Molecular basis for the dominant white phenotype in the domestic pig

Efficient generation of cloned cats with altered coat colour by editing of the KIT gene

Coat colour largely determines the market demand for several cat breeds. The KIT proto-oncogene (KIT) gene is a key gene controlling melanoblast differentiation and melanogenesis. KIT mutations usually cause varied changes in coat colour in mammalian species. In this study, we used a pair of single-guide RNAs (sgRNAs) to delete exon 17 of KIT in somatic cells isolated from two different Chinese Li Hua feline foetuses. Edited cells were used as donor nuclei for somatic cell nuclear transfer (SCNT) to generate cloned embryos presenting an average cleavage rate exceeding 85%, and an average blastocyst formation rate exceeding 9.5%. 131 cloned embryos were transplanted into four surrogates, and all surrogates carried their pregnancies to term, and delivered 4.58% (6/131) alive cloned kittens, with 1.53% (2/131) being KIT-edited heterozygotes (KITD17/+). The KITD17/+ cats presented an obvious darkness reduction in the mackerel tabby coat. Immunohistochemical analysis (IHC) of skin tissues indicated impaired proliferation and differentiation of melanoblasts caused by the lack of exon17 in feline KIT. To our knowledge, this is the first report on coat colour modification of cats through gene editing. The findings could facilitate further understanding of the regulatory role of KIT on feline coat colour and provide a basis for the breeding of cats with commercially desired coat colour.

Genetic and Phenotypic Characteristics of Belted Pig Breeds: A Review

Belted pig breeds have unique, distinguishing phenotypic characteristics. This review summarises the current knowledge on pig breeds displaying a belted coat pattern. Belts of different widths and positions around the animal's trunk characterise specific pig breeds from all around the world. All the breeds included in the present paper have been searched through the FAO domestic animal diversity information system (DAD-IS), Every country was checked to identify all breeds described as having black or red piebald coat pattern variations. Advances in genomic technologies have made it possible to identify the specific genes and genetic markers associated with the belted phenotype and explore the genetic relationships between different local breeds. Thus, the origin, history, and production traits of these breeds, together with all the genomic information related to the mechanism of skin pigmentation, are discussed. By increasing our understanding of these breeds, we can appreciate the richness of our biological and cultural heritage and work to preserve the biodiversity of the world's animals.

Analysis of the genetic loci of pigment pattern evolution in vertebrates

Vertebrate pigmentation patterns are amongst the best characterised model systems for studying the genetic basis of adaptive evolution. The wealth of available data on the genetic basis for pigmentation evolution allows for analysis of trends and quantitative testing of evolutionary hypotheses. We employed Gephebase, a database of genetic variants associated with natural and domesticated trait variation, to examine trends in how cis-regulatory and coding mutations contribute to vertebrate pigmentation phenotypes, as well as factors that favour one mutation type over the other. We found that studies with lower ascertainment bias identified higher proportions of cis-regulatory mutations, and that cis-regulatory mutations were more common amongst animals harbouring a higher number of pigment cell classes. We classified pigmentation traits firstly according to their physiological basis and secondly according to whether they affect colour or pattern, and identified that carotenoid-based pigmentation and variation in pattern boundaries are preferentially associated with cis-regulatory change. We also classified genes according to their developmental, cellular, and molecular functions. We found a greater proportion of cis-regulatory mutations in genes implicated in upstream developmental processes compared to those involved in downstream cellular functions, and that ligands were associated with a higher proportion of cis-regulatory mutations than their respective receptors. Based on these trends, we discuss future directions for research in vertebrate pigmentation evolution.

Genome-wide detection of CNV regions between Anqing six-end-white and Duroc pigs

BACKGROUND

Anqing six-end-white pig is a native breed in Anhui Province. The pigs have the disadvantages of a slow growth rate, low proportion of lean meat, and thick back fat, but feature the advantages of strong stress resistance and excellent meat quality. Duroc pig is an introduced pig breed with a fast growth rate and high proportion of lean meat. With the latter breed featuring superior growth characteristics but inferior meat quality traits, the underlying molecular mechanism that causes these phenotypic differences between Chinese and foreign pigs is still unclear.

RESULTS

In this study, copy number variation (CNV) detection was performed using the re-sequencing data of Anqing Six-end-white pigs and Duroc pigs, A total of 65,701 CNVs were obtained. After merging the CNVs with overlapping genomic positions, 881 CNV regions (CNVRs) were obtained. Based on the obtained CNVR information combined with their positions on the 18 chromosomes, a whole-genome map of the pig CNVs was drawn. GO analysis of the genes in the CNVRs showed that they were primarily involved in the cellular processes of proliferation, differentiation, and adhesion, and primarily involved in the biological processes of fat metabolism, reproductive traits, and immune processes.

CONCLUSION

The difference analysis of the CNVs between the Chinese and foreign pig breeds showed that the CNV of the Anqing six-end-white pig genome was higher than that of the introduced pig breed Duroc. Six genes related to fat metabolism, reproductive performance, and stress resistance were found in genome-wide CNVRs (DPF3, LEPR, MAP2K6, PPARA, TRAF6, NLRP4).

Signatures of Admixture and Genetic Uniqueness in the Autochthonous Greek Black Pig Breed Deduced from Gene Polymorphisms Affecting Domestication-Derived Traits

The Greek Black Pig (or Greek Pig) is the only recognized autochthonous pig breed raised in Greece, usually in extensive or semi-extensive production systems. According to its name, the characteristic breed coat color is solid black. In this study, with the aim to start a systematic genetic characterization of the Greek Black Pig breed, we investigated polymorphisms in major genes well known to affect exterior and production traits (MC1R, KIT, NR6A1, VRTN and IGF2) and compared these data with population genetic information available in other Mediterranean and Western Balkan pig breeds and wild boars. None of the investigated gene markers were fixed for one allele, suggesting that, in the past, this breed experienced introgression from wild boars and admixture from cosmopolitan pig breeds, enriching the breed genetic pool that should be further investigated to design appropriate conservation genetic strategies. We identified a new MC1R allele, containing two missense mutations already reported in two other independent alleles, but here present in the same haplotype. This allele might be useful to disclose biological information that can lead to better understanding the cascade transmission of signals to produce melanin pigments. This study demonstrated that autochthonous genetic resources can be an interesting reservoir of unexpected genetic variants.

