Naturally occurring horse model of miscarriage reveals temporal relationship between chromosomal aberration type and point of lethality.

Chromosomal abnormalities are a common cause of human miscarriage but rarely reported in any other species. As a result, there are currently inadequate animal models available to study this condition. Horses present one potential model since mares receive intense gynecological care. This allowed us to investigate the prevalence of chromosomal copy number aberrations in 256 products of conception (POC) in a naturally occurring model of pregnancy loss (PL). Triploidy (three haploid sets of chromosomes) was the most common aberration, found in 42% of POCs following PL over the embryonic period. Over the same period, trisomies and monosomies were identified in 11.6% of POCs and subchromosomal aberrations in 4.2%. Whole and subchromosomal aberrations involved 17 autosomes, with chromosomes 3, 4, and 20 having the highest number of aberrations. Triploid fetuses had clear gross developmental anomalies of the brain. Collectively, data demonstrate that alterations in chromosome number contribute to PL similarly in women and mares, with triploidy the dominant ploidy type over the key period of organogenesis. These findings, along with highly conserved synteny between human and horse chromosomes, similar gestation lengths, and the shared single greatest risk for PL being advancing maternal age, provide strong evidence for the first animal model to truly recapitulate many key features of human miscarriage arising due to chromosomal aberrations, with shared benefits for humans and equids.

Novel Complex Unbalanced Dicentric X-Autosome Rearrangement in a Thoroughbred Mare with a Mild Effect on the Phenotype.

Complex structural X chromosome abnormalities are rare in humans and animals, and not recurrent. Yet, each case provides a fascinating opportunity to evaluate X chromosome content and functional status in relation to the effect on the phenotype. Here, we report the first equine case of a complex unbalanced X-autosome rearrangement in a healthy but short in stature Thoroughbred mare. Studies of about 200 cells by chromosome banding and FISH revealed an abnormal 2n = 63,X,der(X;16) karyotype with a large dicentric derivative chromosome (der). The der was comprised of normal Xp material, a palindromic duplication of Xq12q21, and a translocation of chromosome 16 to the inverted Xq12q21 segment by the centromere, whereas the distal Xq22q29 was deleted from the der. Microsatellite genotyping determined a paternal origin of the der. While there was no option to experimentally investigate the status of X chromosome inactivation (XCI), the observed mild phenotype of this case suggested the following scenario to retain an almost normal genetic balance: active normal X, inactivated X-portion of the der, but without XCI spreading into the translocated chromosome 16. Cases like this present unique resources to acquire information about species-specific features of X regulation and the role of X-linked genes in development, health, and disease.

Two Novel Cases of Autosomal Translocations in the Horse: Warmblood Family Segregating t(4;30) and a Cloned Arabian with a de novo t(12;25).

We report 2 novel autosomal translocations in the horse. In Case 1, a breeding stallion with a balanced t(4p;30) had produced normal foals and those with congenital abnormalities. Of his 9 phenotypically normal offspring, 4 had normal karyotypes, 4 had balanced t(4p;30), and 1 carried an unbalanced translocation with tertiary trisomy of 4p. We argue that unbalanced forms of t(4p;30) are more tolerated and result in viable congenital abnormalities, without causing embryonic death like all other known equine autosomal translocations. In Case 2, two stallions produced by somatic cell nuclear transfer from the same donor were karyotyped because of fertility issues. A balanced translocation t(12q;25) was found in one, but not in the other clone. The findings underscore the importance of routine cytogenetic screening of breeding animals and animals produced by assisted reproductive technologies. These cases will contribute to molecular studies of translocation breakpoints and their genetic consequences in the horse.

A missense variant in the coil1A domain of the keratin 25 gene is associated with the dominant curly hair coat trait (Crd) in horse.

