Artyku³ przegl¹dowy Review
White markings are patches of white hairs growing on unpigmented skin on the horses peripheral areas: the head and limbs. Rarely, the markings appear in central areas, which in the majority of cases indicates that the horse carries a gene producing one of the pied coat color patterns. The unpigmented skin and hairs result from a lack of melanocytes (17). The white markings are present at the foals birth and persist throughout the life of the horse. Besides the markings, vitiligo may develop in adult horses due to the loss of
melanocytes from the epidermis (24). The patches of depigmentation usually appear around the anus and genital regions, as well as in the face (fig. 1). Vitiligo is observed in many animal species: mostly mammals, but also in chickens. The studies performed in humans show that besides genetic effects it may result from an autoimmune disorder (20).
In many horse breeds, white markings are assumed to be flashy, highlighting the limb action which is particularly important in shows but not desired in
White markings in horses
ANNA STACHURSKA, ANNE PHAFF USSING*Department of Horse Breeding and Use, Faculty of Biology and Animal Breeding, University of Life Sciences, Akademicka 13, 20-950 Lublin, Poland
*The Royal Library, P.O. Box 2149, DK-1016 Copenhagen K, Denmark Stachurska A., Phaff Ussing A.
White markings in the horse
Summary
White markings in horses result from the lack of melanocytes in the skin and hairs. The trait is characteristic of most breeds of domestic horses. In the breeding of the Polish Konik horse, which should resemble its wild ancestors, the lack of white markings was accepted as a selection criterion. The heritability of the markings is high. MC1R and ASIP loci, as well as two other putative QTLs are involved in the appearance of white markings. A foal should inherit a number of genes from both parents to make it possible for the markings to appear. The ultimate extent of markings is caused by the genes, as well as by intrauterine factors. The latter consist of accidental and random events which disturb the survival, migration and clonal proliferation of melanoblasts. The recessive e allele from MC1R locus and the dominant A allele from ASIP locus cause a greater extent of the markings, behaving as major genes. The most desired genotype of the Polish Konik horse, aaEEDD, which in inter se matings produces solely blue dun foals, is also the most desired with regard to the low amount of markings. To increase the progress in the Polish Konik breeding with regard to the absence of the markings, marked individuals and identified carriers of e allele should be consistently eliminated from reproduction.
Keywords: white markings, inheritance, horse
dressage. Moreover, the markings enable the recogni-tion of the horse from a distance. Unpigmented hoof horn associated with the markings is equally hard and elastic as the pigmented one (9, 28). However, because of the unjustified opinion of the inferior quality of the unpigmented horn, horses with white markings are unfavorable in many breeds.
Occurrence of white markings
Archive sources on the Tarpan (Equus caballus gmelini) living in Poland and Lithuania do not men-tion any white markings. It may be supposed that if the old chronicles name such details as the dorsal stripe, white markings would not be passed over, either. Gmelin (1745-1774) (quoted after 21) states that the wild horses are similar to a mouse with regard to the coat color and their legs beneath the middle to the hooves are black. The lack of white markings in the Przewalski horse (Equus caballus przewalskii), the last living wild representative of the species, and in the Paleolithic cave paintings illustrating the horse, as well as the markings rarely occurring in primeval Asiatic horses (12), indicate the trait is characteristic of domestication. Hence, in the breeding of the Polish Konik horse, which should resemble its wild ancestors, the absence of white markings was accepted as a se-lection criterion. Similarly, in the Norwegian Fjord horse which is bred for a yellow dun color, white mar-kings seldom occur and are undesired (29). A facial star, up to 5 cm diameter, is solely accepted in brood mares, but not in stallions, in Norway. The reason for the official acceptance is that such a marking was pre-sent in three famous Fjord ancestors in the nineteenth century. In Denmark, only white hairs on the forehead in Fjords are accepted.
Breeders of Polish Konik, Fjord and other horses which should be without white markings always face the dilemma if it is worth eliminating an otherwise valuable horse due to, for instance, a small white spot. Hence, usually some exceptions are considered. In Polish Konik breeding, two programs are in effect. According to the Programme of Conservation Breeding accepted by the Minister of Agriculture in 1999, Polish Koniks ( ) without white markings may be registered in the studbook ( ) temporary in mares little markings on head (white hairs, snip) are allowed. The Breeding Programme of Genetic Resources Conservation effective from 2000 is more strict, since it allows only horses without any white markings. Analyzing the stud book, it may be noticed that up to Volume VI some individuals with a marking, includ-ing the fetlock joint, occur. In Volume VII, mares with a blaze, stripe or white pastern, as well as stallions with a star are registered. There are still five stallions with white hairs, one with a snip and two with a star in Volume IX, which is the latest volume. The percentage of horses without markings is lower in Volumes III and IV due to the fact that individuals from the
preli-minary stud book were then included in the main stud book (26; fig. 2). The number of the horses without markings is higher in the following volumes; however, it still only slightly exceeds 80%. These data show that the constant occurrence of marked horses, despite the selection, is a great problem in the breeding of Polish Koniks.
