Thoracic wall deformity (TWD) is reported in several cat breeds. The Burmese and Bengal cats appear predisposed, suggesting a genetic background. Thoracic changes in Bengal kittens were investigated using whole-body computed tomography, macro- and microscopic examination of the rib cage. By using a questionnaire, breeding performance and kitten mortality among Bengal cats in Sweden during a two-year period was analyzed. TWD comprised a spectrum of symmetrical and asymmetrical rib cage changes. Macroscopically, lordosis, flat chest and pectus excavatum were frequent findings. When asymmetric, the right rib cage was typically most severely affected. Microscopic rib changes included angled rib cartilages, uneven growth plates, enlarged perichondrial ring of Ranvier, ectopic cartilage formation, fracture of primary trabeculae, and abundant vascular structures in rib cartilages. The questionnaire revealed TWD-affected kittens in 38 out of a total of 311 litters (12%). Mean litter size was 4.0 (SD 1.8). Stillborn kittens and kitten deaths within the first 24 h occurred significantly more often in litters with than without TWD (20% vs. 12%, respectively; P = 0.025). Breeder-reported signs included lack of weight gain and dyspnoea. Male kittens were over-represented (72%). Two thirds of owner-reported kittens affected by TWD survived. The proportion of litters with TWD affected kittens varied between sires. The increased perinatal mortality and proportion of stillborn kittens in TWD-affected litters suggest that both pre- and postnatal factors contribute to TWD. The variation in frequency between sires suggests a genetic background. Strategic selection of breeding animals may reduce the frequency of TWD. Identification of genetic risk variants warrants further investigation.

The aim of this study was to better understand the genomic architecture behind performance-related traits in sport horses. In this study, we conducted a haplotype-based genome-wide association study (GWAS) for 36 conformation and free jumping phenotypes recorded during routinely conducted young horse evaluation tests involving 380 Swedish Warmblood (SWB) horses. The horses were evaluated by expert judges using both traditional and linear evaluation systems. All samples were genotyped using the 670K Affymetrix® Axiom® Equine Genotyping Array, haplotypes were first phased, and haplotype blocks were calculated for a total of 78,000 haplotypes. To assess the association between the haplotypes and studied traits, a single-trait linear mixed model was used, correcting for sex and the date-location in which the evaluation took place. In the analysis, a total of 11 haplotype blocks were found to be significantly associated with a total of six traits: height at withers, the conformation traits hooves and correctness in movement, and the free jumping traits technique: haunches, carefulness, and distance estimation. In the proximity of those haplotypes (windows size ± 500 kb), 33 protein-coding genes, 31 IncRNAs, and one miRNA were found. Within those regions, key candidate genes were located such as LCORL and KHDRBS3, associated with body size and growth, as well as COL12A1, MYO6, and FILIP1, involved in musculoskeletal development and muscle elasticity and strength. The haplotype-based GWAS approach proved to be a useful method since it helped in the detection of aggregated genetic effects. Future studies with larger sample sizes and using novel tools for objective phenotyping will be essential to further investigate the genetic mechanisms behind sports performance.

Sustainable breeding of native breeds is essential to preserve genetic diversity and cultural heritage. Several native Nordic horse breeds are at risk of extinction and lack genetic characterization. This study aimed to analyze genetic variation and kinship within and among native Nordic horse breeds using whole genome sequence data, and to compare results from using a Finnhorse genome assembly to that of the EquCab3.0 (Thoroughbred) reference genome. The breeds Dola horse, North Swedish horse and Coldblooded Trotter showed close genetic relationship for fixation index (0.04-0.10), and in principal component analysis. The other breeds showed stronger genetic differentiation, especially the Faroese horse with fixation index above 0.21 to all other breeds. This breed had the highest genomic inbreeding of 33% and a heterozygosity of 12%. The Swedish Ardennes showed the lowest inbreeding at 14% and a heterozygosity of 16%. The North Swedish horses had the highest historical Ne of 96, estimated 13 generations back in time, and the Faroese horse the lowest (23). The mean identity by descent varied from 17% for Swedish Ardennes to 40% for Faroese horses. The choice of reference genomes gave minor to moderate differences, suggesting that a closer related reference improves precision for fine mapping and understanding of genetic landscapes of Nordic breeds. Together, the different analyses showed low genetic diversity in all breeds, and the general pattern of relatedness largely agreed with the known breed history. The results underline the importance of maintaining genetic diversity for the survival of the breeds.