BACKGROUND: Curly horses present a variety of curl phenotypes that are associated with various degrees of curliness of coat, mane, tail and ear hairs. Their origin is still a matter of debate and several genetic hypotheses have been formulated to explain the diversity in phenotype, including the combination of autosomal dominant and recessive alleles. Our purpose was to map the autosomal dominant curly hair locus and identify the causal variant using genome-wide association study (GWAS) and whole-genome sequencing approaches. RESULTS: A GWAS was performed using a Bayesian sparse linear mixed model, based on 51 curly and 19 straight-haired French and North American horses from 13 paternal families genotyped on the Illumina EquineSNP50 BeadChip. A single strong signal was observed on equine chromosome 11, in a region that encompasses the type I keratin gene cluster. This region was refined by haplotype analysis to a segment including 36 genes, among which are 10 keratin genes (KRT-10, -12, -20, -23, -24, -25, -26, -27, -28, -222). To comprehensively identify candidate causal variants within all these genes, whole-genome sequences were obtained for one heterozygous curly stallion and its straight-haired son. Among the four non-synonymous candidate variants identified and validated in the curly region, only variant g.21891160G>A in the KRT25 gene (KRT25:p.R89H) was in perfect agreement with haplotype status in the whole pedigree. Genetic association was then confirmed by genotyping a larger population consisting of 353 horses. However, five discordant curly horses were observed, which carried neither the variant nor the main haplotype associated with curliness. Sequencing of KRT25 for two discordant horses did not identify any other deleterious variant, which suggests locus rather than allelic heterogeneity for the curly phenotype. CONCLUSIONS: We identified the KRT25:p.R89H variant as responsible for the dominant curly trait, but a second dominant locus may also be involved in the shape of hairs within North American Curly horses.

Copy number variation in the horse genome.

We constructed a 400K WG tiling oligoarray for the horse and applied it for the discovery of copy number variations (CNVs) in 38 normal horses of 16 diverse breeds, and the Przewalski horse. Probes on the array represented 18,763 autosomal and X-linked genes, and intergenic, sub-telomeric and chrY sequences. We identified 258 CNV regions (CNVRs) across all autosomes, chrX and chrUn, but not in chrY. CNVs comprised 1.3% of the horse genome with chr12 being most enriched. American Miniature horses had the highest and American Quarter Horses the lowest number of CNVs in relation to Thoroughbred reference. The Przewalski horse was similar to native ponies and draft breeds. The majority of CNVRs involved genes, while 20% were located in intergenic regions. Similar to previous studies in horses and other mammals, molecular functions of CNV-associated genes were predominantly in sensory perception, immunity and reproduction. The findings were integrated with previous studies to generate a composite genome-wide dataset of 1476 CNVRs. Of these, 301 CNVRs were shared between studies, while 1174 were novel and require further validation. Integrated data revealed that to date, 41 out of over 400 breeds of the domestic horse have been analyzed for CNVs, of which 11 new breeds were added in this study. Finally, the composite CNV dataset was applied in a pilot study for the discovery of CNVs in 6 horses with XY disorders of sexual development. A homozygous deletion involving AKR1C gene cluster in chr29 in two affected horses was considered possibly causative because of the known role of AKR1C genes in testicular androgen synthesis and sexual development. While the findings improve and integrate the knowledge of CNVs in horses, they also show that for effective discovery of variants of biomedical importance, more breeds and individuals need to be analyzed using comparable methodological approaches.

Identification of copy number variants in horses.

Copy number variants (CNVs) represent a substantial source of genetic variation in mammals. However, the occurrence of CNVs in horses and their subsequent impact on phenotypic variation is unknown. We performed a study to identify CNVs in 16 horses representing 15 distinct breeds (Equus caballus) and an individual gray donkey (Equus asinus) using a whole-exome tiling array and the array comparative genomic hybridization methodology. We identified 2368 CNVs ranging in size from 197 bp to 3.5 Mb. Merging identical CNVs from each animal yielded 775 CNV regions (CNVRs), involving 1707 protein- and RNA-coding genes. The number of CNVs per animal ranged from 55 to 347, with median and mean sizes of CNVs of 5.3 kb and 99.4 kb, respectively. Approximately 6% of the genes investigated were affected by a CNV. Biological process enrichment analysis indicated CNVs primarily affected genes involved in sensory perception, signal transduction, and metabolism. CNVs also were identified in genes regulating blood group antigens, coat color, fecundity, lactation, keratin formation, neuronal homeostasis, and height in other species. Collectively, these data are the first report of copy number variation in horses and suggest that CNVs are common in the horse genome and may modulate biological processes underlying different traits observed among horses and horse breeds.

