Analyses of pig genomes provide insight into porcine demography and evolution

For 10,000 years pigs and humans have shared a close and complex relationship. From domestication to modern breeding practices, humans have shaped the genomes of domestic pigs. Here we present the assembly and analysis of the genome sequence of a female domestic Duroc pig (Sus scrofa) and a comparison with the genomes of wild and domestic pigs from Europe and Asia. Wild pigs emerged in South East Asia and subsequently spread across Eurasia. Our results reveal a deep phylogenetic split between European and Asian wild boars ∼1 million years ago, and a selective sweep analysis indicates selection on genes involved in RNA processing and regulation. Genes associated with immune response and olfaction exhibit fast evolution. Pigs have the largest repertoire of functional olfactory receptor genes, reflecting the importance of smell in this scavenging animal. The pig genome sequence provides an important resource for further improvements of this important livestock species, and our identification of many putative disease-causing variants extends the potential of the pig as a biomedical model.

Allele-specific primer polymerase chain reaction for a single nucleotide polymorphism (C1205T) of swine toll-like receptor 5 and comparison of the allelic frequency among several pig breeds in Japan and the Czech Republic

In the present study, an allele-specific primer-polymerase chain reaction (ASP-PCR) for genotyping a single nucleotide polymorphism (SNP) of swine Toll-like receptor 5 (TLR5) (C1205T; P402L) that is related to the impaired recognition of Salmonella enterica serovar Choleraesuis (SC) was developed. The allele frequencies in several pig breeds in Japan and the Czech Republic were also compared. The swine TLR5 C1205T mutation was successfully determined by ASP-PCR using genomic DNA samples in Japan that had previously been genotyped by a sequencing method. Using the PCR condition determined, genomic DNA samples from blood obtained from 110 pigs from seven different breeds in the Czech Republic were genotyped by the ASP-PCR. The genotyping results from the ASP-PCR completely matched the results from the sequencing method. The allele frequency of the swine TLR5 C1205T mutation was 27.5% in the Landrace breed of the Czech Republic compared with 50.0% in Japanese Landrace. In Japan, the C1205T mutation was found only in the Landrace breed, whereas in the Czech Republic it was found in both the Landrace and Piétrain breeds. These results indicate the usefulness of ASP-PCR for detecting a specific SNP for swine TLR5 affecting ligand recognition. They also suggest the possibility of genetically improving pigs to enhance their resistance against SC infection by eliminating or selecting this specific SNP of swine TLR5.

MBL1 genotypes in wild boar populations from Sweden, Austria, the Czech Republic, and Japan

The single nucleotide polymorphism (SNP) G949T in the mannose-binding lectin ( MBL ) 1 gene has been associated with low MBL-A concentration in serum and detected at different frequencies in various European pig populations. However, the origin of this SNP is not known. Part of the MBL1 gene was sequenced in 12 wild boar/Large White crossbred pigs from the second backcross (BC 2 ) generation in a family material originating from two wild boar x Large White intercrosses. Also, MBL-A serum concentration was measured in the entire BC 2 generation (n = 45). Furthermore, the genotypes of 68 wild boars from Sweden, Austria, the Czech Republic, and Japan were determined in regard to five previously described SNPs in MBL1 . The T allele of G949T was present among the BC 2 animals. MBL-A serum concentration in the BC 2 animals showed a bimodal distribution, with one-third of the animals at levels between 0.7 and 1.6 μg mL(-1) and the remaining pigs at levels around 13 μg mL(-1) . There was a co-variation between the presence of the T allele and low MBL-A concentration in serum. The genotyping of the wild boars revealed differences between populations. The T allele of G949T was not detected in the Austrian and Japanese samples and is thus unlikely to be an original feature of wild boars. In contrast, it was present at high frequency (0.35) among the Swedish wild boars, probably representing a founder effect. Five MBL1 haplotypes were resolved. Only two of these were present among the Japanese wild boars compared to four in each of the European populations. This difference may reflect differences in selection pressure and population history.