ABSTRACT The wild boar (Sus scrofa) has been a part of the Swedish fauna for thousands of years. Although it became extinct in the 17th century, populations reemerged in the 1970s and 1980s after escapes from enclosures. The country of origin of the wild boars kept in these enclosures is unknown. This study aims to investigate the origin and level of genetic diversity of Swedish wild boars, as well as to compare them with Swedish domestic pigs. Swedish wild boar samples were collected from four regions of Sweden: Skåne, Blekinge, Södermanland and Uppland. In total, 107 wild boars and 427 domestic pigs were genotyped using the 80 K SNP chip. Wild boars from Skåne and Blekinge were shown to belong to the same genetic group, whereas the Södermanland and Uppland formed separate clusters. Results from both principal component analysis (PCA) and fixation index (FST) analyses demonstrate that Swedish wild boars are genetically distinct from Swedish domestic pigs. Moreover, the ADMIXTURE analysis revealed no evidence of introgression from domestic pigs into Swedish wild boar populations. The genetic differentiation observed among Swedish wild boar populations further indicates that they originate from more than one external source population.

Large-effect recessive damaging variants found at relatively high frequencies are common enough in domestic animals that they cause problems in breeding programs. There are several potential reasons. Deleterious variants may rise in frequency because of genetic drift, be driven to intermediate frequency by balancing selection by heterozygote advantage, or be spread by exceptionally influential parents, usually sires, that sometimes occur in animal breeding programs. Further, animal populations are often connected by migration. In this paper, we use a series of population genetic models to address how large-effect damaging alleles establish themselves in small populations, such as domestic animal populations or wild populations of mammals. The results confirm that a lethal allele under directional selection is fairly unlikely to become common, unless there are influential sire events with very large contributions or the population size is very small. On the other hand, a lethal allele under balancing selection can easily become common, especially if there are influential parents. When there is balancing selection, migration between connected populations effectively spreads alleles. The results accord with the many examples of recessive damaging alleles in domestic animals. ### Competing Interest Statement The authors have declared no competing interest. Swedish Research Council for Environment Agricultural Sciences and Spatial Planning, https://ror.org/03pjs1y45, 2020-01637 Kjell och Märta Beijers Stiftelse, https://ror.org/00fmf4r59

Fragile foal syndrome (FFS) is a disease caused by a recessive lethal missense mutation in the PLOD1 gene located on ECA2. Despite its harmful effect, a relatively high frequency of FFS carriers was observed in Warmblood breeds spanning from 7.4% in a random sample of Swedish Warmblood breed to 17% in the Hanoverian and Danish Warmblood, indicating potential heterozygous advantage. Balancing selection can be further studied based on haplotype blocks and via detection of heterozygosity-rich region (ROHet) around the target of selection. In this study we evaluated the presence of haplotype blocks and ROHet on ECA2 in 380 Swedish Warmblood horses. We compared the results of ROHet with the rest of the genome. On average, 11.7 heterozygosity rich regions were identified per horse on ECA2, with no significant difference in numbers and length compared to what was found in other chromosomes. A unique haplotype block containing 28 markers was found in the FFS haplotype, while there were several haplotype blocks in the non-carrier haplotype. This unique haplotype block mostly spanned the region upstream of the PLOD1 gene and included the MFN2 gene. The presence of this extended haplotype, shared by multiple individuals and including both the FFS variant and the MFN2 gene, suggests that this region may be under selection. While we did not find a clear heterozygosity-rich region around the FFS variant, the extended haplotype may reflect either a signature of balancing selection or linkage disequilibrium with a positively selected variant in MFN2, PLOD1, or nearby loci.