The legend of the Canadian horse: genetic diversity and breed origin.

The Canadian breed of horse invokes a fascinating chapter of North American history and as such it is now a heritage breed and the national horse of Canada. The aims of this study were to determine the level of genetic diversity in the Canadian, investigate the possible foundation breeds and the role it had in the development of the US horse breeds, such as Morgan Horse. We tested a total of 981 horses by using 15 microsatellite markers. We found that Canadian horses have high values of genetic diversity indices and show no evidence of a serious loss of genetic diversity and the inbreeding coefficient was not significantly different from zero. Belgian, Percheron, Breton and Dales Pony, unlike the light French horses, may have common ancestries with the Canadian and could be important founders. However, the Shire and Clydesdale influenced the Canadian to a lesser extent than French and Belgian draft breeds. Furthermore, our finding indicated that there was no evidence of a clear relationship between Canadian and Oriental or Iberian breeds. Also, the Canadian likely contributed to the early development of the Morgan. Finally, these findings support the ancient legends of the Canadian Horse as North America's first equine breed and the foundation bloodstock to many American breeds and may help in the management and breeding program of this outstanding breed in North America.

Relatedness and genomic inbreeding in a sample of Timor ponies.

Timor ponies (TP) were first shipped to Australia in the early 1800s and were highly valued as transport and pack animals, which resulted in TPs contributing to the development of Australian horse breeds. Today, while the exact number of TPs in Australia is currently unknown, there has been recent interest in establishing a domestic breeding program for Australian TPs. The aim of this study was to evaluate the relatedness of a sample of TPs, as well as provide estimates of genomic inbreeding levels to better inform the feasibility of using these animals as founders for a domestic breeding program. Hair samples from each horse were genotyped using the Illumina 80K Infinium Equine genotyping array and data were analysed using PLINK v1.90b7, KING 2.3.2 and R v4.3.1. The results illustrate that there are distantly related and minimally inbred horses within the sampled TPs. Lengths of the ROH segments also indicated that recent inbreeding events are likely to only have occurred in a third of the horses. Overall, these results are promising for the success of a domestic TP breeding program; however, considering the low number of domestic TPs known to reside in Australia, there would certainly still be substantial benefits to incorporating additional TPs either directly from Timor or from areas in Australia that are believed to contain wild descendants of TPs.

Microsatellite analysis of genetic diversity and population structure of Arabian horse populations.

The Arabian horse ignites imagination throughout the world. Populations of this breed exist in many countries, and recent genetic work has examined the diversity and ancestry of a few of these populations in isolation. Here, we explore 7 different populations of Arabians represented by 682 horses. Three of these are Middle Eastern populations from near the historical origin of the breed, including Syrian, Persian, and Saudi Arabian. The remaining Western populations are found in Europe (the Shagya Arabian and Polish Arabian) and in America (American Arabian). Analysis of genetic structure was carried out using 15 microsatellite loci. Genetic distances, analysis of molecular variance, factorial correspondence analysis, and a Bayesian method were applied. The results consistently show higher level of diversity within the Middle Eastern populations than the Western populations. The Western Arabian populations were the main source among population variation. Genetic differentiation was not strong among all Middle Eastern populations, but all American Arabians showed differentiation from Middle Eastern populations and were somewhat uniform among themselves. Here, we explore the diversities of many different populations of Arabian horses and find that populations not from the Middle East have noticeably lower levels of diversity, which may adversely affect the health of these populations.