Impact of KIT Editing on Coat Pigmentation and Fresh Meat Color in Yorkshire Pigs

The white coat color of Yorkshire pigs is caused by the dominant white I allele, which has been associated with at least one copy of the 450-kb duplication encompassing the entire KIT gene and a splice mutation (G > A) at the first base of intron 17. The splice mutation in KIT has an adverse effect on pigmentation in mice. Therefore, removing the 450 kb duplications harboring the KIT copy with splice mutations is expected to affect Yorkshire pig pigmentation. In this study, we describe the use of a Yorkshire pig kidney cell strain with the I^?/I^Be-ed genotype, previously created by CRISPR-Cas9, as donor cells for somatic cell nuclear transfer to generate gene-edited Yorkshire pigs. The removal of the 450 kb duplications harboring the KIT copy with splice mutation did not alter the white coat color of Yorkshire pigs, which was confirmed by the absence of fully mature melanocytes and melanin accumulation in the hair follicles. Except for the improved transcription of tyrosinase, and slight increase in microphthalmia transcription factor and tyrosinase-related protein 1 protein expression, there was no significant impact of the removal of splice mutations on genes and signaling pathways (PI3K/AKT) involved in melanogenesis. However, the removal of the 450 kb duplications harboring the KIT copy with splice mutation substantially improved fresh meat color accompanied by significantly increased red blood cell number, which merits further investigation. Our study provides new insights into the role of structural mutations of the KIT gene in the formation of white coat color and erythropoiesis in Yorkshire pigs.

Pig Coat Color Manipulation by MC1R Gene Editing

Black coat color in pigs is determined by the dominant E allele at the MC1R locus. Through comparing MC1R gene sequences between recessive e and dominant ED1 alleles, we identified four missense mutations that could affect MC1R protein function for eumelanin synthesis. With the aim of devising a genetic modification method for pig coat color manipulation, we mutated the e allele in the Duroc breed to the dominant ED1 allele using CRISPR-mediated homologous recombination for the four mutation substitutions at the MC1R locus. The MC1R-modified Duroc pigs generated using the allele replacement strategy displayed uniform black coat color across the body. A genotyping assay showed that the MC1R-modified Duroc pigs had a heterozygous ED1/e allele at the MC1R locus; in addition, the pigs remained in the Duroc genetic background. Our work offers a gene editing method for pig coat color manipulation, which could value the culture of new pig varieties meeting the needs of diversified market.

A composite strategy of genome-wide association study and copy number variation analysis for carcass traits in a Duroc pig population

BACKGROUND

Carcass traits are important in pig breeding programs for improving pork production. Understanding the genetic variants underlies complex phenotypes can help explain trait variation in pigs. In this study, we integrated a weighted single-step genome-wide association study (wssGWAS) and copy number variation (CNV) analyses to map genetic variations and genes associated with loin muscle area (LMA), loin muscle depth (LMD) and lean meat percentage (LMP) in Duroc pigs.

RESULTS

Firstly, we performed a genome-wide analysis for CNV detection using GeneSeek Porcine SNP50 Bead chip data of 3770 pigs. A total of 11,100 CNVs were detected, which were aggregated by overlapping 695 CNV regions (CNVRs). Next, we investigated CNVs of pigs from the same population by whole-genome resequencing. A genome-wide analysis of 21 pigs revealed 23,856 CNVRs that were further divided into three categories (851 gain, 22,279 loss, and 726 mixed), which covered 190.8 Mb (~ 8.42%) of the pig autosomal genome. Further, the identified CNVRs were used to determine an overall validation rate of 68.5% for the CNV detection accuracy of chip data. CNVR association analyses identified one CNVR associated with LMA, one with LMD and eight with LMP after applying stringent Bonferroni correction. The wssGWAS identified eight, six and five regions explaining more than 1% of the additive genetic variance for LMA, LMD and LMP, respectively. The CNVR analyses and wssGWAS identified five common regions, of which three regions were associated with LMA and two with LMP. Four genes (DOK7, ARAP1, ELMO2 and SLC13A3) were highlighted as promising candidates according to their function.

CONCLUSIONS

We determined an overall validation rate for the CNV detection accuracy of low-density chip data and constructed a genomic CNV map for Duroc pigs using resequencing, thereby proving a value genetic variation resource for pig genome research. Furthermore, our study utilized a composite genetic strategy for complex traits in pigs, which will contribute to the study for elucidating the genetic architecture that may be influenced and regulated by multiple forms of variations.

Molecular genetic variation of animals and plants under domestication

Domesticated plants and animals played crucial roles as models for evolutionary change by means of natural selection and for establishing the rules of inheritance, originally proposed by Charles Darwin and Gregor Mendel, respectively. Here, we review progress that has been made during the last 35 y in unraveling the molecular genetic variation underlying the stunning phenotypic diversity in crops and domesticated animals that inspired Mendel and Darwin. We notice that numerous domestication genes, crucial for the domestication process, have been identified in plants, whereas animal domestication appears to have a polygenic background with no obvious “domestication genes” involved. Although model organisms, such as Drosophila and Arabidopsis, have replaced domesticated species as models for basic research, the latter are still outstanding models for evolutionary research because phenotypic change in these species represents an evolutionary process over thousands of years. A consequence of this is that some alleles contributing to phenotypic diversity have evolved by accumulating multiple changes in the same gene. The continued molecular characterization of crops and farm animals with ever sharper tools is essential for future food security.

Roan coat color in livestock

Since domestication, a wide variety of phenotypes including coat color variation has developed in livestock. This variation is mostly based on selective breeding. During the beginning of selective breeding, potential negative consequences did not become immediately evident due to low frequencies of homozygous animals and have been occasionally neglected. However, numerous studies of coat color genetics have been carried out over more than a century and, meanwhile, pleiotropic effects for several coat color genes, including disorders of even lethal impact, were described. Similar coat color phenotypes can often be found across species, caused either by conserved genes or by different genes. Even in the same species, more than one gene could cause the same or similar coat color phenotype. The roan coat color in livestock species is characterized by a mixture of white and colored hair in cattle, pig, sheep, goat, alpaca, and horse. So far, the genetic background of this phenotype is not fully understood, but KIT and its ligand KITLG (MGF) are major candidate genes in livestock species. For some of these species, pleiotropic effects such as subfertility in homozygous roan cattle or homozygous embryonic lethality in certain horse breeds have been described. This review aims to point out the similarities and differences of the roan phenotype across the following livestock species: cattle, pig, sheep, goat, alpaca, and horse; and provides the current state of knowledge on genetic background and pleiotropic effects.