Inheritance of white markings
For over 80 years geneticists have been studying the inheritance of white markings. In the beginning, the most extensive research performed in Germany enabled the suggestion of the hypotheses of two pairs of genes controlling the trait (18, 30). Crew and Buchannan Smith (8) put forward a theory of a domi-nant M factor and modifiers. Afterwards, German stu-dies assumed four genes responsible for the markings (3-6). According to another hypothesis the markings were considered as the least developed pied pattern (2). Following that approach, Lauvergne et al. (15) suggested horse classification related to the extent of white patches on the body. None of the hypotheses were confirmed later.
In the early 1980s, monozygotic horse twins produ-ced by embryo micromanipulation attracted the gene-ticists attention (1). One colt had a marking on all four limbs and another colt had it solely on the left limbs. In a pair of fillies one foal had a marking on the left forelimb and the other did not have any markings. The result was not consistent with expectations, since if the trait is genetically controlled in monozygotic twins the genotype and consequently the markings should be identical (35). A similar riddle was noticed in the case of individual horses with asymmetrical markings. The same genotype produces differently expressed traits on two sides of the body. The findings indicated that additional non-genetic factors determine the extension of the trait.
Fig. 2. Percentage of Polish Konik horses without markings registered in I to VIII volumes of the studbook (24)
Extensive studies on the occurrence and amount of markings brought the solution to the problem. A high or medium correlation between the extension of the facial and limb markings (10, 19, 23, 30, 35) showed that they were due to one genetic mechanism. Results documenting a high sire-foal and dam-foal regression proved the mechanism could not be founded on one pair of genes which would cause the segregation in the offspring. It was also found that stallions were slightly more marked than mares, hence a gene producing the trait might be associa-ted with the horses gender (10, 23, 35).
A hypothesis of the multifactorial mode of inheritance of white markings was put forward by Nebe (19) on the basis of data collected in over 8,000 foals of Hessian saddle horses. Broad investigations of Ara-bian horses registered in the American
stud-book, conducted by Woolf at the turn of the 1980s and 1990s (31-36) explored the hypothesis on the multi-factorial inheritance of the markings. Particular analy-ses concerned 2.5 thousand to 12 thousand horanaly-ses. The markings were scored from 0 to 5 depending on the amount of whiteness at five areas on the head and separately on each limb. According to those studies, the heritability of the markings on the head is 0.69, on the limbs 0.68, and combining the facial score and four limb score amounts to 0.77 (35). Other authors (19, 22, 23) also reported high indices, but compared with those mentioned above they were lower. MC1R and ASIP loci, which control basic coat colors, show a pleiotropic function affecting the white marking expression. Recent studies on Swiss Franches-Mon-tagnes horses show that other two putative QTLs greatly influence the trait (14, 22).
A heritability of approximately 0.75 is very high. That means the phenotypic variance is determined in ¾ by the genotype and in ¼ by environmental factors. The latter finding was the most difficult to interpret. It has always been known that the markings were innate and constant, hence they did not seem to be influen-ced by the environment. To explain the phenomenon, Mintzes (17) and earlier hypotheses on the ordinary developmental noise in the intrauterine life were used (33, 35). As is known, melanoblasts migrate from the neural crest and enter distal ends of particular body part buds. The melanoblasts proliferate in the presump-tive facial and limb tissues. After metamorphose, the melanocytes that have settled in the skin, epidermis and follicles secrete melanines determining the color of the skin and hairs. During the prenatal development, accidental and random events may affect the survival, migration and clonal proliferation of the melanoblasts. If the melanocytes are absent or did not develop normally, melanogenesis does not occur and the skin and hairs are unpigmented. Similarly, freeze-marking
(breed or identity marks) destroys the melanocytes, resulting in growth of white hairs in the involved area. Factors disturbing melanoblast migration and pro-liferation are not known. Although the effects should similarly influence both sides of the organism, stocha-stic events appear. It must be emphasized that the migration and proliferation of the melanoblasts occur autonomously in each side of the embryo because of the mid-dorsal separation (33). Sponenberg (25) pre-sented a photograph of a horse with white narrow vertical stripes on one side of the body. Such horses occur extremely rarely in the world and they do not reproduce those stripes. Occasionally, vertical black stripes on one side of the body occur (fig. 3). They also may be caused by specific factors in the prenatal life.
Woolfs articles formed a view of how to explain the inheritance of white markings (33, 35). A number of genes of equal and unequal effects on the markings is inherited from the parents by a foal. A parent with markings may be heterozygous to some extent, hence it produces gametes both with genes increasing the markings and with genes limiting the expression of the trait. Heavily marked parents carry more genes pro-ducing markings and in some loci are homozygous, hence the offspring will show a tendency towards the presence of the markings. In contrast, parents less marked or without any markings carry few genes for the markings and the progeny will show a tendency not to be marked. According to Woolf (33, 35), a foal should inherit a threshold number of genes from both parents to make it possible that the markings appear. If the threshold is not reached, the markings do not appear. Horses without markings may differ with respect to the number of potential genes producing the markings. Ultimate extension of markings is influen-ced by the genes, as well as by intrauterine factors. The polygenic inheritance is consistent with results of Fig. 3. Vertical black stripes in Polish Konik stallion. The stripes are not present on the other side of the body. Roztocze National Park
matings of marked horses producing non-marked foals and, conversely, marked offspring originating from non-marked parents.