Chromosome-level reference genome for North American bison (Bison bison) and variant database aids in identifying albino mutation.

We developed a highly contiguous chromosome-level reference genome for North American bison to provide a platform to evaluate the conservation, ecological, evolutionary, and population genomics of this species. Generated from a F1 hybrid between a North American bison dam and a domestic cattle bull, completeness and contiguity exceed that of other published bison genome assemblies. To demonstrate the utility for genome-wide variant frequency estimation, we compiled a genomic variant database consisting of 3 true albino bison and 44 wild-type pelage color bison. Through the examination of genomic variants fixed in the albino cohort and absent in the controls, we identified a nonsynonymous single nucleotide polymorphism (SNP) mutation on chromosome 29 in exon 3 of the tyrosinase gene (c.1114C>T). A TaqMan SNP Genotyping Assay was developed to genotype this SNP in a total of 283 animals across 29 herds. This assay confirmed the absence of homozygous variants in all animals except 7 true albino bison included in this study. In addition, the only heterozygous animals identified were 2 wild-type pelage color dams of albino offspring. Therefore, we propose that this new high-quality bison genome assembly and incipient variant database provides a highly robust and informative resource for genomics investigations for this iconic North American species.

Refining the evolutionary tree of the horse Y chromosome.

The Y chromosome carries information about the demography of paternal lineages, and thus, can prove invaluable for retracing both the evolutionary trajectory of wild animals and the breeding history of domesticates. In horses, the Y chromosome shows a limited, but highly informative, sequence diversity, supporting the increasing breeding influence of Oriental lineages during the last 1500 years. Here, we augment the primary horse Y-phylogeny, which is currently mainly based on modern horse breeds of economic interest, with haplotypes (HT) segregating in remote horse populations around the world. We analyze target enriched sequencing data of 5 Mb of the Y chromosome from 76 domestic males, together with 89 whole genome sequenced domestic males and five Przewalski's horses from previous studies. The resulting phylogeny comprises 153 HTs defined by 2966 variants and offers unprecedented resolution into the history of horse paternal lineages. It reveals the presence of a remarkable number of previously unknown haplogroups in Mongolian horses and insular populations. Phylogenetic placement of HTs retrieved from 163 archaeological specimens further indicates that most of the present-day Y-chromosomal variation evolved after the domestication process that started around 4200 years ago in the Western Eurasian steppes. Our comprehensive phylogeny significantly reduces ascertainment bias and constitutes a robust evolutionary framework for analyzing horse population dynamics and diversity.

The Second Case of Non-Mosaic Trisomy of Chromosome 26 with Homologous Fusion 26q;26q in the Horse.

We present cytogenetic and genotyping analysis of a Thoroughbred foal with congenital neurologic disorders and its phenotypically normal dam. We show that the foal has non-mosaic trisomy for chromosome 26 (ECA26) but normal 2n = 64 diploid number because two copies of ECA26 form a metacentric derivative chromosome der(26q;26q). The dam has normal 64,XX karyotype indicating that der(26q;26q) in the foal originates from errors in parental meiosis or post-fertilization events. Genotyping ECA26 microsatellites in the foal and its dam suggests that trisomy ECA26 is likely of maternal origin and that der(26q;26q) resulted from Robertsonian fusion. We demonstrate that conventional and molecular cytogenetic approaches can accurately identify aneuploidy with a derivative chromosome but determining the mechanism and parental origin of the rearrangement requires genotyping with chromosome-specific polymorphic markers. Most curiously, this is the second case of trisomy ECA26 with der(26q;26q) in the horse, whereas all other equine autosomal trisomies are 'traditional' with three separate chromosomes. We discuss possible ECA26 instability as a contributing factor for the aberration and likely ECA26-specific genetic effects on the clinical phenotype. Finally, because ECA26 shares evolutionary homology with human chromosome 21, which trisomy causes Down syndrome, cytogenetic, molecular, and phenotypic similarities between trisomies ECA26 and HSA21 are discussed.