Identification of Candidate Genes for Pigmentation in Camels Using Genotyping-by-Sequencing

The coat color of dromedary is usually uniform and varies from black to white, although dark- to light-brown colors are the most common phenotypes. This project was designed to gain knowledge on novel color-related variants using genotyping-by-sequencing (GBS). The association between the SNPs and coat color was tested using MLM (mixed linear models) with kinship matrix. Three GWAS models including white color vs. non-white color, black vs. non-black color, and light-brown vs. dark-brown color were performed. There were no distinct genetic clusters detected based on the color phenotypes. However, admixture occurred among all individuals of the four different coat color groups. We identified nine significant SNPs associated with white color after Bonferroni correction, located close to ANKRD26, GNB1, TSPYL4, TEKT5, DEXI, CIITA, TVP23B, CLEC16A, TMPRSS13, FXYD6, MPZL3, ANKRD26, HFM1, CDC7, TGFBR3, and HACE1 genes in neighboring flanking regions. The 13 significant SNPs associated with black color and the candidate genes were: CAPN7, CHRM4, CIITA, CLEC16A, COL4A4, COL6A6, CREB3L1, DEXI, DGKZ, DGKZ, EAF1, HDLBP, INPP5F, MCMBP, MDK, SEC23IP, SNAI1, TBX15, TEKT5, TMEM189, trpS, TSPYL4, TVP23B, and UBE2V1. The SNAI1 gene interacted with MCIR, ASIP and KIT genes. These genes play a key role in the melanin biosynthetic and pigmentation biological process and melanogenesis biological pathway. Further research using a larger sample size and pedigree data will allow confirmation of associated SNPs and the identified candidate genes.

Whole-genome resequencing reveals genetic structure and introgression in Pudong White pigs

Pudong White (PDW) pigs, historically originating from Shanghai, are the only Chinese indigenous pigs characterised by their completely white coats, with the exception of Rongchang pigs. However, there is limited information concerning their overall genetic structure or relationship with other breeds, especially the East Chinese (ECN) and European pigs. To uncover the genetic structure, selection signatures, and potential exotic introgression in PDW pigs, we sampled 15 PDW pigs using whole-genome sequencing (~20×). We then conducted in-depth population genetic analyses in 320 pigs from 27 global pig groups, namely, European wild boars, Chinese wild boars, and outgroup. Neighbour-joining tree and principal component analysis confirmed that PDW pigs belonged to the ecotype of ECN pigs. Both f3, D-statistics, and structure analysis showed that PDW pigs shared apparent alleles with Large White (LW) pigs. Three statistics, rIBD, a haplotype heat map and copy number variation, further indicated that PDW pigs shared apparent alleles with LW pigs at the KIT Proto-Oncogene, Receptor Tyrosine Kinase (KIT) and PARG-MARCHF8 loci, suggesting that the lineage of European pigs in PDW originated from LW pigs. After further detecting the KIT mutations in different pig breeds, PDW was confirmed to have the same duplication region 1, duplication region 2, and the splicing mutation on intron 17 of KIT as LW pigs that determine the white coat colour phenotype in European white pigs. We hypothesised that LW pigs were imported to China ∼110-160 years ago according to the admixture time estimate and then crossed with ECN pigs, resulting in the introgression of the KIT alleles that produce the white coat colour phenotype in the PDW pig breed. To our knowledge, this study presents the first thorough description of the genetic structure of PDW pigs via whole-genome resequencing data; moreover, the results provide a basis for the national project for the conservation of this unique Chinese local population.

Exon-1 skipping and intron-1 retaining by alternative splicing of the c-KIT gene encodes a novel splice variant in the skin of Merino sheep (Ovis aries)

c-KIT, a type III receptor protein tyrosine kinase, plays an essential role in melanocyte development, migration, and survival. Mutations within the c-KIT gene are previously shown to cause the white coat color phenotypes in pigs, mice, goats, and humans. However, up so far, the splicing isoform(s), genomic architecture of c-KIT have not been characterized well in merino sheep. Reverse transcriptase (RT)-PCR analysis with molecular prediction identified two basic splice variants: Transcript Variant-1, 2 for 12 bp insertion coding sequences (CDS) corresponding to the four amino acids 'GNSK', respectively. Using 5' RACE, here we report for the first time a novel c-KIT 'Transcript Variant-3' from the skin of merino sheep by comparative genome analyses at exon(1)-intron(1)-exon(2) boundaries. In contrast, a single product of 795 bp was characterized by 3' RACE. We also demonstrated that the c-KIT gene expression at the transcript level is not mediated via an intron-9 splicing event. Overall, beyond what was observed in other mammals, our data provide novel insights into the molecular structure of the c-KIT gene in sheep.

Characterization of a haplotype-reference panel for genotyping by low-pass sequencing in Swiss Large White pigs

BACKGROUND

The key-ancestor approach has been frequently applied to prioritize individuals for whole-genome sequencing based on their marginal genetic contribution to current populations. Using this approach, we selected 70 key ancestors from two lines of the Swiss Large White breed that have been selected divergently for fertility and fattening traits and sequenced their genomes with short paired-end reads.

RESULTS

Using pedigree records, we estimated the effective population size of the dam and sire line to 72 and 44, respectively. In order to assess sequence variation in both lines, we sequenced the genomes of 70 boars at an average coverage of 16.69-fold. The boars explained 87.95 and 95.35% of the genetic diversity of the breeding populations of the dam and sire line, respectively. Reference-guided variant discovery using the GATK revealed 26,862,369 polymorphic sites. Principal component, admixture and fixation index (F_ST) analyses indicated considerable genetic differentiation between the lines. Genomic inbreeding quantified using runs of homozygosity was higher in the sire than dam line (0.28 vs 0.26). Using two complementary approaches, we detected 51 signatures of selection. However, only six signatures of selection overlapped between both lines. We used the sequenced haplotypes of the 70 key ancestors as a reference panel to call 22,618,811 genotypes in 175 pigs that had been sequenced at very low coverage (1.11-fold) using the GLIMPSE software. The genotype concordance, non-reference sensitivity and non-reference discrepancy between thus inferred and Illumina PorcineSNP60 BeadChip-called genotypes was 97.60, 98.73 and 3.24%, respectively. The low-pass sequencing-derived genomic relationship coefficients were highly correlated (r > 0.99) with those obtained from microarray genotyping.

CONCLUSIONS

We assessed genetic diversity within and between two lines of the Swiss Large White pig breed. Our analyses revealed considerable differentiation, even though the split into two populations occurred only few generations ago. The sequenced haplotypes of the key ancestor animals enabled us to implement genotyping by low-pass sequencing which offers an intriguing cost-effective approach to increase the variant density over current array-based genotyping by more than 350-fold.

Genomic scan revealed KIT gene underlying white/gray plumage color in Chinese domestic geese

Goose is an important type of domesticated poultry. The wild geese that are regarded as the ancestors of modern domestic geese present gray plumage. Domesticated, geese have both white and gray feathers. To elucidate the genetic mechanisms underlying the formation of white and gray plumage in geese, we resequenced the whole genome of 18 geese from six populations including white and gray goose breeds. The average sequencing depth per individual was 9.81× and the average genome coverage was 96.8%. A total of 346 genes were detected in the top 1% of F_ST scores of gray- and white-feathered geese, and a significant F_ST site was located in the intron region within the KIT gene, the 18 bp deletion in KIT having the strongest potential association with white feathers. It has been reported that a number of genes are associated with plumage colors in birds. However, no studies have identified the relationship between KIT and plumage color in birds at present, although the white coat can be attributed to mutations in KIT in some mammals. Our study showed that that KIT is a plausible candidate gene for white/gray plumage color in Chinese domestic geese.

