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A Survey of Functional Genomic Variation in Domesticated Chickens Martijn F

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A Survey of Functional Genomic Variation in Domesticated Chickens Martijn F Derks et al. Genet Sel Evol (2018) 50:17 https://doi.org/10.1186/s12711-018-0390-1 Genetics Selection Evolution RESEARCH ARTICLE Open Access A survey of functional genomic variation in domesticated chickens Martijn F. L. Derks1*, Hendrik‑Jan Megens1, Mirte Bosse1, Jeroen Visscher2, Katrijn Peeters2, Marco C. A. M. Bink2, Addie Vereijken2, Christian Gross3,4, Dick de Ridder3, Marcel J. T. Reinders4 and Martien A. M. Groenen1 Abstract Background: Deleterious genetic variation can increase in frequency as a result of mutations, genetic drift, and genetic hitchhiking. Although individual efects are often small, the cumulative efect of deleterious genetic variation can impact population ftness substantially. In this study, we examined the genome of commercial purebred chicken lines for deleterious and functional variations, combining genotype and whole-genome sequence data. Results: We analysed over 22,000 animals that were genotyped on a 60 K SNP chip from four purebred lines (two white egg and two brown egg layer lines) and two crossbred lines. We identifed 79 haplotypes that showed a signif‑ cant defcit in homozygous carriers. This defcit was assumed to stem from haplotypes that potentially harbour lethal recessive variations. To identify potentially deleterious mutations, a catalogue of over 10 million variants was derived from 250 whole-genome sequenced animals from three purebred white-egg layer lines. Out of 4219 putative delete‑ rious variants, 152 mutations were identifed that likely induce embryonic lethality in the homozygous state. Inferred deleterious variation showed evidence of purifying selection and deleterious alleles were generally overrepresented in regions of low recombination. Finally, we found evidence that mutations, which were inferred to be evolutionally intolerant, likely have positive efects in commercial chicken populations. Conclusions: We present a comprehensive genomic perspective on deleterious and functional genetic variation in egg layer breeding lines, which are under intensive selection and characterized by a small efective population size. We show that deleterious variation is subject to purifying selection and that there is a positive relationship between recombination rate and purging efciency. In addition, multiple putative functional coding variants were discovered in selective sweep regions, which are likely under positive selection. Together, this study provides a unique molecular perspective on functional and deleterious variation in commercial egg-laying chickens, which can enhance current genomic breeding practices to lower the frequency of undesirable variants in the population. Background increase in frequency, mainly due to genetic drift [1]. In animal breeding, the number of deleterious genetic Deleterious alleles are expected to be purged from the variants that are segregating in a population is afected population by purifying selection, and thus, generally by several factors, e.g. genetic drift, mutation rate, and remain at low frequencies in a population [2]. However, selection. As a result, small efective population size many evolutionary forces shape the landscape of deleteri- and artifcial selection can impact population ftness ous alleles in a population, including recombination and in domesticated populations substantially [1] and can genetic hitchhiking, which is a change in allele frequency lead to a high risk of inbreeding depression, which is due to the allele being passed along together with a vari- the result of the accumulation of deleterious alleles that ant that is under selection [3]. Recent examples have shown a large impact of such deleterious alleles in several livestock populations [4, 5]. Terefore, efective purging *Correspondence: [email protected] 1 Wageningen University & Research Animal Breeding and Genomics, P.O. of these deleterious variants is desired. However, most of Box 338, 6700 AH Wageningen, The Netherlands these variants are rare, and selection on rare variants is Full list of author information is available at the end of the article © The Author(s) 2018. This 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. Derks et al. Genet Sel Evol (2018) 50:17 Page 2 of 14 usually inefcient, especially if the relationship between under selection) in purebred commercial layer lines. genotype and phenotype is poorly characterised [6, 7]. First, we showed that missing homozygosity can result In this study, we examined chicken layer lines that have from early embryonic lethality. Second, we mined the been primarily selected for production traits, including genomes of 250 whole-genome-sequenced individuals mortality, egg production, egg composition, shell quality for deleterious (including embryonic lethal) and func- [8], and traits related to animal welfare [9]. In spite of the tional variants. Te result is a comprehensive catalogue many positive consequences of this artifcial selection, of putative deleterious and functional variants, which will several health issues are associated with intense selection be an important resource for future functional studies in for production traits in laying hens, including excessive chicken and should facilitate the purging of deleterious comb growth, brittle bones, feather pecking, and ovarian variants in breeding populations. cancer [10–12]. To date, the underlying genetic architec- ture of these deleterious efects has not been character- Methods ised. Terefore, it is essential to better understand the Animals, genotypes and pre‑processing relationship between genotype and phenotype, which is, We genotyped six diferent commercial chicken breeds to a large extent, still a black box [13]. using the 60 K Illumina SNP BeadChip: one purebred Purebred chickens are routinely genotyped by breed- white layer dam line (WA), one purebred white layer sire ing companies using SNP genotyping panels to acceler- line (W1), two crossbred lines (CB: W1-WA, W1-WD) ate genetic progress by applying genomic selection [14]. and two brown layer lines (B1, B2) (see Additional fle 1: Although genomic selection itself may not be very ef- Table S1). All animals from multiple generations were cient in eliminating low-frequency deleterious variants, genotyped as part of a routine data collection from the large number of routinely genotyped and pedigreed Hendrix-Genetics breeding programs. Chromosomal individuals does allow for the identifcation of deleterious positions were determined based on the Gallus gallus variation. A powerful method is to systematically assess GalGal5 reference assembly [18]. SNPs with an unknown missing homozygosity in the genome by identifying hap- position on the Galgal5 reference assembly and SNPs on lotypes that cause early lethality by statistical depletion, sex chromosomes were discarded. Pre-processing was or even absence, of the homozygous state, suggesting that performed using PLINK v1.90b3.30 [19, 20] based on the they carry a lethal recessive mutation [4]. Tis approach following criteria: each SNP had to have a minor allele can detect even very rare (frequency < 2%) deleterious frequency higher than 0.01 (1%) and a call rate higher haplotypes if a large number, at least several thousands, than 0.85 and animals with a call rate lower than 0.7 were of animals are genotyped in a population. One disadvan- discarded from the analysis. We did not flter for devia- tage of this method is that low-frequency deleterious var- tions from Hardy–Weinberg equilibrium (HWE) because iants that reside on common haplotypes will be missed haplotypes that exhibit a defcit in homozygosity were [5]. An alternative method that does allow such rare del- expected to deviate from HWE. eterious alleles to be identifed is to sequence the entire genome of tens to hundreds of animals from a popula- Phasing and identifcation of missing homozygous tion. Whole-genome sequencing (WGS) can be used to haplotypes identify potential phenotype-altering variants, which We used the BEAGLE version 4.0 genetic analysis soft- can range from embryonic lethal (EL) to only mildly ware for phasing of the SNP genotypes [21]. We used deleterious mutations in coding regions, and to predict a sliding-window approach using window sizes rang- their efects using various tools [15]. Te use of WGS ing from 0.25 to 1 Mb in steps of 0.5 times the window data from a population can lead to the discovery of vari- size. Haplotypes with a frequency higher than 0.5% were ants that are benefcial for breeding programs [16, 17], retained for identifcation of missing homozygotes. Te e.g. by looking for regions in the genome that are under expected number of homozygotes was estimated using (recent) positive selection. A challenge for this approach the parental haplotype information with the formula is to diferentiate true selected variants and variants that described by Fritz et al. [22]. Te number of heterozy- increased in frequency as a result of genetic drift. In gous ofspring from carrier matings was also calculated addition, the incompleteness of current genome
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