Swedish Warmblood horses (SWB) are bred for show jumping and/or dressage with young horse test scores as indicator traits. This study aimed to investigate possible candidate genes and regions of importance for evaluated and linearly scored young horse test traits. A single-step genome-wide association study (ssGWAS) was done using the BLUPF90 suite of programs for factors scores from factor analysis of traits assessed at young horse tests together with height at withers. The ssGWAS included 20,814 SWB with factors scores for four factors for evaluated traits. A total of 6436 of these horses also had factor scores for 13 factors for linearly scored traits. Genotypes from a 670K SNP array were available for 380 of the horses in this study. All genotyped horses had factor scores for evaluated traits, and 379 also had factors scores for linearly scored traits. Significant SNPs associated with three factors related to size were located on ECA3 within or nearby a well-known region, including the genes ligand dependent nuclear receptor corepressor like (LCORL), non-SMC condensin I complex subunit G (NCAPG), DDB1 and CUL4 Associated Factor 16 (DCAF16), and the Family with Sequence Similarity 184 Member B (FAM184B). Significant SNPs were also detected for two factors for evaluated traits representing conformation and jumping, and four factors for linearly scored traits related to body length, neck conformation, walk and trot (hindleg position and activity), respectively. Among nearby genes, calcium/calmodulin-dependent protein kinase type 1D (CAMK1D) for the factor for linearly scored traits related to neck conformation and GLI Family Zinc Finger 2 (GLI2) for the factor for evaluated jumping traits, were most promising. For these, top associated SNPs were detected within the genes, and the known gene functions seems to be related to the phenotypes. In conclusion, ssGWAS is beneficial to detect plausible candidate genes/regions for desired traits in warmblood horses.

Abstract Background The Franches-Montagnes (FM) is the last native horse breed of Switzerland, established at the end of the 19th century by cross-breeding local mares with Anglo-Norman stallions. We collected high-density SNP genotype data (Axiom™ 670 K Equine genotyping array) from 522 FM horses, including 44 old-type horses (OF), 514 European Warmblood horses (WB) from Sweden and Switzerland (including a stallion used for cross-breeding in 1990), 136 purebred Arabians (AR), 32 Shagya Arabians (SA), and 64 Thoroughbred (TB) horses, as introgressed WB stallions showed TB origin in their pedigrees. The aim of the study was to ascertain fine-scale population structures of the FM breed, including estimation of individual admixture levels and genomic inbreeding (FROH) by means of Runs of Homozygosity. Results To assess fine-scale population structures within the FM breed, we applied a three-step approach, which combined admixture, genetic contribution, and FROH of individuals into a high-resolution network visualization. Based on this approach, we were able to demonstrate that population substructures, as detected by model-based clustering, can be either associated with a different genetic origin or with the progeny of most influential sires. Within the FM breed, admixed horses explained most of the genetic variance of the current breeding population, while OF horses only accounted for a small proportion of the variance. Furthermore, we illustrated that FM horses showed high TB admixture levels and we identified inconsistencies in the origin of FM horses descending from the Arabian stallion Doktryner. With the exception of WB, FM horses were less inbred compared to the other breeds. However, the relatively few but long ROH segments suggested diversity loss in both FM subpopulations. Genes located in FM- and OF-specific ROH islands had known functions involved in conformation and behaviour, two traits that are highly valued by breeders. Conclusions The FM remains the last native Swiss breed, clearly distinguishable from other historically introgressed breeds, but it suffered bottlenecks due to intensive selection of stallions, restrictive mating choices based on arbitrary definitions of pure breeding, and selection of rare coat colours. To preserve the genetic diversity of FM horses, future conservation managements strategies should involve a well-balanced selection of stallions (e.g., by integrating OF stallions in the FM breeding population) and avoid selection for rare coat colours.