Red and blond Mangalitza pigs display a signature of divergent directional selection in the SLC45A2 gene

The Mangalitza lard-type pig breed is well known for its fat appearance and curly hair, and it is mainly distributed in Eastern Europe. Four main lines were created in the nineteenth century by artificial selection: Blond Mangalitza, Black Mangalitza, Swallow-Belly Mangalitza and Red Mangalitza. The Swallow-Belly line has a black coat combined with yellow-blond throat and underbelly. In the current work, we aimed to investigate if the colourations of Mangalitza pigs are genetically determined by one or a few loci whose frequencies have been modified by artificial selection. The results of selection scans, with HapFLK and BayeScan, and of a GWAS for coat colour highlighted the existence of one region on SSC16 (18-20 Mb) with potential effects on hair pigmentation (Red vs. Blond contrast). The analysis of the gene content of this region allowed us to detect the solute carrier family 45 member 2 (SLC45A2) locus as a candidate gene for this trait. The polymorphism of the SLC45A2 locus has been associated with reduced levels or the absence of melanin in several mammalian species. The genotyping of four missense polymorphisms evidenced that rs341599992:G > A and rs693695020:G > A SNPs are strongly but not fully associated with the red and blond coat colours of Mangalitza pigs, a result that was confirmed by performing a haplotype association test. The near fixation of alternative SLC45A2 genotypes in Red and Blond Mangalitza pigs provides a compelling example of the consequences of a divergent directional selection for coat colour in a domestic species.

Whole genome variants across 57 pig breeds enable comprehensive identification of genetic signatures that underlie breed features

Background

A large number of pig breeds are distributed around the world, their features and characteristics vary among breeds, and they are valuable resources. Understanding the underlying genetic mechanisms that explain across-breed variation can help breeders develop improved pig breeds.

Results

In this study, we performed GWAS using a standard mixed linear model with three types of genome variants (SNP, InDel, and CNV) that were identified from public, whole-genome, sequencing data sets. We used 469 pigs of 57 breeds, and we identified and analyzed approximately 19 million SNPs, 1.8 million InDels, and 18,016 CNVs. We defined six biological phenotypes by the characteristics of breed features to identify the associated genome variants and candidate genes, which included coat color, ear shape, gradient zone, body weight, body length, and body height. A total of 37 candidate genes was identified, which included 27 that were reported previously (e.g., PLAG1 for body weight), but the other 10 were newly detected candidate genes (e.g., ADAMTS9 for coat color).

Conclusion

Our study indicated that using GWAS across a modest number of breeds with high density genome variants provided efficient mapping of complex traits.

Supplementary Information

Supplementary information accompanies this paper at 10.1186/s40104-020-00520-8.

Genome-wide detection of copy number variants in European autochthonous and commercial pig breeds by whole-genome sequencing of DNA pools identified breed-characterising copy number states

In this study, we identified copy number variants (CNVs) in 19 European autochthonous pig breeds and in two commercial breeds (Italian Large White and Italian Duroc) that represent important genetic resources for this species. The genome of 725 pigs was sequenced using a breed-specific DNA pooling approach (30-35 animals per pool) obtaining an average depth per pool of 42×. This approach maximised CNV discovery as well as the related copy number states characterising, on average, the analysed breeds. By mining more than 17.5 billion reads, we identified a total of 9592 CNVs (~683 CNVs per breed) and 3710 CNV regions (CNVRs; 1.15% of the reference pig genome), with an average of 77 CNVRs per breed that were considered as private. A few CNVRs were analysed in more detail, together with other information derived from sequencing data. For example, the CNVR encompassing the KIT gene was associated with coat colour phenotypes in the analysed breeds, confirming the role of the multiple copies in determining breed-specific coat colours. The CNVR covering the MSRB3 gene was associated with ear size in most breeds. The CNVRs affecting the ELOVL6 and ZNF622 genes were private features observed in the Lithuanian Indigenous Wattle and in the Turopolje pig breeds respectively. Overall, the genome variability unravelled here can explain part of the genetic diversity among breeds and might contribute to explain their origin, history and adaptation to a variety of production systems.

Discovery of selection-driven genetic differences of Duroc, Landrace, and Yorkshire pig breeds by EigenGWAS and Fst analyses

Pigs are one of the earliest domesticated animals and multiple breeds have been developed to meet the various demands of consumers. EigenGWAS is a novel strategy to identify candidate genes that underlying population genetic differences and to infer candidate regions under selection as well. In this study, EigenGWAS and F_st analyses were performed using the public re-sequencing data of three typical commercial pig breeds, Duroc, Landrace and Yorkshire. The intersection of genome-wide significant SNPs detected by EigenGWAS and top-ranked 1% SNPs of F_st results were treated as signals under selection. Using the data of all three breeds, 3062 signals under selection were detected and the nearby genomic regions within 300 kb upstream and downstream covered 6.54% of whole genome. Pairs of breeds were analysed along with the pathway analysis. The gene function enrichment results indicated that many candidate genes located in the genomic regions of the signals under selection were associated with biological processes related to growth, metabolism, reproduction, sensory perception, etc. Among the candidate genes, the FSHB, AHR, PTHLH, KDR and FST genes were reported to be associated with reproductive performance; the KIT, KITLG, MITF, MC1R and EDNRB genes were previously identified to affect coat colour; the RETREG1, TXNIP, BMP5, PPARD and RBP4 genes were reported to be associated with lipid metabolism and growth traits. The identified genetic differences across the three commercial breeds will advance understanding of the artificial selection history of pigs and the signals under selection will suggest potential uses in pig genomic breeding programmes.

A quantitative trait locus on chromosome 2 was identified that accounts for a substantial proportion of phenotypic variance of the yellow plumage color in chicken

Chicken plumage color is an important economical trait in poultry breeding, as triple-yellow indigenous broilers are preferred over western commercial broilers in the Chinese market. However, the studies on the pigmentation of plumage coloration are relatively rare at present. Here, we performed a genome-wide mapping study on an F₂ intercross, whose 2 founders were one hybrid commercial line “High Quality chicken Line A” that originated from the Anak red chicken and one indigenous line “Huiyang Beard” chicken that is a classical “triple-yellow” Chinese indigenous breed. Moreover, we used an automatic colorimeter that can quantitatively assess the colorations in L∗, a∗, and b∗ values. One major quantitative trait locus (QTL) on chromosome 2 was thus identified by both genome-wide association and linkage analyses, which could explain 10 to 20% of the total phenotypic variance of the b∗ measurements of the back plumage color. Using linkage analysis, 2 additional QTL on chromosome 1 and 20 were also found to be significantly associated with the plumage coloration in this cross. With additional samples from Anak red and Huiyang Beard chickens as well as pooled resequencing data from the 2 founders of this cross, we then further narrowed down the QTL regions and identified several candidate genes, such as CABLES1, CHST11, BCL2L1, and CHD22. As the effects of QTL found in this study were substantial, quantitatively measuring the coloration rather than the descriptive measurements provides stronger statistical power for the analyses. In addition, this major QTL on chromosome 2 that was associated with feather pigmentation at the genome-wide level will facilitate the future chicken breeding for yellow plumage color. In conclusions, we mapped 3 associated QTL on chromosome 1, 2, and 20. The candidate genes identified in this study shed light in the genetic basis of yellow plumage color in chicken.

