Runs of homozygosity (ROH) are contiguous homozygous genotype segments in the genome that are present in an individual since the identical haplotypes are inherited from each parent. The aim of this study was to investigate the frequency and distribution of ROH in the genomes of Landrace, Songliao black and Yorkshire pigs. We calculated two types of genome inbreeding coefficients and their correlation, including the inbreeding coefficient based on ROH (F_ROH) and the inbreeding coefficient based on the difference between the observed and expected number of homozygous genotypes (F_HOM). Furthermore, we identified candidate genes in the genomic region most associated with ROH. We identified 21,312 ROH in total. The average number of ROH per individual was 32.99 ± 0.38 and the average length of ROH was 6.40 ± 0.070 Mb in the three breeds. The F_ROH results showed that Yorkshire pigs exhibited the highest level of inbreeding (0.092 ± 0.0015) and that Landrace pigs exhibited the lowest level of inbreeding (0.073 ± 0.0047). The average correlation between F_ROH and F_HOM was high (0.94) within three breeds. The length of ROH provides insight into the inbreeding history of these three pig breeds. In this study, Songliao black pigs presented a higher frequency and average length of long ROH (>40 Mb) compared with those of Landrace and Yorkshire pigs, which indicated greater inbreeding in recent times. Genes related to reproductive traits (GATM, SPATA46, HSD17B7, VANGL2, DAXX, CPEB1), meat quality traits (NR1I3, APOA2, USF1) and energy conversion (NDUFS2) were identified within genomic regions with a high frequency of ROH. These genes could be used as target genes for further marker-assisted selection and genome selection.

Background: A run of homozygosity (ROH) is a consecutive tract of homozygous genotypes in an individual that indicates it has inherited the same ancestral haplotype from both parents. Genomic inbreeding can be quantified based on ROH. Genomic regions enriched with ROH may be indicative of selection sweeps and are known as ROH islands. We carried out ROH analyses in five Chinese indigenous sheep breeds; Altay sheep (n = 50 individuals), Large-tailed Han sheep (n = 50), Hulun Buir sheep (n = 150), Short-tailed grassland sheep (n = 150), and Tibetan sheep (n = 50), using genotypes from an Ovine Infinium HD SNP BeadChip.Results: A total of 18,288 ROH were identified. The average number of ROH per individual across the five sheep breeds ranged from 39 (Hulun Buir sheep) to 78 (Large-tailed Han sheep) and the average length of ROH ranged from 0.929 Mb (Hulun Buir sheep) to 2.544 Mb (Large-tailed Han sheep). The effective population size (Ne) of Altay sheep, Large-tailed Han sheep, Hulun Buir sheep, Short-tailed grassland sheep and Tibetan sheep were estimated to be 81, 78, 253, 238 and 70 five generations ago. The highest ROH-based inbreeding estimate (F_ROH) was 0.0808 in Large-tailed Han sheep, whereas the lowestF_ROHwas 0.0148 in Hulun Buir sheep. Furthermore, the highest proportion of long ROH fragments (> 5 Mb) was observed in the Large-tailed Han sheep breed which indicated recent inbreeding. In total, 49 ROH islands (the top 0.1% of the SNPs most commonly observed in ROH) were identified in the five sheep breeds. Three ROH islands were common to all the five sheep breeds, and were located on OAR2: 12.2–12.3 Mb, OAR12: 78.4–79.1 Mb and OAR13: 53.0–53.6 Mb. Three breed-specific ROH islands were observed in Altay sheep (OAR15: 3.4–3.8 Mb), Large-tailed Han sheep (ORA17: 53.5–53.8 Mb) and Tibetan sheep (ORA5:19.8–20.2 Mb). Collectively, the ROH islands harbored 78 unique genes, including 19 genes that have been documented as having associations with tail types, adaptation, growth, body size, reproduction or immune response.Conclusion: Different ROH patterns were observed in five Chinese indigenous sheep breeds, which reflected their different population histories. Large-tailed Han sheep had the highest genomic inbreeding coefficients and the highest proportion of long ROH fragments indicating recent inbreeding. Candidate genes in ROH islands could be used to illustrate the genetic characteristics of these five sheep breeds. Our findings contribute to the understanding of genetic diversity and population demography, and help design and implement breeding and conservation strategies for Chinese sheep.

Background: Quantifying and controlling the inbreeding level in livestock populations is crucial for the long-term sustainability of animal husbandry. However, the extent of inbreeding has not been fully understood in sheep populations on a global scale. Methods: Here, we analyzed high-depth genomes of 210 sheep from 20 worldwide breeds to identify the pattern and distribution of genome-wide runs of homozygosity (ROH) and detect candidate selected genes in ROH islands for agronomic and phenotypic traits. Results: Leveraging whole-genome sequencing data, we found a large number of short ROH (e.g., <1.0 Mb) in all breeds and observed the overall higher values of ROH statistics and inbreeding coefficient in European breeds than in Asian breeds and Dorper sheep. We identified some well-known candidate genes (e.g., CAMK4, HOXA gene family, ALOX12, FGF11, and MTOR) and 40 novel genes (e.g., KLHL1, FGFRL1, WDR62, GDF6, KHDRBS2, and PAX1) that are functionally associated with fecundity, body size, and wool-related traits in sheep. Based on the candidate genes, we revealed different genetic bases for the fecundity traits of European and Asian sheep. Conclusions: This study improves the resolution of ROH detection and provides new insights into genomic inbreeding and trait architecture in sheep as well as useful markers for future breeding practice.