Dynamic evolutionary processes and complex structure make the Y chromosome among the most diverse and least understood regions in mammalian genomes. Here, we present an annotated assembly of the male specific region of the horse Y chromosome (eMSY), representing the first comprehensive Y assembly in odd-toed ungulates. The eMSY comprises single-copy, equine specific multi-copy, PAR transposed, and novel ampliconic sequence classes. The eMSY gene density approaches that of autosomes with the highest number of retained X–Y gametologs recorded in eutherians, in addition to novel Y-born and transposed genes. Horse, donkey and mule testis RNAseq reveals several candidate genes for stallion fertility. A novel testis-expressed XY ampliconic sequence class, ETSTY7, is shared with the parasite Parascaris genome, providing evidence for eukaryotic horizontal transfer and inter-chromosomal mobility. Our study highlights the dynamic nature of the Y and provides a reference sequence for improved understanding of equine male development and fertility.

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. Genomes of individuals in a species vary in many ways, one of which is DNA copy number variation (CNV). This includes deletions, duplications, and complex rearrangements typically larger than 50 base-pairs. CNVs are part of normal genetic variation contributing to phenotypic diversity but can also be pathogenic and associated with diseases and disorders. In order to distinguish between the two, detailed knowledge about CNVs in the species of interest is needed. Here we studied the genomes of 38 normal horses of 16 diverse breeds, and identified 258 CNV regions. We integrated our findings with previously published horse CNVs and generated a composite dataset of ∼1400 CNVRs. Despite this large number, our analysis shows that CNV research in horses needs further improvement because the current data are based on 10% of horse breeds and that most CNVRs are study-specific and require validation. Finally, we analyzed CNVs in horses with disorders of sexual development and found in two male pseudo-hermaphrodites a large deletion disrupting a group of genes involved in sex hormone metabolism and sexual differentiation. The findings underline the possible role of CNVs in complex disorders such as development and reproduction.

Skin injuries in horses frequently lead to chronic wounds that lack a keratinocyte cover essential for healing. The limited proliferation of equine keratinocytes using current protocols has limited their use for regenerative medicine. Previously, equine induced pluripotent stem cells (eiPSCs) have been produced, and eiPSCs could be differentiated into equine keratinocytes suitable for stem cell-based skin constructs. However, the procedure is technically challenging and time-consuming. The present study was designed to evaluate whether conditional reprogramming (CR) could expand primary equine keratinocytes rapidly in an undifferentiated state but retain their ability to differentiate normally and form stratified epithelium. Conditional reprogramming was used to isolate and propagate two equine keratinocyte cultures. PCR and FISH were employed to evaluate the equine origin of the cells and karyotyping to perform a chromosomal count. FACS analysis and immunofluorescence were used to determine the purity of equine keratinocytes and their proliferative state. Three-dimensional air-liquid interphase method was used to test the ability of cells to differentiate and form stratified squamous epithelium. Conditional reprogramming was an efficient method to isolate and propagate two equine keratinocyte cultures. Cells were propagated at the rate of 2.39 days/doubling for more than 40 population doublings. A feeder-free culture method was also developed for long-term expansion. Rock-inhibitor is critical for both feeder and feeder-free conditions and for maintaining the proliferating cells in a stem-like state. PCR and FISH validated equine-specific markers in the cultures. Karyotyping showed normal equine 64, XY chromosomes. FACS using pan-cytokeratin antibodies showed a pure population of keratinocytes. When ROCK inhibitor was withdrawn and the cells were transferred to a three-dimensional air-liquid culture, they formed a well-differentiated stratified squamous epithelium, which was positive for terminal differentiation markers. Our results prove that conditional reprogramming is the first method that allows for the rapid and continued in vitro propagation of primary equine keratinocytes. These unlimited supplies of autologous cells could be used to generate transplants without the risk of immune rejection. This offers the opportunity for treating recalcitrant horse wounds using autologous transplantation.

Disorders of sex development (DSD) and reproduction are not uncommon among horses, though knowledge about their molecular causes is sparse. Here we characterized a ~200 kb homozygous deletion in chromosome 29 at 29.7–29.9 Mb. The region contains AKR1C genes which function as ketosteroid reductases in steroid hormone biosynthesis, including androgens and estrogens. Mutations in AKR1C genes are associated with human DSDs. Deletion boundaries, sequence properties and gene content were studied by PCR and whole genome sequencing of select deletion homozygotes and control animals. Deletion analysis by PCR in 940 horses, including 622 with DSDs and reproductive problems and 318 phenotypically normal controls, detected 67 deletion homozygotes of which 79% were developmentally or reproductively abnormal. Altogether, 8–9% of all abnormal horses were homozygous for the deletion, with the highest incidence (9.4%) among cryptorchids. The deletion was found in ~4% of our phenotypically normal cohort, ~1% of global warmblood horses and ponies, and ~7% of draught breeds of general horse population as retrieved from published data. Based on the abnormal phenotype of the carriers, the functionally relevant gene content, and the low incidence in general population, we consider the deletion in chromosome 29 as a risk factor for equine DSDs and reproductive disorders.