Functional Analysis of KIT Gene Structural Mutations Causing the Porcine Dominant White Phenotype Using Genome Edited Mouse Models

The dominant white phenotype in pigs is thought to be mainly due to a structural mutation in the KIT gene, a splice mutation (G > A) at the first base in intron 17 which leads to the deletion of exon 17 in the mature KIT mRNA. However, this hypothesis has not yet been validated by functional studies. Here, we created two mouse models, KIT D17/+ to mimic the splice mutation, and KIT Dup/+ to partially mimic the duplication mutation of KIT gene in dominant white pigs using CRISPR/Cas9 technology. We found that the splice mutation homozygote is lethal and the heterozygous mice have a piebald coat. Slightly increased expression of KIT in KIT Dup/+ mice did not confer the patched phenotype and had no obvious impact on coat color. Interestingly, the combination of these two mutations reduced the phosphorylation of PI3K and MAPK pathway associated proteins, which may be related to the impaired migration of melanoblasts observed during embryonic development that eventually leads to the dominant white phenotype.

Selection signatures in goats reveal copy number variants underlying breed-defining coat color phenotypes

Domestication and human selection have formed diverse goat breeds with characteristic phenotypes. This process correlated with the fixation of causative genetic variants controlling breed-specific traits within regions of reduced genetic diversity, so called selection signatures or selective sweeps. Using whole genome sequencing of DNA pools (pool-seq) from 20 genetically diverse modern goat breeds and bezoars, we identified 2,239 putative selection signatures. In two Pakistani goat breeds, Pak Angora and Barbari, we found selection signatures in a region harboring KIT, a gene involved in melanoblast development, migration, and survival. The search for candidate causative variants responsible for these selective sweeps revealed two different copy number variants (CNVs) downstream of KIT that were exclusively present in white Pak Angora and white-spotted Barbari goats. Several Swiss goat breeds selected for specific coat colors showed selection signatures at the ASIP locus encoding the agouti signaling protein. Analysis of these selective sweeps revealed four different CNVs associated with the white or tan (A^Wt), Swiss markings (A^sm), badgerface (A^b), and the newly proposed peacock (A^pc) allele. RNA-seq analyses on skin samples from goats with the different CNV alleles suggest that the identified structural variants lead to an altered expression of ASIP between eumelanistic and pheomelanistic body areas. Our study yields novel insights into the genetic control of pigmentation by identifying six functionally relevant CNVs. It illustrates how structural changes of the genome have contributed to phenotypic evolution in domestic goats.

Domestic animals have been selected for hundreds or sometimes even thousands of years for traits that were appreciated by their human owners. This process correlated with the fixation of causative genetic variants controlling breed-specific traits within regions of reduced genetic diversity, so called selection signatures or selective sweeps. We conducted a comprehensive screen for selection signatures in 20 phenotypically and genetically diverse modern goat breeds and identified a total of 2,239 putative selection signatures in our dataset. Follow-up experiments on selection signatures harboring known candidate genes for coat color revealed six different copy number variants (CNVs). Two of these CNVs were located in the 3’-flanking region of KIT and associated with a completely white coat color phenotype in Pak Angora goats and a white-spotted coat color phenotype in Barbari goats, respectively. The other four CNVs were located at the ASIP locus. They were associated with four different types of coat color patterning in seven Swiss goat breeds. Their functional effect is mediated by region-specific quantitative changes in ASIP mRNA expression. Our study illustrates how structural changes of the genome have contributed to phenotypic evolution in domestic goats.

Highly efficient correction of structural mutations of 450 kb KIT locus in kidney cells of Yorkshire pig by CRISPR/Cas9

The white coat colour of Yorkshire and Landrace pig breeds is caused by the dominant white I allele of KIT, associated with 450-kb duplications and a splice mutation (G > A) at the first base in intron 17. To test whether genome editing can be employed to correct this structural mutation, and to investigate the role of KIT in the control of porcine coat colour, we designed sgRNAs targeting either intron 16 or intron 17 of KIT, and transfected Cas9/sgRNA co-expression plasmids into the kidney cells of Yorkshire pigs. The copy number of KIT was reduced by about 13%, suggesting the possibility of obtaining cells with corrected structural mutations of the KIT locus. Using single cell cloning, from 24 successfully expanded single cell clones derived from cells transfected with sgRNA targeting at intron 17, we obtained 3 clones with a single copy of KIT without the splice mutation. Taken together, the 12.5% (3/24) efficiency of correction of structural mutations of 450 kb fragments is highly efficient, providing a solid basis for the generation of genome edited Yorkshire pigs with a normal KIT locus. This provides an insight into the underlying genetic mechanisms of porcine coat colour.

A comparison of the association between large haplotype blocks under selection and the presence/absence of inversions

Inversions may contribute to ecological adaptation and phenotypic diversity, and with the advent of "second" and "third" generation sequencing technologies, the ability to detect inversion polymorphisms has been enhanced dramatically. A key molecular consequence of an inversion is the suppression of recombination allowing independent accumulation of genetic changes between alleles over time. This may lead to the development of divergent haplotype blocks maintained by balancing selection. Thus, divergent haplotype blocks are often considered as indicating the presence of an inversion. In this paper, we first review the features of a 7.7 Mb inversion causing the Rose-comb phenotype in chicken, as a model for how inversions evolve and directly affect phenotypes. Second, we compare the genetic basis for alternative mating strategies in ruff and timing of reproduction in Atlantic herring, both associated with divergent haplotype blocks. Alternative male mating strategies in ruff are associated with a 4.5 Mb inversion that occurred about 4 million years ago. In fact, the ruff inversion shares some striking features with the Rose-comb inversion such as disruption of a gene at one of the inversion breakpoints and generation of a new allele by recombination between the inverted and noninverted alleles. In contrast, inversions do not appear to be a major reason for the fairly large haplotype blocks (range 10-200 kb) associated with ecological adaptation in the herring. Thus, it is important to note that divergent haplotypes may also be maintained by natural selection in the absence of structural variation.