Background Warmblood fragile foal syndrome (WFFS) is a monogenetic defect caused by a recessive lethal missense point mutation in the procollagen-lysine, 2-oxoglutarate 5-dioxygenase 1 gene (PLOD1, c.2032G>A). The majority of homozygous WFFS horses are aborted during gestation. Clinical signs of affected horses include fragile skin, skin and mucosa lacerations, hyperextension of the articulations, and hematomas. In spite of its harmful effect, a relatively high frequency of WFFS carriers has been found in Warmblood horses, suggesting a heterozygote advantage. Thus, in this study our aims were to: (1) estimate the frequency of WFFS carriers in the Swedish Warmblood breed (SWB), (2) estimate the effect of WFFS carrier genotype on performance traits in two SWB subpopulations bred for different disciplines, and (3) simulate the potential effects of balancing selection and different selection strategies on the frequency of carriers. Methods In total, 2288 SWB sport horses born between 1971 and 2020 were tested for the WFFS mutation and had estimated breeding values (EBV) for ten traditional evaluating and 50 linear descriptive traits. Results The frequency of WFFS carriers calculated from a pool of 511 randomly selected SWB horses born in 2017 was equal to 7.4% and ranged from 0.0 to 12.0% among the whole set of tested SWB horses, starting from 1971 till 2020. The effect of the WFFS carrier genotype was significant for several EBV mainly related to movements and dressage traits and especially for horses not bred for the show jumping discipline. Using simulation, we showed that balancing selection can maintain a recessive lethal allele in populations such as the SWB breed over generations and that the frequency is expected to slowly decrease in absence of balancing selection. Finally, we showed that selection against carrier sires can result in a more rapid decrease of the frequency of the mutant allele over time. Conclusion Further research is needed to confirm the apparent association between equine performance and the WFFS carrier genotype. Identification of such associations or new causative mutations for horse performance traits can serve as new tools in horse breeding to select for healthy, sustainable, and better performing horses.

The Fragile Foal Syndrome (FFS) is a disease caused by a recessive lethal missense mutation in the PLOD1 gene (ECA2). Despite its harmful effect, a relatively high frequency of the FFS allele carriers has been found in the Swedish Warmblood (SWB), suggesting a heterozygous advantage. Balancing selection can be further studied in haplotype blocks and increased heterozygosity around the target of selection. In this study we evaluated the presence of haplotype blocks and Runs of Heterozygosity on ECA2 in 380 SWB horses divided by sport discipline. In horses mainly bred for dressage a haplotype block comprising the FFS variant was found. On average 11.7 Runs of Heterozygosity were identified per horse on ECA2, with no significant difference in numbers between the sport disciplines. This study provides a preliminary characterization of haplotype blocks and heterozygosity rich regions on ECA2 which may further corroborate the potential presence of balancing selection for the FFS allele.

Additional file 4: Table S3. Descriptive statistics of the traditional evaluating traits in the SJ horses. The following values are reported for each traditional evaluating trait in the SJ horses: number of observations, mean, standard deviation, minimum, 25th percentile, 75th percentile and maximum.

The nematode Haemonchus contortus is one of the most prevalent and pathogenic parasites in small ruminants. Although usually controlled using anthelmintics, the development of drug resistance by the parasite has become a major issue in livestock production. While the molecular detection of benzimidazole resistance in H. contortus is well developed, the molecular tools and protocols are far less advanced for the detection of levamisole resistance. The hco-acr-8 gene encodes a critical acetylcholine susceptible subunit that confers levamisole-sensitivity to the receptor. Here, we report the development of a droplet digital PCR assay as a molecular tool to detect a 63 bp deletion in the hco-acr-8 that has been previously associated with levamisole resistance. Sanger sequencing of single adult H. contortus yielded 56 high-quality consensus sequences surrounding the region containing the deletion. Based on the sequencing data, new primers and probes were designed and validated with a novel droplet digital PCR assay for the quantification of the deletion containing “resistant” allele in genomic DNA samples. Single adult worms from six phenotypically described isolates (n = 60) and from two Swedish sheep farms (n = 30) where levamisole was effective were tested. Even though a significant difference in genotype frequencies between the resistant and susceptible reference isolates was found (p = 0.01), the homozygous “resistant” genotype was observed to be abundantly present in both the susceptible isolates as well as in some Swedish H. contortus samples. Furthermore, field larval culture samples, collected pre- (n = 7) and post- (n = 6) levamisole treatment on seven Swedish sheep farms where levamisole was fully efficacious according to Fecal Egg Count Reduction Test results, were tested to evaluate the frequency of the “resistant” allele in each. Frequencies of the deletion ranged from 35 to 80% in the pre-treatment samples, whereas no amplifiable H. contortus genomic DNA was detected in the post-treatment samples. Together, these data reveal relatively high frequencies of the 63 bp deletion in the hco-acr-8 both on individual H. contortus and field larval culture scales, and cast doubt on the utility of the deletion in the hco-acr-8 as a molecular marker for levamisole resistance detection on sheep farms.