Copy number variation of horse Y chromosome genes in normal equine populations and in horses with abnormal sex development and subfertility: relationship of copy number variations with Y haplogroups.

Structural rearrangements like copy number variations in the male-specific Y chromosome have been associated with male fertility phenotypes in human and mouse but have been sparsely studied in other mammalian species. Here, we designed digital droplet PCR assays for 7 horse male-specific Y chromosome multicopy genes and SRY and evaluated their absolute copy numbers in 209 normal male horses of 22 breeds, 73 XY horses with disorders of sex development and/or infertility, 5 Przewalski's horses and 2 kulans. This established baseline copy number for these genes in horses. The TSPY gene showed the highest copy number and was the most copy number variable between individuals and breeds. SRY was a single-copy gene in most horses but had 2-3 copies in some indigenous breeds. Since SRY is flanked by 2 copies of RBMY, their copy number variations were interrelated and may lead to SRY-negative XY disorders of sex development. The Przewalski's horse and kulan had 1 copy of SRY and RBMY. TSPY and ETSTY2 showed significant copy number variations between cryptorchid and normal males (P < 0.05). No significant copy number variations were observed in subfertile/infertile males. Notably, copy number of TSPY and ETSTY5 differed between successive male generations and between cloned horses, indicating germline and somatic mechanisms for copy number variations. We observed no correlation between male-specific Y chromosome gene copy number variations and male-specific Y chromosome haplotypes. We conclude that the ampliconic male-specific Y chromosome reference assembly has deficiencies and further studies with an improved male-specific Y chromosome assembly are needed to determine selective constraints over horse male-specific Y chromosome gene copy number and their relation to stallion reproduction and male biology.

Y-Chromosomal Insights into Breeding History and Sire Line Genealogies of Arabian Horses.

The Y chromosome is a valuable genetic marker for studying the origin and influence of paternal lineages in populations. In this study, we conducted Y-chromosomal lineage-tracing in Arabian horses. First, we resolved a Y haplotype phylogeny based on the next generation sequencing data of 157 males from several breeds. Y-chromosomal haplotypes specific for Arabian horses were inferred by genotyping a collection of 145 males representing most Arabian sire lines that are active around the globe. These lines formed three discrete haplogroups, and the same haplogroups were detected in Arabian populations native to the Middle East. The Arabian haplotypes were clearly distinct from the ones detected in Akhal Tekes, Turkoman horses, and the progeny of two Thoroughbred foundation sires. However, a haplotype introduced into the English Thoroughbred by the stallion Byerley Turk (1680), was shared among Arabians, Turkomans, and Akhal Tekes, which opens a discussion about the historic connections between Oriental horse types. Furthermore, we genetically traced Arabian sire line breeding in the Western World over the past 200 years. This confirmed a strong selection for relatively few male lineages and uncovered incongruences to written pedigree records. Overall, we demonstrate how fine-scaled Y-analysis contributes to a better understanding of the historical development of horse breeds.

Targeted analysis of four breeds narrows equine Multiple Congenital Ocular Anomalies locus to 208 kilobases.

The syndrome Multiple Congenital Ocular Anomalies (MCOA) is the collective name ascribed to heritable congenital eye defects in horses. Individuals homozygous for the disease allele (MCOA phenotype) have a wide range of eye anomalies, while heterozygous horses (Cyst phenotype) predominantly have cysts that originate from the temporal ciliary body, iris, and/or peripheral retina. MCOA syndrome is highly prevalent in the Rocky Mountain Horse but the disease is not limited to this breed. Affected horses most often have a Silver coat color; however, a pleiotropic link between these phenotypes is yet to be proven. Locating and possibly isolating these traits would provide invaluable knowledge to scientists and breeders. This would favor maintenance of a desirable coat color while addressing the health concerns of the affected breeds, and would also provide insight into the genetic basis of the disease. Identical-by-descent mapping was used to narrow the previous 4.6-Mb region to a 264-kb interval for the MCOA locus. One haplotype common to four breeds showed complete association to the disease (Cyst phenotype, n = 246; MCOA phenotype, n = 83). Candidate genes from the interval, SMARCC2 and IKZF4, were screened for polymorphisms and genotyped, and segregation analysis allowed the MCOA syndrome region to be shortened to 208 kb. This interval also harbors PMEL17, the gene causative for Silver coat color. However, by shortening the MCOA locus by a factor of 20, 176 other genes have been unlinked from the disease and only 15 genes remain.