Whole-Genome Resequencing Identifies KIT New Alleles That Affect Coat Color Phenotypes in Pigs

The Duroc × (Landrace × Large White) hybrid pig (DLY) is the most popular commercial pig used in the Chinese pig industry. DLY pigs are usually white but sometimes show colored phenotypes. Colored DLY pigs are not favored by slaughterhouses and retailers, thus causing certain economic losses to farmers in China. In this study, we first conducted a genome-wide association study and RNA sequencing to demonstrate that KIT variants are responsible for diversifying coat color phenotypes segregating in a DLY population. We then defined the precise sizes and locations of four duplications (DUP1-4), four candidate causative mutations at the KIT locus, in the pig reference genome using the whole-genome sequence data of representative colored individuals. The sequence data also enabled us to identify a list of new KIT alleles. By investigating the association between these new alleles and coat color phenotypes, we provide further evidence that DUP2 is another causative mutation for the solid white coat color in pigs. DUP1 (the KIT gene duplication), DUP2 and the splice mutation are all required for the manifestation of a solid white coat color. DUP4 had a more significant effect on the formation of the belt phenotype compared with DUP3. Given the necessity of DUP2 for the solid white coat color, we detected I^N/I^N homozygotes lacking DUP2 in Large White and Landrace pigs and found that French Landrace pigs had the highest frequency (8.98%) of I^N/I^N individuals. This study not only advances our understanding of the molecular mechanism of the color phenotype in pigs, but also establishes a simple and accurate method for the screening of KIT I^N/I^N homozygotes in Large White and Landrace that would cause colored DLY pigs.

Comparison of the histology of the skin of the Windsnyer, Kolbroek and Large White pigs

The skin is a protective barrier, and an endocrine, sensory and thermoregulatory organ. We investigated whether the skin of local pigs had beneficial anatomical traits compared to exotic pigs to withstand the increased heat loads predicted under future climate change scenarios. Full-thickness skin specimens were obtained from the dorsal interscapular, lateral thoraco-abdominal and ventral abdominal regions of intact boars (age 6-8 months) of two local breeds of pigs (Windsnyer [n = 5] and Kolbroek [n = 4]) and an exotic pig breed (Large White [n = 7]). The skin sections were stained with a one-step Mallory-Heidenhain stain and Fontana stain (melanin). Sweat gland perimeter was measured using Image J software. The Windsnyer breed had the thinnest dermis layer while the Large White had the thickest dermis layer across all the three body regions (analysis of variance [ANOVA]; p < 0.001). The Windsnyers had widely spaced dermal pegs compared to the other breeds. The Windsnyers had significantly more superficial and larger (~1 mm depth; 4.4 mm perimeter) sweat glands than the Kolbroek (~3 mm depth; 2.2 mm perimeter) and Large White (~4 mm depth; 2.0 mm perimeter) pigs (ANOVA; p < 0.001). The Windsnyers had visibly more melanin in the basal layer, the Kolbroek pigs had very little and the Large Whites had none. The functionality of the sweat glands of the Windsnyer breed needs to be established. The skin from the Windsnyer breed possesses traits that may confer a protective advantage for the increased solar radiation and ambient temperatures predicted with climate change.

Divergent Evolutional Mode and Purifying Selection of the KIT Gene in European and Asian Domestic Pig Breeds

The recent geographic expansion of wild boars and the even more recent development of numerous domestic pigs have spurred exploration on pig domestic origins. The porcine KIT gene has been showed to affect pleiotropic effects, blood parameters, and coat colour phenotypes, especially the white colour phenotype formation in European commercial breeds. Here, we described the use of SNPs to identify different selection patterns on the porcine KIT gene and the phylogenetic relationships of the inferred haplotypes. The phylogenetic tree revealed four clades in European and Asian wild and domestic pigs: two major clades with European and Asian origins and one minor clade with Iberian origins as well as the other minor clade in Asia, consistent with the major introgression of domestic Asian pigs in Europe around 18th -19th century. The domestication history of pigs, which occurred in the domestication centers (Europe and Asia), has also been demonstrated by mtDNA analysis. Furthermore, both Asian and European domestic pigs evolved under purifying selection. This study indicated that domestic pigs in Europe and Asia have different lineage origins but the porcine KIT gene was undergoing a purifying selection during their evolutional histories.

Genome-Wide Association Studies Identify Candidate Genes for Coat Color and Mohair Traits in the Iranian Markhoz Goat

The Markhoz goat provides an opportunity to study the genetics underlying coat color and mohair traits of an Angora type goat using genome-wide association studies (GWAS). This indigenous Iranian breed is valued for its quality mohair used in ceremonial garments and has the distinction of exhibiting an array of coat colors including black, brown, and white. Here, we performed 16 GWAS for different fleece (mohair) traits and coat color in 228 Markhoz goats sampled from the Markhoz Goat Research Station in Sanandaj, Kurdistan province, located in western Iran using the Illumina Caprine 50K beadchip. The Efficient Mixed Model Linear analysis was used to identify genomic regions with potential candidate genes contributing to coat color and mohair characteristics while correcting for population structure. Significant associations to coat color were found within or near the ASIP, ITCH, AHCY, and RALY genes on chromosome 13 for black and brown coat color and the KIT and PDGFRA genes on chromosome 6 for white coat color. Individual mohair traits were analyzed for genetic association along with principal components that allowed for a broader perspective of combined traits reflecting overall mohair quality and volume. A multitude of markers demonstrated significant association to mohair traits highlighting potential candidate genes of POU1F1 on chromosome 1 for mohair quality, MREG on chromosome 2 for mohair volume, DUOX1 on chromosome 10 for yearling fleece weight, and ADGRV1 on chromosome 7 for grease percentage. Variation in allele frequencies and haplotypes were identified for coat color and differentiated common markers associated with both brown and black coat color. This demonstrates the potential for genetic markers to be used in future breeding programs to improve selection for coat color and mohair traits. Putative candidate genes, both novel and previously identified in other species or breeds, require further investigation to confirm phenotypic causality and potential epistatic relationships.

Genome-wide genetic structure and differentially selected regions among Landrace, Erhualian, and Meishan pigs using specific-locus amplified fragment sequencing

As typical Chinese indigenous pig breeds, Erhualian and Meishan have been widely used to produce new strain or breed in the world. However, the genetic basis of characteristics of these pig breeds is still limited. Moreover, considering cost and output of sequencing, it is necessary to further develop cost-effective method for pig genome screening. To contribute on this issue, we developed a SLAF-seq (specific-locus amplified fragment sequencing) method for pigs and applied it to analyze the genetic difference among Landrace, Erhualian, and Meishan pigs. A total of 453.75 million reads were produced by SLAF-seq. After quality-control, 165,670 SNPs (single nucleotide polymorphisms) were used in further analysis. The results showed that Landrace had distinct genetic relationship compared to Erhualian (F_ST = 0.5480) and Meishan (F_ST = 0.5800), respectively, while Erhualian and Meishan held the relatively close genetic relationship (F_ST = 0.2335). Furthermore, a genome-wide scanning revealed 268 differentially selected regions (DSRs) with 855 genes and 256 DSRs with 347 genes between Landrace and the two Chinese indigenous pig breeds and between Erhualian and Meishan, respectively. This study provides a new cost-effective method for pig genome study and might contribute to a better understanding on the formation mechanism of genetic difference among pigs with different geographical origins.