Horses are nowadays mainly used for sport and leisure activities, and several local breeds, traditionally used in agriculture, have been exposed to a dramatic loss in population size and genetic diversity. The loss of genetic diversity negatively impacts individual fitness and reduces the potential long-term survivability of a breed. Recent advances in molecular biology and bioinformatics have allowed researchers to explore biodiversity one step further. This study aimed to evaluate the loss of genetic variability and identify genomic regions under selection pressure in the Bardigiano breed based on GGP Equine70k SNP data. The effective population size based on Linkage Disequilibrium (Ne) was equal to 39 horses, and it showed a decline over time. The average inbreeding based on runs of homozygosity (ROH) was equal to 0.17 (SD = 0.03). The majority of the ROH were relatively short (91% were ≤ 2Mbp long), highlighting the occurrence of older inbreeding, rather than a more recent occurrence. A total of eight ROH islands, shared among more than 70% of the Bardigiano horses, were found. Four of them mapped to known quantitative trait loci related to morphological traits (e.g., body size and coat color) and disease susceptibility. This study provided the first genome-wide scan of genetic diversity and selection signatures in an Italian native horse breed.

Domestic animal populations are often characterised by high rates of inbreeding and low effective population sizes due to selective breeding practices. These practices can result in otherwise rare recessive deleterious alleles drifting to high frequencies, resulting in reduced fertility rates. This study aimed to identify potential recessive lethal haplotypes in the Thoroughbred horse breed, a closed population that has been selectively bred for racing performance. In this study, we identified a haplotype in the LY49B gene that shows strong evidence of being homozygous lethal, despite having high frequencies of heterozygotes in Thoroughbreds and other domestic horse breeds. Variant analysis of whole-genome sequence data identified two SNPs in the 3′UTR of the LY49B gene that may result in loss of function. Analysis of transcriptomic data from equine embryonic tissue revealed that LY49B is expressed in the trophoblast during placentation stage of development. These findings suggest that LY49B may have an essential, but as yet unknown function in the implantation stage of equine development. Further investigation of this region may allow for the development of a genetic test to improve fertility rates in horse populations. Identification of other lethal variants could assist in improving natural levels of fertility in horse populations.

AbstractRecessive lethal variants often segregate at low frequencies in animal populations, such that two randomly selected individuals are unlikely to carry the same mutation. However, the likelihood of an individual inheriting two copies of a recessive lethal mutation is dramatically increased by inbreeding events. Such occurrences are particularly common in domestic animal populations, which are often characterised by high rates of inbreeding and low effective population sizes. To date there have been no published investigations into the presence of specific variants at high frequencies in domestic horse populations. This study aimed to identify potential recessive lethal haplotypes in the Thoroughbred horse breed, a closed population that has been selectively bred for racing performance.In this study, we scanned genotype data from Thoroughbred horses(n= 526) for adjacent single nucleotide polymorphisms (SNPs) at high heterozygote frequencies, but with a complete absence of homozygotes. Two SNPs that matched these criteria were mapped to an intronic region in theLY49Bgene, indicating that a closely linked mutation may cause lethality in homozygous state. Despite a complete absence of homozygotes, almost 35% of Thoroughbreds included in these analyses were heterozygous for both SNPs. A similar loss or absence of homozygotes was observed in genotype data from other domestic horse breeds (n= 2030). Variant analysis of whole-genome sequence data (n= 90) identified two SNPs in the 3’UTR region of theLY49Bgene that may result in loss of function. Analysis of transcriptomic data from equine embryonic tissue revealed thatLY49Bis expressed in the trophoblast during placentation stage of development.In this study, a region in theLY49Bgene was identified as a strong candidate for harbouring a variant causing lethality in homozygous state. These findings suggest thatLY49Bmay have an essential, but as yet unknown function in the implantation stage of equine development. Further investigation of this region may allow for the development of a genetic test to improve fertility rates in horse populations. Identification of other lethal variants could assist in improving natural levels of fertility in horse populations.Author SummaryRecessive lethal mutations may reach high frequencies in livestock populations due to selective breeding practices, resulting in reduced fertility rates. In this study, we characterise recessive lethal mutations at high frequencies in the Thoroughbred horse population, a breed with high rates of inbreeding and low genetic diversity. We identified a haplotype in theLY49Bgene that shows strong evidence of being homozygous lethal, despite having high frequencies of heterozygotes in Thoroughbreds and other domestic horse breeds. Two 3’UTR variants were identified as most likely to cause loss of function in theLY49Bgene, resulting in lethality. This finding provides novel insights into the potential importance ofLY49Bin equine development. Additionally, this study may assist with breeding strategies to improve fertility rates in the Thoroughbred and other domestic horse breeds.

