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      Genomic measures of inbreeding coefficients and genome-wide scan for runs of homozygosity islands in Iranian river buffalo, Bubalus bubalis


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          Consecutive homozygous fragments of a genome inherited by offspring from a common ancestor are known as runs of homozygosity (ROH). ROH can be used to calculate genomic inbreeding and to identify genomic regions that are potentially under historical selection pressure. The dataset of our study consisted of 254 Azeri (AZ) and 115 Khuzestani (KHZ) river buffalo genotyped for ~ 65,000 SNPs for the following two purposes: 1) to estimate and compare inbreeding calculated using ROH (F ROH), excess of homozygosity (F HOM), correlation between uniting gametes (F UNI), and diagonal elements of the genomic relationship matrix (F GRM); 2) to identify frequently occurring ROH (i.e. ROH islands) for our selection signature and gene enrichment studies.


          In this study, 9102 ROH were identified, with an average number of 21.2 ± 13.1 and 33.2 ± 15.9 segments per animal in AZ and KHZ breeds, respectively. On average in AZ, 4.35% (108.8 ± 120.3 Mb), and in KHZ, 5.96% (149.1 ± 107.7 Mb) of the genome was autozygous. The estimated inbreeding values based on F HOM, F UNI and F GRM were higher in AZ than they were in KHZ, which was in contrast to the F ROH estimates. We identified 11 ROH islands (four in AZ and seven in KHZ). In the KHZ breed, the genes located in ROH islands were enriched for multiple Gene Ontology (GO) terms ( P ≤ 0.05). The genes located in ROH islands were associated with diverse biological functions and traits such as body size and muscle development (BMP2), immune response (CYP27B1), milk production and components (MARS, ADRA1A, and KCTD16), coat colour and pigmentation (PMEL and MYO1A), reproductive traits (INHBC, INHBE, STAT6 and PCNA), and bone development (SUOX).


          The calculated F ROH was in line with expected higher inbreeding in KHZ than in AZ because of the smaller effective population size of KHZ. Thus, we find that F ROH can be used as a robust estimate of genomic inbreeding. Further, the majority of ROH peaks were overlapped with or in close proximity to the previously reported genomic regions with signatures of selection. This tells us that it is likely that the genes in the ROH islands have been subject to artificial or natural selection.

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          Coefficients of Inbreeding and Relationship

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            Genomic Runs of Homozygosity Record Population History and Consanguinity

            The human genome is characterised by many runs of homozygous genotypes, where identical haplotypes were inherited from each parent. The length of each run is determined partly by the number of generations since the common ancestor: offspring of cousin marriages have long runs of homozygosity (ROH), while the numerous shorter tracts relate to shared ancestry tens and hundreds of generations ago. Human populations have experienced a wide range of demographic histories and hold diverse cultural attitudes to consanguinity. In a global population dataset, genome-wide analysis of long and shorter ROH allows categorisation of the mainly indigenous populations sampled here into four major groups in which the majority of the population are inferred to have: (a) recent parental relatedness (south and west Asians); (b) shared parental ancestry arising hundreds to thousands of years ago through long term isolation and restricted effective population size (Ne), but little recent inbreeding (Oceanians); (c) both ancient and recent parental relatedness (Native Americans); and (d) only the background level of shared ancestry relating to continental Ne (predominantly urban Europeans and East Asians; lowest of all in sub-Saharan African agriculturalists), and the occasional cryptically inbred individual. Moreover, individuals can be positioned along axes representing this demographic historic space. Long runs of homozygosity are therefore a globally widespread and under-appreciated characteristic of our genomes, which record past consanguinity and population isolation and provide a distinctive record of the demographic history of an individual's ancestors. Individual ROH measures will also allow quantification of the disease risk arising from polygenic recessive effects.
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              Extended tracts of homozygosity in outbred human populations.

              Long tracts of consecutive homozygous single nucleotide polymorphisms (SNPs) can arise in the genome through a number of mechanisms. These include inbreeding in which an individual inherits chromosomal segments that are identical by descent from each parent. However, recombination and other processes break up chromosomal segments over generations. The longest tracts are therefore to be expected in populations with an appreciable degree of inbreeding. We examined the length, number and distribution of long tracts of homozygosity in the apparently outbred HapMap populations. We observed 1393 tracts exceeding 1 Mb in length among the 209 unrelated HapMap individuals. The longest was an uninterrupted run of 3922 homozygous SNPs spanning 17.9 Mb in a Japanese individual. We find that homozygous tracts are significantly more common in regions with high linkage disequilibrium and low recombination, and the location of tracts is similar across all populations. The Yoruba sample has the fewest long tracts per individual, consistent with a larger number of generations (and hence amount of recombination) since the founding of that population. Our results suggest that multiple-megabase-scale ancestral haplotypes persist in outbred human populations in broad genomic regions which have lower than average recombination rates. We observed three outlying individuals who have exceptionally long and numerous homozygous tracts that are not associated with recombination suppressed areas of the genome. We consider that this reflects a high level of relatedness in their ancestry which is too recent to have been influenced by the local recombination intensity. Possible alternative mechanisms and the implications of long homozygous tracts in the genome are discussed.

                Author and article information

                BMC Genet
                BMC Genet
                BMC Genetics
                BioMed Central (London )
                10 February 2020
                10 February 2020
                : 21
                : 16
                [1 ]ISNI 0000 0004 0612 7950, GRID grid.46072.37, Department of Animal Science, , University College of Agriculture and Natural Resources, University of Tehran, ; Karaj, 31587-11167 Iran
                [2 ]ISNI 0000 0004 0612 7950, GRID grid.46072.37, Departments of Animal and Poultry Science, College of Aburaihan, , University of Tehran, ; Pakdasht, 33916-53755 Iran
                [3 ]AgriBio Centre for AgriBioscience, Agriculture Victoria, Bundoora, VIC 3083 Australia
                Author information
                © The Author(s). 2020

                Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License ( http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver ( http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.

                : 27 October 2019
                : 4 February 2020
                Research Article
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                © The Author(s) 2020

                water buffalo,river buffalo,genetic diversity,inbreeding,gene enrichment,runs of homozygosity,selection signatures


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