An 8.22 Mb Assembly and Annotation of the Alpaca (Vicugna pacos) Y Chromosome.

The unique evolutionary dynamics and complex structure make the Y chromosome the most diverse and least understood region in the mammalian genome, despite its undisputable role in sex determination, development, and male fertility. Here we present the first contig-level annotated draft assembly for the alpaca (Vicugna pacos) Y chromosome based on hybrid assembly of short- and long-read sequence data of flow-sorted Y. The latter was also used for cDNA selection providing Y-enriched testis transcriptome for annotation. The final assembly of 8.22 Mb comprised 4.5 Mb of male specific Y (MSY) and 3.7 Mb of the pseudoautosomal region. In MSY, we annotated 15 X-degenerate genes and two novel transcripts, but no transposed sequences. Two MSY genes, HSFY and RBMY, are multicopy. The pseudoautosomal boundary is located between SHROOM2 and HSFY. Comparative analysis shows that the small and cytogenetically distinct alpaca Y shares most of MSY sequences with the larger dromedary and Bactrian camel Y chromosomes. Most of alpaca X-degenerate genes are also shared with other mammalian MSYs, though WWC3Y is Y-specific only in alpaca/camels and the horse. The partial alpaca Y assembly is a starting point for further expansion and will have applications in the study of camelid populations and male biology.

Efficient correction of a deleterious point mutation in primary horse fibroblasts with CRISPR-Cas9.

Phenotypic selection during animal domestication has resulted in unwanted incorporation of deleterious mutations. In horses, the autosomal recessive condition known as Glycogen Branching Enzyme Deficiency (GBED) is the result of one of these deleterious mutations (102C > A), in the first exon of the GBE1 gene (GBE1102C>A). With recent advances in genome editing, this type of genetic mutation can be precisely repaired. In this study, we used the RNA-guided nuclease CRISPR-Cas9 (clustered regularly-interspaced short palindromic repeats/CRISPR-associated protein 9) to correct the GBE1102C>A mutation in a primary fibroblast cell line derived from a high genetic merit heterozygous stallion. To correct this mutation by homologous recombination (HR), we designed a series of single guide RNAs (sgRNAs) flanking the mutation and provided different single-stranded donor DNA templates. The distance between the Cas9-mediated double-stranded break (DSB) to the mutation site, rather than DSB efficiency, was the primary determinant for successful HR. This framework can be used for targeting other harmful diseases in animal populations.

Major inconsistencies of inferred population genetic structure estimated in a large set of domestic horse breeds using microsatellites.

STRUCTURE remains the most applied software aimed at recovering the true, but unknown, population structure from microsatellite or other genetic markers. About 30% of structure-based studies could not be reproduced (Molecular Ecology, 21, 2012, 4925). Here we use a large set of data from 2,323 horses from 93 domestic breeds plus the Przewalski horse, typed at 15 microsatellites, to evaluate how program settings impact the estimation of the optimal number of population clusters K opt that best describe the observed data. Domestic horses are suited as a test case as there is extensive background knowledge on the history of many breeds and extensive phylogenetic analyses. Different methods based on different genetic assumptions and statistical procedures (dapc, flock, PCoA, and structure with different run scenarios) all revealed general, broad-scale breed relationships that largely reflect known breed histories but diverged how they characterized small-scale patterns. structure failed to consistently identify K opt using the most widespread approach, the ΔK method, despite very large numbers of MCMC iterations (3,000,000) and replicates (100). The interpretation of breed structure over increasing numbers of K, without assuming a K opt, was consistent with known breed histories. The over-reliance on K opt should be replaced by a qualitative description of clustering over increasing K, which is scientifically more honest and has the advantage of being much faster and less computer intensive as lower numbers of MCMC iterations and repetitions suffice for stable results. Very large data sets are highly challenging for cluster analyses, especially when populations with complex genetic histories are investigated.

