Assessment and Distribution of Runs of Homozygosity in Horse Breeds Representing Different Utility Types

SIMPLE SUMMARY: In recent years, the concept of runs of homozygosity (ROH) has become increasingly important in the genetic studies of various animal species, due to its advantages in estimating inbreeding and identifying regions under selection pressure. In this study, ROH distribution in six horse breeds belonging to three horse types (primitive, light, and draft) was examined. The presented results showed diverse differences in the length, number, and frequency of ROH between the analyzed breeds. Moreover, genomic inbreeding coefficients (F(ROH)) showed a higher level of inbreeding in light and primitive horses in comparison to draft horses. Regarding ROH islands, which are regions of the genome characterized by a high frequency of ROH and thus may represent signals of selection events, we observed several genes with confirmed effects on major horse breed characteristics. In addition, ROH regions of zero frequency were analyzed and these regions also spanned across genes involved in important breed characteristics. The presented results, especially regarding ROH islands and no-ROH regions, can be used as a basis for further research concerning the identification of genes and markers that determine important horse breed characteristics. ABSTRACT: 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.