Background A growing demand for improved physical skills and mental attitude in modern sport horses has led to strong selection for performance in many warmblood studbooks. The aim of this study was to detect genomic regions with low diversity, and therefore potentially under selection, in Swedish Warmblood horses (SWB) by analysing high-density SNP data. To investigate if such signatures could be the result of selection for equestrian sport performance, we compared our SWB SNP data with those from Exmoor ponies, a horse breed not selected for sport performance traits. Results The genomic scan for homozygous regions identified long runs of homozygosity (ROH) shared by more than 85% of the genotyped SWB individuals. Such ROH were located on ECA4, ECA6, ECA7, ECA10 and ECA17. Long ROH were instead distributed evenly across the genome of Exmoor ponies in 77% of the chromosomes. Two population differentiation tests (F ST and XP-EHH) revealed signatures of selection on ECA1, ECA4, and ECA6 in SWB horses. Conclusions Genes related to behaviour, physical abilities and fertility, appear to be targets of selection in the SWB breed. This study provides a genome-wide map of selection signatures in SWB horses, and ground for further functional studies to unravel the biological mechanisms behind complex traits in horses.

Background Native cattle breeds are important genetic resources given their adaptation to the local environment in which they are bred. However, the widespread use of commercial cattle breeds has resulted in a marked reduction in population size of several native cattle breeds worldwide. Therefore, conservation management of native cattle breeds requires urgent attention to avoid their extinction. To this end, we genotyped nine Swedish native cattle breeds with genome-wide 150 K single nucleotide polymorphisms (SNPs) to investigate the level of genetic diversity and relatedness between these breeds. Results We used various SNP-based approaches on this dataset to connect the demographic history with the genetic diversity and population structure of these Swedish cattle breeds. Our results suggest that the Väne and Ringamåla breeds originating from southern Sweden have experienced population isolation and have a low genetic diversity, whereas the Fjäll breed has a large founder population and a relatively high genetic diversity. Based on the shared ancestry and the constructed phylogenetic trees, we identified two major clusters in Swedish native cattle. In the first cluster, which includes Swedish mountain cattle breeds, there was little differentiation among the Fjäll, Fjällnära, Swedish Polled, and Bohus Polled breeds. The second cluster consists of breeds from southern Sweden: Väne, Ringamåla and Swedish Red. Interestingly, we also identified sub-structuring in the Fjällnära breed, which indicates different breeding practices on the farms that maintain this breed. Conclusions This study represents the first comprehensive genome-wide analysis of the genetic relatedness and diversity in Swedish native cattle breeds. Our results show that different demographic patterns such as genetic isolation and cross-breeding have shaped the genomic diversity of Swedish native cattle breeds and that the Swedish mountain breeds have retained their authentic distinct gene pool without significant contribution from any of the other European cattle breeds that were included in this study.