As an example, data on matings of Polish Konik horses are presented (27; tab. 1). For parents without markings, the progeny usually was not marked either (88.5%). For parents with markings, the number of progeny without markings was distinctly lower (75.0% and 69.2%). The relation is visible in spite of select-ing the herd against the markselect-ings.
The markings are more frequent on hind limbs than on fore limbs and on left side limbs compared to right side limbs (10, 23, 35). Fig. 4 illustrates average white marking size scored on the 0-5 scale in over 16 thousand Purebred Arabian and half-breed Arabian horses (33). The marking amount in forelimbs was significantly lower than in hind limbs. In turn, the trait extent on right side limbs was considerably lower than on left side limbs, which was found in both breeds, except for the hind right limb in chestnut Purebred Arabians. It turned out that half-breed Arabians were less marked compared to Purebred Arabians. It was also shown in the case of each limb that bay horses were generally less heavily marked than chestnut horses. The fact that chestnut horses are more marked was established a long time ago (3-6, 10, 19, 35). Further
research proved that chestnuts were more heavily mar-ked not only when compared to bays, but primarily in regard to blacks (32, 33). On that basis, a hypothesis was suggested that the recessive e allele from the MC1R locus (mapped to ECA3p12) causes a greater size of the markings behaving as a major gene. The dominant A allele from the ASIP locus (ECA22q15) shows a similar although weaker function. Hence, a genotype producing the greatest amount of markings is AAee, whereas the fewest markings is produced by aaEE.
The strong MC1R locus effect causes that horses of all homozygous ee colors have a greater amount of markings, i.e. besides chestnuts, those are red duns and palominos. ASIP locus implies that homozygous aa horses are relatively less heavily marked, i.e. besides blacks, this includes blue duns. Bays, yellow duns and buckskins are located at a medium position. Thus, the most desired genotype of the Polish Konik horse, aaEEDD, which produces solely blue dun foals, is also the most desired with regard to the low amount of its markings. Blue dun Koniks of aaEeDD genotype car-rying the e allele, with a similar parent, may produce a red dun foal and in 50% of all kinds of matings will transfer the tendency to the presence of the markings. MC1R locus is mapped in a group linked with KIT locus controlling tobiano, roan and other white pat-terns (7, 13, 16); thus in the region important for the melanogenesis in horses. Recent studies by Rieder et al. (22) show another putative major locus mapped to ECA3q at or near the KIT locus. A recessive single gene could account for 20-80% of the total heritability for the trait present at different parts of the body, parti-cularly in pheomelanic (ee) horses. One more QTL (MITF) responsible for the expression of white mar-kings, mainly on eumelanic (E_) coat color, has been found on ECA16q (14). These findings may indicate that not many genes (at least four) produce the trait.
White markings versus breeding criteria The high heritability and QTLs involved mean that selection both towards and against the markings is effective. Simultaneously, the polygenic inheritance is associated with the impossibility of the total elimina-tion of markings. As shown, the genes affecting the markings may be recessive and masked by dominant genes, hence they are not easy to identify. Moreover, mutations are always possible. To increase the progress in the Polish Konik breeding with regard to the absence of markings, the marked individuals should be con-sequently eliminated. Moreover, identified carriers of e allele should not be considered in reproduction either. It seems that soon molecular studies can almost enti-rely explain the white markings inheritance. If the model includes only a few genes, it will allow the matings to be more directed.
The high effectiveness of the selection resulted in great differentiation of horse breeds with regard to the Tab. 1. Results of matings (%) of Polish Konik horses in 2002
with regard to the presence of white markings (25)
s t n e r a P g n ir p s ff O r e b m u N s l a o f f o mWartihkionugts amWartikhing t u o h ti W s g n i k r a m mWartihkoinugts 209 88.5 11.5 t u o h ti W s g n i k r a m amWartikhing 092 75.0 25.0 h ti W g n i k r a m a amWartikhing 013 69.2 30.8
Fig. 4. Mean white limb marking size in bay and chestnut Purebred Arabians and half-breed Arabians (32)
extent of white markings. Besides the Polish Koniks and Fjords, marked horses are eliminated with various intensity in, e.g. Hucul, Friesian, Cleveland Bay, Percherons and Suffolk Punch breeding. Conversely, the markings are desired in Hackneys, Clydesdales, Shires and Welsh ponies (25). Arabian horses had white markings from the beginning and Bedouins favored them (11). In many breeds the feature is not a selec-tion criterion.
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Corresponding author: prof. Anna Stachurska, PhD, Department of Horse Breeding and Use, University of Life Sciences, Akademicka 13, 20-950 Lublin, Poland; e-mail: anna.stachurska@up.lublin.pl