A genome-wide scan for quantitative trait loci affecting respiratory disease and immune capacity in Landrace pigs

Respiratory disease is the most important health concern for the swine industry. Genetic improvement for disease resistance is challenging because of the difficulty in obtaining good phenotypes related with disease resistance; however, identification of genes or markers associated with disease resistance can help in the genetic improvement of pig health. The purpose of our study was to investigate whether quantitative trait loci (QTL) associated with disease resistance were segregated in a purebred population of Landrace pigs that had been selected for meat production traits and mycoplasmal pneumonia of swine (MPS) scores over five generations. We analysed 1395 pigs from the base to the fifth generation of this population. Two respiratory disease traits [MPS scores and atrophic rhinitis (AR) scores] and 11 immune-capacity traits were measured in 630-1332 animals at 7 weeks of age and when the animal's body weight reached 105 kg. Each of the pigs, except sires in the base population, was genotyped using 109 microsatellite markers, and then, QTL analysis of the full-sib family population with a multi-generational pedigree structure was performed. Variance component analysis was used to detect QTL associated with MPS or AR scores, and the logarithm of odds (LOD) score and genotypic heritability of the QTL were estimated. Five significant (LOD > 2.51) and 18 suggestive (LOD > 1.35) QTL for respiratory disease traits and immune-capacity traits were detected. The significant QTL for Log-MPS score, located on S. scrofa chromosome 2, could explain 87% of the genetic variance of this score in this analysis. This is the first report of QTL associated with respiratory disease lesions.

Changes in gene expression in a porcine preadipocyte cell line during differentiation

Adipocyte differentiation plays an important role in the formation of fat tissues in pigs and affects meat quality and productivity. Clarification of the nature of the pig genes that participate in adipocyte differentiation will provide a clue to the regulation of fat content and thickness in pig carcases by dietary control; it will also help to find target genes for exploring potentially useful polymorphisms for molecular breeding aimed at fat traits. We constructed a DNA oligomer microarray based on pig transcripts, and we used the array to investigate time-dependent changes in gene expression in the PSPA porcine preadipocyte cell line during differentiation into adipocytes. We selected genes with markedly altered expression (at least fivefold difference in comparison with expression in undifferentiated cells) and classified them into five groups according to gene expression pattern. In the early stage after stimulation of adipocyte differentiation, we observed up-regulation of many genes encoding proteins involved in regulating cell proliferation and transcription. Among the probes corresponding to transcripts that showed marked changes in expression, 27 were located within previously reported QTL regions for traits related to adipose tissues. These results will be valuable resources for finding the genes responsible for fat-related traits that have been identified in previous studies using various pig resource families.

Porcine single nucleotide polymorphism (SNP) development and population structure of pigs assessed by validated SNPs

In this study, we identified porcine single nucleotide polymorphisms (SNPs) by aligning eight sequences generated with two approaches: amplification of 665 intronic regions using one sample from each of eight breeds, including three East Asian pigs, and amplification of 289 3'-UTR regions using two samples from each of four major commercial breeds. The 1,760 and 599 SNPs were validated using two 384-sample DNA panels by matrix-assisted laser desorption ionization time-of-flight mass spectrometry. The phylogenetic tree and Structure analyses classified the pigs into two large clusters: Euro-American and East Asian populations. The membership proportions, however, differed between inferred clusters for K = 2 generated by the two approaches. With intronic SNPs, Euro-American breeds constituted about 100% of the Euro-American cluster, but with 3'-UTR SNPs, about 17% of the East Asian cluster comprised five Euro-American breeds. The differences in the SNP discovery panels may affect population structure found in study panels of large samples.

Development of allele-specific primer PCR for a swine TLR2 SNP and comparison of the frequency among several pig breeds of Japan and the Czech Republic

In the present study, we have developed an allele-specific primer-polymerase chain reaction (ASP-PCR) for genotyping a single nucleotide polymorphism (SNP) of swine Toll-like receptor 2 (TLR2) (C406G), which is related to the prevalence of pneumonia caused by Mycoplasma hyopneumoniae. We also compared the allele frequency among several pig breeds of Japan and the Czech Republic. Allele-specific primers were constructed by introducing 1-base mismatch sequence before the SNP site. The swine TLR2 C406G mutation was successfully determined by the ASP-PCR using genomic DNA samples in Japan as previously genotyped by a sequencing method. Using the PCR condition determined, genomic DNA samples from pig blood obtained from 110 pigs from 7 different breeds in the Czech Republic were genotyped by the ASP-PCR. The genotyping results from the ASP-PCR were completely matched with the results from the sequencing method. The allele frequency of the swine TLR2 C406G mutation was 27.5% in the Czech Republic and 3.6% in Japan. The C406G mutation was only found in the Landrace breed in Japan, and was almost exclusively found in the Landrace breed in the Czech Republic as well. These results indicated the usefulness of ASP-PCR for detecting a specific SNP for swine TLR2.