Estimation of genome-wide and locus-specific breed composition in pigs

Advances in pig genomic technologies enable implementation of new methods to estimate breed composition, allowing innovative and efficient ways to evaluate and ensure breed and line background. Existing methods to test for homozygosity at key loci involve test mating the animal in question and observing phenotypic patterns among offspring, requiring extensive resources. In this study, whole-genome pig DNA microarray data from over 8,000 SNP was used to profile the composition of U.S. registered purebred pigs using a refined linear regression method that enhances the interpretation of coefficients. In a simulation analysis, a strong correlation between true and estimated breed composition was observed (R² = 0.94). Applying these methods to 930 Yorkshire animals registered with the National Swine Registry, 95% were estimated to have a “genome-wide” Yorkshire breed composition of at least 0.825 or 82.5%, with similar performance for evaluating datasets of registered Duroc (n = 88) Landrace (n = 129), and Hampshire (n = 17) breeds. We also developed new methods to evaluate locus-based breed probabilities. Such methods have been applied to multi-locus SNP genotypes flanking the KIT gene known to predominantly control coat color, thereby inferring the probability that an animal has haplotypes in the KIT region that are predominant in white breeds. These methods have been adopted by the National Swine Registry as a means to identify purebred Yorkshire animals.

Domestic animals as models for biomedical research

Domestic animals are unique models for biomedical research due to their long history (thousands of years) of strong phenotypic selection. This process has enriched for novel mutations that have contributed to phenotype evolution in domestic animals. The characterization of such mutations provides insights in gene function and biological mechanisms. This review summarizes genetic dissection of about 50 genetic variants affecting pigmentation, behaviour, metabolic regulation, and the pattern of locomotion. The variants are controlled by mutations in about 30 different genes, and for 10 of these our group was the first to report an association between the gene and a phenotype. Almost half of the reported mutations occur in non-coding sequences, suggesting that this is the most common type of polymorphism underlying phenotypic variation since this is a biased list where the proportion of coding mutations are inflated as they are easier to find. The review documents that structural changes (duplications, deletions, and inversions) have contributed significantly to the evolution of phenotypic diversity in domestic animals. Finally, we describe five examples of evolution of alleles, which means that alleles have evolved by the accumulation of several consecutive mutations affecting the function of the same gene.

Genome-Wide Copy Number Variations Using SNP Genotyping in a Mixed Breed Swine Population

Copy number variations (CNVs) are increasingly understood to affect phenotypic variation. This study uses SNP genotyping of trios of mixed breed swine to add to the catalog of known genotypic variation in an important agricultural animal. PorcineSNP60 BeadChip genotypes were collected from 1802 pigs that combined to form 1621 trios. These trios were from the crosses of 50 boars with 525 sows producing 1621 piglets. The pigs were part of a population that was a mix of ¼ Duroc, ½ Landrace and ¼ Yorkshire breeds. Merging the overlapping CNVs that were observed in two or more individuals to form CNV regions (CNVRs) yielded 502 CNVRs across the autosomes. The CNVRs intersected genes, as defined by RefSeq, 84% of the time – 420 out of 502. The results of this study are compared and contrasted to other swine studies using similar and different methods of detecting CNVR. While progress is being made in this field, more work needs to be done to improve consistency and confidence in CNVR results.

Two variants in the KIT gene as candidate causative mutations for a dominant white and a white spotting phenotype in the donkey

White spotting phenotypes have been intensively studied in horses, and although similar phenotypes occur in the donkey, little is known about the molecular genetics underlying these patterns in donkeys. White spotting in donkeys can range from only a few white areas to almost complete depigmentation and is characterised by a loss of pigmentation usually progressing from a white spot in the hip area. Completely white-born donkeys are rare, and the phenotype is characterised by the complete absence of pigment resulting in pink skin and a white coat. A dominant mode of inheritance has been demonstrated for spotting in donkeys. Although the mode of inheritance for the completely white phenotype in donkeys is not clear, the phenotype shows similarities to dominant white in horses. As variants in the KIT gene are known to cause a range of white phenotypes in the horse, we investigated the KIT gene as a potential candidate gene for two phenotypes in the donkey, white spotting and white. A mutation analysis of all 21 KIT exons identified a missense variant in exon 4 (c.662A>C; p.Tyr221Ser) present only in a white-born donkey. A second variant affecting a splice donor site (c.1978+2T>A) was found exclusively in donkeys with white spotting. Both variants were absent in 24 solid-coloured controls. To the authors' knowledge, this is the first study investigating genetic mechanisms underlying white phenotypes in donkeys. Our results suggest that two independent KIT alleles are probably responsible for white spotting and white in donkeys.

Platinum coat color in red fox (Vulpes vulpes) is caused by a mutation in an autosomal copy of KIT

The red fox (Vulpes vulpes) demonstrates a variety of coat colors including platinum, a common phenotype maintained in farm-bred fox populations. Foxes heterozygous for the platinum allele have a light silver coat and extensive white spotting, whereas homozygosity is embryonic lethal. Two KIT transcripts were identified in skin cDNA from platinum foxes. The long transcript was identical to the KIT transcript of silver foxes, whereas the short transcript, which lacks exon 17, was specific to platinum. The KIT gene has several copies in the fox genome: an autosomal copy on chromosome 2 and additional copies on the B chromosomes. To identify the platinum-specific KIT sequence, the genomes of one platinum and one silver fox were sequenced. A single nucleotide polymorphism (SNP) was identified at the first nucleotide of KIT intron 17 in the platinum fox. In platinum foxes, the A allele of the SNP disrupts the donor splice site and causes exon 17, which is part of a segment that encodes a conserved tyrosine kinase domain, to be skipped. Complete cosegregation of the A allele with the platinum phenotype was confirmed by linkage mapping (LOD 25.59). All genotyped farm-bred platinum foxes from Russia and the US were heterozygous for the SNP (A/G), whereas foxes with different coat colors were homozygous for the G allele. Identification of the platinum mutation suggests that other fox white-spotting phenotypes, which are allelic to platinum, would also be caused by mutations in the KIT gene.

Genes on B chromosomes of vertebrates

BACKGROUND

There is a growing body of evidence that B chromosomes, once regarded as totally heterochromatic and genetically inert, harbor multiple segmental duplications containing clusters of ribosomal RNA genes, processed pseudogenes and protein-coding genes. Application of novel molecular approaches further supports complex composition and possible phenotypic effects of B chromosomes.