By studying genes associated with coat colour, we can understand the role of these genes in pigmentation but also gain insight into selection history. North European short-tailed sheep, including Swedish breeds, have variation in their coat colour, making them good models to expand current knowledge of mutations associated with coat colour in sheep. We studied ASIP and MC1R, two genes with known roles in pigmentation, and their association with black coat colour. We did this by sequencing the coding regions of ASIP in 149 animals and MC1R in 129 animals from seven native Swedish sheep breeds in individuals with black, white or grey fleece. Previously known mutations in ASIP [recessive black allele: g.100_105del (D5 ) and/or g.5172T>A] were associated with black coat colour in Klövsjö and Roslag sheep breeds and mutations in both ASIP and MC1R (dominant black allele: c.218T>A and/or c.361G>A) were associated with black coat colour in Swedish Finewool. In Gotland, Gute, Värmland and Helsinge sheep breeds, coat colour inheritance was more complex: only 11 of 16 individuals with black fleece had genotypes that could explain their black colour. These breeds have grey individuals in their populations, and grey is believed to be a result of mutations and allelic copy number variation within the ASIP duplication, which could be a possible explanation for the lack of a clear inheritance pattern in these breeds. Finally, we found a novel missense mutation in MC1R (c.452G>A) in Gotland, Gute and Värmland sheep and evidence of a duplication of MC1R in Gotland sheep.

Background Copy Number Variation (CNV) is a common form of genetic variation underlying animal evolution and phenotypic diversity across a wide range of species. In the mammalian genome, high frequency of CNV differentiation between breeds may be candidates for population-specific selection. However, CNV differentiation, selection and its population genetics have been poorly explored in horses. Results We investigated the patterns, population variation and gene annotation of CNV using the Axiom® Equine Genotyping Array (670,796 SNPs) from a large cohort of individuals ( N = 1755) belonging to eight European horse breeds, varying from draught horses to several warmblood populations. After quality control, 152,640 SNP CNVs (individual markers), 18,800 segment CNVs (consecutive SNP CNVs of same gain/loss state or both) and 939 CNV regions (CNVRs; overlapping segment CNVs by at least 1 bp) compared to the average signal of the reference (Belgian draught horse) were identified. Our analyses showed that Equus caballus chromosome 12 (ECA12) was the most enriched in segment CNV gains and losses (~ 3% average proportion of the genome covered), but the highest number of segment CNVs were detected on ECA1 and ECA20 (regardless of size). The Friesian horses showed private SNP CNV gains (> 20% of the samples) on ECA1 and Exmoor ponies displayed private SNP CNV losses on ECA25 (> 20% of the samples). The Warmblood cluster showed private SNP CNV gains located in ECA9 and Draught cluster showed private SNP CNV losses located in ECA7. The length of the CNVRs ranged from 1 kb to 21.3 Mb. A total of 10,612 genes were annotated within the CNVRs. The PANTHER annotation of these genes showed significantly under- and overrepresented gene ontology biological terms related to cellular processes and immunity (Bonferroni P -value < 0.05). We identified 80 CNVRs overlapping with known QTL for fertility, coat colour, conformation and temperament. We also report 67 novel CNVRs. Conclusions This work revealed that CNV patterns, in the genome of some European horse breeds, occurred in specific genomic regions. The results provide support to the hypothesis that high frequency private CNVs residing in genes may potentially be responsible for the diverse phenotypes seen between horse breeds.

The equestrian sport horse Swedish Warmblood (SWB) originates from versatile cavalry horses. Most modern SWB breeders have specialized their breeding either towards show jumping or dressage disciplines. The aim of this study was to explore the genomic structure of SWB horses to evaluate the presence of genomic subpopulations, and to search for signatures of selection in subgroups of SWB with high or low breeding values (EBVs) for show jumping. We analyzed high density genotype information from 380 SWB horses born in the period 2010–2011, and used Principal Coordinates Analysis and Discriminant Analysis of Principal Components to detect population stratification. Fixation index and Cross Population Extended Haplotype Homozygosity scores were used to scan the genome for potential signatures of selection. In accordance with current breeding practice, this study highlights the development of two separate breed subpopulations with putative signatures of selection in eleven chromosomes. These regions involve genes with known function in, e.g., mentality, endogenous reward system, development of connective tissues and muscles, motor control, body growth and development. This study shows genetic divergence, due to specialization towards different disciplines in SWB horses. This latter evidence can be of interest for SWB and other horse studbooks encountering specialized breeding.