Structural analysis of MHC alleles in an RSV tumour regression chicken using a BAC library

The chicken major histocompatibility complex (MHC-B locus) has a strong association with resistance and susceptibility to numerous diseases. We have found a B haplotype designated WLA that associated with the regression of tumours caused by Rous sarcoma virus J strain (RSV-J). Haplotype WLA was identical to the regressive B6 haplotype when partial genotyping was performed (Poultry Science, 89, 2010, 651). We then constructed a bacterial artificial chromosome (BAC) library from a WLA homozygote chicken to evaluate the structure of this regression haplotype and compared it to those of the B6 haplotype. Comparison between WLA and B6 above 59 kb within the 167 kb, including 14 genes from BG1 to BF2, revealed 75 SNPs and 14 indels. However, several genes were identical between WLA and B6, including the BF1 and BF2 genes, which encode a class I molecule previously suggested to be related to the regression phenotype. The BLB2 gene encoding the MHC class II beta chain showed the greatest diversity, with 19 non-synonymous SNPs. A comparison of WLA and B6 haplotpyes that are associated with tumour regression and RIRa and B24 haplotypes associated with tumour progression suggests that DMA1, DMA2, BRD2, TAPBP and BLB2 genes are not involved in the intensity of RSV J tumour regression.

Population structure of pigs determined by single nucleotide polymorphisms observed in assembled expressed sequence tags

We have collected more than 190000 porcine expressed sequence tags (ESTs) from full-length complementary DNA (cDNA) libraries and identified more than 2800 single nucleotide polymorphisms (SNPs). In this study, we tentatively chose 222 SNPs observed in assembled ESTs to study pigs of different breeds; 104 were selected by comparing the cDNA sequences of a Meishan pig and samples of three-way cross pigs (Landrace, Large White, and Duroc: LWD), and 118 were selected from LWD samples. To evaluate the genetic variation between the chosen SNPs from pig breeds, we determined the genotypes for 192 pig samples (11 pig groups) from our DNA reference panel with matrix-assisted laser desorption ionization time-of-flight mass spectrometry. Of the 222 reference SNPs, 186 were successfully genotyped. A neighbor-joining tree showed that the pig groups were classified into two large clusters, namely, Euro-American and East Asian pig populations. F-statistics and the analysis of molecular variance of Euro-American pig groups revealed that approximately 25% of the genetic variations occurred because of intergroup differences. As the F(IS) values were less than the F(ST) values(,) the clustering, based on the Bayesian inference, implied that there was strong genetic differentiation among pig groups and less divergence within the groups in our samples.

Polymorphisms in pattern recognition receptors and their relationship to infectious disease susceptibility in pigs

Background

Pattern recognition receptors (PRRs), including Toll-like receptors (TLRs), are censoring receptors for molecules derived from bacteria, viruses, and fungi. The PRR system is a prerequisite for proper responses to pathogens, for example by cytokine production, resulting in pathogen eradication. Many cases of polymorphisms in PRR genes affecting the immune response and disease susceptibility are known in humans and mice.

Methods

We surveyed polymorphisms in pig genes encoding PRRs and investigated the relationship between some of the detected polymorphisms and molecular function or disease onset.

Results

Nonsynonymous polymorphisms abounded in pig TLR genes, particularly in the region corresponding to the ectodomains of TLRs expressed on the cell surface. Intracellular TLRs such as TLR3, TLR7, and TLR8, and other intracellular PRRs, such as the peptidoglycan receptor NOD2 and viral RNA receptors RIG-I and MDA5, also possessed nonsynonymous polymorphisms. Several of the polymorphisms influenced molecular functions such as ligand recognition. Polymorphisms in the PRR genes may be related to disease susceptibility in pigs: pigs with a particular allele of TLR2 showed an increased tendency to contract pneumonia.