Population Genetic Analysis of the Estonian Native Horse Suggests Diverse and Distinct Genetics, Ancient Origin and Contribution from Unique Patrilines.

The Estonian Native Horse (ENH) is a medium-size pony found mainly in the western islands of Estonia and is well-adapted to the harsh northern climate and poor pastures. The ancestry of the ENH is debated, including alleged claims about direct descendance from the extinct Tarpan. Here we conducted a detailed analysis of the genetic makeup and relationships of the ENH based on the genotypes of 15 autosomal short tandem repeats (STRs), 18 Y chromosomal single nucleotide polymorphisms (SNPs), mitochondrial D-loop sequence and lateral gait allele in DMRT3. The study encompassed 2890 horses of 61 breeds, including 33 ENHs. We show that the expected and observed genetic diversities of the ENH are among the highest within 52 global breeds, and the highest among 8 related Northern European ponies. The genetically closest breeds to the ENH are the Finn Horse, and the geographically more distant primitive Hucul and Konik. ENH matrilines are diverse and relate to draught and Pontic-Caspian breeds. ENH patrilines relate to draught breeds, and to a unique haplogroup not described before. None of the 33 ENHs carried the "gait" mutation, but the mutation was found in 2 Huculs. The study demonstrates that the ENH is a genetically distinct and diverse breed of ancient origin with no notable pressure of selective breeding.

Comparative FISH-Mapping of MC1R, ASIP, and TYRP1 in New and Old World Camelids and Association Analysis With Coat Color Phenotypes in the Dromedary (Camelus dromedarius).

Melanocortin 1 receptor (MC1R), the agouti signaling protein (ASIP), and tyrosinase related protein 1 (TYRP1) are among the major regulators of pigmentation in mammals. Recently, MC1R and ASIP sequence variants were associated with white and black/dark brown coat colors, respectively, in the dromedary. Here we confirmed this association by independent sequencing and mutation discovery of MC1R and ASIP coding regions and by TaqMan genotyping in 188 dromedaries from Saudi Arabia and United States, including 38 black, 53 white, and 97 beige/brown/red animals. We showed that heterozygosity for a missense mutation c.901C > T in MC1R is sufficient for the white coat color suggesting a possible dominant negative effect. Likewise, we confirmed that the majority of black dromedaries were homozygous for a frameshift mutation in ASIP exon 2, except for 4 animals, which were heterozygous. In search for additional mutations underlying the black color, we identified another frameshift mutation in ASIP exon 4 and 6 new variants in MC1R including a significantly associated SNP in 3'UTR. In pursuit of sequence variants that may modify dromedary wild-type color from dark-reddish brown to light beige, we identified 4 SNPs and one insertion in TYRP1 non-coding regions. However, none of these were associated with variations in wild-type colors. Finally, the three genes were cytogenetically mapped in New World (alpaca) and Old World (dromedary and Bactrian camel) camelids. The MC1R was assigned to chr21, ASIP to chr19 and TYRP1 to chr4 in all 3 species confirming extensive conservation of camelid karyotypes. Notably, while the locations of ASIP and TYRP1 were in agreement with human-camelid comparative map, mapping MC1R identified a new evolutionary conserved synteny segment between camelid chromosome 21 and HSA16. The findings contribute to coat color genomics and the development of molecular tests in camelids and toward the chromosome level reference assemblies of camelid genomes.