'Tur' (read like eng. or even better french "tour" but with real 'r') just means "aurochs". So, basicaly, 'turica' means "(little/young/cute) aurochs cow (female heifer?)". And just look how seeds of Bidens tripartita, as European native, often looks like (despite its Latin name it commonly has two "teeth", or better to say "horns". Right as a little cows' heads. Of course, plural is becose there are many of them in each seed head. But, the thing is they have not "the right aurochs horn ankle and form", right, bos Daniel? ;) Their shape is much more similar to heads of (many) podolian cows. Not bulls, cows! And in slavic languages the word 'tur', as I am aware, can't be misunderstood for domesticated stock (Croatian: 'bik' bull, 'krava' cow, 'tele' calf, 'govedo' bovine, 'stoka' or 'skot' cattle...). So, could it be an argument (indirect proof) for the view in wich "more upward horns" are also a wild form, and consequently podolics are not so far from its old 'primigenius' status as it is claimed in recent decades..?

      (Sorry for the language quality, my English is not representative... I hope it is good enough to understand my point.)

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