RESULTS

Here we review recent findings of gene-carrying genomic segments on B chromosomes from different vertebrate groups. We demonstrate that the genetic content of B chromosomes is highly heterogeneous and some B chromosomes contain multiple large duplications derived from various chromosomes of the standard karyotype. Although B chromosomes seem to be mostly homologous to each other within a species, their genetic content differs between species. There are indications that some genomic regions are more likely to be located on B chromosomes.

CONCLUSIONS

The discovery of multiple autosomal genes on B chromosomes opens a new discussion about their possible effects ranging from sex determination to fitness and adaptation, their complex interactions with host genome and role in evolution.

Genomic regions influencing coat color saturation and facial markings in Fleckvieh cattle

Genomic regions associated with coat color and pigmented areas of the head were identified for Fleckvieh (dual-purpose Simmental), a red-spotted and white-headed cattle breed. Coat color was measured with a chromameter, implementing the CIELAB color space and resulting in numerical representation of lightness, color intensity, red/green and blue/yellow color components, rather than subjective classification. Single marker regression analyses with fixed effects of the sex and barn were applied, and significant regions were determined with the local false discovery rate methodology. The PMEL and ERBB3 genes on chromosome 5 were in the most significant region for the color measurements. In addition to the blue/yellow color component and color intensity, the AP3B2 gene on chromosome 21 was identified. Its function was confirmed for similar traits in a range of model species. The KIT gene on chromosome 6 was found to be strongly associated with the inhibition of circum-ocular pigmentation and pigmented spots on the cheek.

Analysis of genome-wide copy number variations in Chinese indigenous and western pig breeds by 60 K SNP genotyping arrays

Copy number variations (CNVs) represent a substantial source of structural variants in mammals and contribute to both normal phenotypic variability and disease susceptibility. Although low-resolution CNV maps are produced in many domestic animals, and several reports have been published about the CNVs of porcine genome, the differences between Chinese and western pigs still remain to be elucidated. In this study, we used Porcine SNP60 BeadChip and PennCNV algorithm to perform a genome-wide CNV detection in 302 individuals from six Chinese indigenous breeds (Tongcheng, Laiwu, Luchuan, Bama, Wuzhishan and Ningxiang pigs), three western breeds (Yorkshire, Landrace and Duroc) and one hybrid (Tongcheng×Duroc). A total of 348 CNV Regions (CNVRs) across genome were identified, covering 150.49 Mb of the pig genome or 6.14% of the autosomal genome sequence. In these CNVRs, 213 CNVRs were found to exist only in the six Chinese indigenous breeds, and 60 CNVRs only in the three western breeds. The characters of CNVs in four Chinese normal size breeds (Luchuan, Tongcheng and Laiwu pigs) and two minipig breeds (Bama and Wuzhishan pigs) were also analyzed in this study. Functional annotation suggested that these CNVRs possess a great variety of molecular function and may play important roles in phenotypic and production traits between Chinese and western breeds. Our results are important complementary to the CNV map in pig genome, which provide new information about the diversity of Chinese and western pig breeds, and facilitate further research on porcine genome CNVs.

Genome-wide analysis reveals artificial selection on coat colour and reproductive traits in Chinese domestic pigs

Pigs from Asia and Europe were independently domesticated from c. 9000 years ago. During this period, strong artificial selection has led to dramatic phenotypic changes in domestic pigs. However, the genetic basis underlying these morphological and behavioural adaptations is relatively unknown, particularly for indigenous Chinese pigs. Here, we performed a genome-wide analysis to screen 196 regions with selective sweep signals in Tongcheng pigs, which are a typical indigenous Chinese breed. Genes located in these regions have been found to be involved in lipid metabolism, melanocyte differentiation, neural development and other biological processes, which coincide with the evolutionary phenotypic changes in this breed. A synonymous substitution, c.669T>C, in ESR1, which colocalizes with a major quantitative trait locus for litter size, shows extreme differences in allele frequency between Tongcheng pigs and wild boars. Notably, the variant C allele in this locus exhibits high allele frequency in most Chinese populations, suggesting a consequence of positive selection. Five genes (PRM1, PRM2, TNP2, GPR149 and JMJD1C) related to reproductive traits were found to have high haplotype similarity in Chinese breeds. Two selected genes, MITF and EDNRB, are implied to shape the two-end black colour trait in Tongcheng pig. Subsequent SNP microarray studies of five Chinese white-spotted breeds displayed a concordant signature at both loci, suggesting that these two genes are responsible for colour variations in Chinese breeds. Utilizing massively parallel sequencing, we characterized the candidate sites that adapt to artificial and environmental selections during the Chinese pig domestication. This study provides fundamental proof for further research on the evolutionary adaptation of Chinese pigs.

The KIT gene is associated with the english spotting coat color locus and congenital megacolon in Checkered Giant rabbits (Oryctolagus cuniculus)

The English spotting coat color locus in rabbits, also known as Dominant white spotting locus, is determined by an incompletely dominant allele (En). Rabbits homozygous for the recessive wild-type allele (en/en) are self-colored, heterozygous En/en rabbits are normally spotted, and homozygous En/En animals are almost completely white. Compared to vital en/en and En/en rabbits, En/En animals are subvital because of a dilated (“mega”) cecum and ascending colon. In this study, we investigated the role of the KIT gene as a candidate for the English spotting locus in Checkered Giant rabbits and characterized the abnormalities affecting enteric neurons and c-kit positive interstitial cells of Cajal (ICC) in the megacolon of En/En rabbits. Twenty-one litters were obtained by crossing three Checkered Giant bucks (En/en) with nine Checkered Giant (En/en) and two en/en does, producing a total of 138 F1 and backcrossed rabbits. Resequencing all coding exons and portions of non-coding regions of the KIT gene in 28 rabbits of different breeds identified 98 polymorphisms. A single nucleotide polymorphism genotyped in all F1 families showed complete cosegregation with the English spotting coat color phenotype (θ = 0.00 LOD  = 75.56). KIT gene expression in cecum and colon specimens of En/En (pathological) rabbits was 5–10% of that of en/en (control) rabbits. En/En rabbits showed reduced and altered c-kit immunolabelled ICC compared to en/en controls. Morphometric data on whole mounts of the ascending colon showed a significant decrease of HuC/D (P<0.05) and substance P (P<0.01) immunoreactive neurons in En/En vs. en/en. Electron microscopy analysis showed neuronal and ICC abnormalities in En/En tissues. The En/En rabbit model shows neuro-ICC changes reminiscent of the human non-aganglionic megacolon. This rabbit model may provide a better understanding of the molecular abnormalities underlying conditions associated with non-aganglionic megacolon.