The present study reports runs of homozygosity (ROH) distribution in the genomes of six horse breeds (571 horses in total) representing three horse types (primitive, light, and draft horses) based on the 65k Equine BeadChip assay. Of major interest was the length, quantity, and frequency of ROH characteristics, as well as differences between horse breeds and types. Noticeable differences in the number, length and distribution of ROH between breeds were observed, as well as in genomic inbreeding coefficients. We also identified regions of the genome characterized by high ROH coverage, known as ROH islands, which may be signals of recent selection events. Eight to fourteen ROH islands were identified per breed, which spanned multiple genes. Many were involved in important horse breed characteristics, including WFIKNN2, CACNA1G, STXBP4, NOG, FAM184B, QDPR, LCORL, and the zinc finger protein family. Regions of the genome with zero ROH occurrences were also of major interest in specific populations. Depending on the breed, we detected between 2 to 57 no-ROH regions and identified 27 genes in these regions that were common for five breeds. These genes were involved in, e.g., muscle contractility (CACNA1A) and muscle development (miR-23, miR-24, miR-27). To sum up, the obtained results can be furthered analyzed in the topic of identification of markers unique for specific horse breed characteristics.
