Assessment of runs of homozygosity islands and estimates of genomic inbreeding in Gyr (Bos indicus) dairy cattle.

Background: Runs of homozygosity (ROH) are continuous homozygous segments of the DNA sequence. They have been applied to quantify individual autozygosity and used as a potential inbreeding measure in livestock species. The aim of the present study was (i) to investigate genome-wide autozygosity to identify and characterize ROH patterns in Gyr dairy cattle genome; (ii) identify ROH islands for gene content and enrichment in segments shared by more than 50% of the samples, and (iii) compare estimates of molecular inbreeding calculated from ROH (F _ROH ), genomic relationship matrix approach (F _GRM ) and based on the observed versus expected number of homozygous genotypes (F _HOM ), and from pedigree-based coefficient (F _PED ).
Results: ROH were identified in all animals, with an average number of 55.12 ± 10.37 segments and a mean length of 3.17 Mb. Short segments (ROH _{1-2 Mb} ) were abundant through the genomes, which accounted for 60% of all segments identified, even though the proportion of the genome covered by them was relatively small. The findings obtained in this study suggest that on average 7.01% (175.28 Mb) of the genome of this population is autozygous. Overlapping ROH were evident across the genomes and 14 regions were identified with ROH frequencies exceeding 50% of the whole population. Genes associated with lactation (TRAPPC9), milk yield and composition (IRS2 and ANG), and heat adaptation (HSF1, HSPB1, and HSPE1), were identified. Inbreeding coefficients were estimated through the application of F _ROH , F _GRM , F _HOM , and F _PED approaches. F _PED estimates ranged from 0.00 to 0.327 and F _ROH from 0.001 to 0.201. Low to moderate correlations were observed between F _PED -F _ROH and F _GRM -F _ROH , with values ranging from -0.11 to 0.51. Low to high correlations were observed between F _ROH -F _HOM and moderate between F _PED -F _HOM and F _GRM -F _HOM . Correlations between F _ROH from different lengths and F _PED gradually increased with ROH length.
Conclusions: Genes inside ROH islands suggest a strong selection for dairy traits and enrichment for Gyr cattle environmental adaptation. Furthermore, low F _PED- F _ROH correlations for small segments indicate that F _PED estimates are not the most suitable method to capture ancient inbreeding. The existence of a moderate correlation between larger ROH indicates that F _ROH can be used as an alternative to inbreeding estimates in the absence of pedigree records.