Conclusions

We propose the possibility of pig breeding aimed at disease resistance by the selection of PRR gene alleles that affect pathogen recognition.

SLA-DRB1 and -DQB1 genotyping by the PCR-SSOP-Luminex method

A simple and novel genotyping method was developed to detect alleles at the swine leukocyte antigen (SLA)-DRB1 and -DQB1 class II loci by using polymerase chain reaction (PCR)-fluorescently labeled sequence-specific oligonucleotide probes (SSOPs) and Luminex 100 xMAP detection. The PCR-SSOP-Luminex method exhibited accuracy of 95% for both SLA-DRB1 and -DQB1 in 6 homozygous and 16 heterozygous pig samples as confirmed by sequencing the PCR products of the same samples. In addition, 12 low-resolution SLA class II haplotypes consisting of 7 and 9 DRB1 and DQB1 alleles were identified, respectively, in one population of 283 Landrace pigs. This genotyping method facilitates the rapid and accurate identification of two- or four-digit alleles at the SLA-DRB1 and -DQB1 loci.

Porcine Toll-like receptors: recognition of Salmonella enterica serovar Choleraesuis and influence of polymorphisms

Salmonella enterica serovar Choleraesuis (SC) is a highly invasive pathogen that causes enteric and septicemic diseases in pigs. Although there have been some reports on gene expression profiles in the course of infection with SC in pigs, little is known about the genes involved in the infection. By measuring activation, as represented by nuclear factor-κB activity, after stimulation by the pathogen, we showed the involvement of Toll-like receptor (TLR) 5 and the TLR2-TLR1 heterodimer in the recognition of SC. We previously found single nucleotide polymorphisms (SNPs) in the TLRs of various pig populations. Here we demonstrated that the polymorphisms resulting in amino acid changes TLR5(R148L), TLR5(P402L), and TLR2(V703M) attenuated the responses to SC by the cells transfected with the TLR genes. Each of these three SNPs was differently restricted in distribution among breeds. These results suggest that there are differences in resistance to salmonellosis among breeds; these differences may be of great importance for the pig industry in terms of breeding and vaccine development.

Cloning, expression, and polymorphisms of natural killer cell receptor NCR1 in pigs

NCR1 (NKp46) is expressed on the surfaces of natural killer cells and recognizes hemagglutinin on the influenza virus. We cloned the NCR1 gene in pigs and found that porcine NCR1 was minimally expressed in the thymus, suggesting that NCR1 could be a useful marker of natural killer cells in pigs. We observed three nonsynonymous single nucleotide polymorphisms and one deletion of three nucleotides in the coding sequence of porcine NCR1; these may affect the function of NCR1. The polymorphisms detected here may be useful markers for breeding for influenza resistance in pigs.

Pig genome sequence--analysis and publication strategy

Background

The pig genome is being sequenced and characterised under the auspices of the Swine Genome Sequencing Consortium. The sequencing strategy followed a hybrid approach combining hierarchical shotgun sequencing of BAC clones and whole genome shotgun sequencing.

Results

Assemblies of the BAC clone derived genome sequence have been annotated using the Pre-Ensembl and Ensembl automated pipelines and made accessible through the Pre-Ensembl/Ensembl browsers. The current annotated genome assembly (Sscrofa9) was released with Ensembl 56 in September 2009. A revised assembly (Sscrofa10) is under construction and will incorporate whole genome shotgun sequence (WGS) data providing > 30× genome coverage. The WGS sequence, most of which comprise short Illumina/Solexa reads, were generated from DNA from the same single Duroc sow as the source of the BAC library from which clones were preferentially selected for sequencing. In accordance with the Bermuda and Fort Lauderdale agreements and the more recent Toronto Statement the data have been released into public sequence repositories (Genbank/EMBL, NCBI/Ensembl trace repositories) in a timely manner and in advance of publication.

Conclusions

In this marker paper, the Swine Genome Sequencing Consortium (SGSC) sets outs its plans for analysis of the pig genome sequence, for the application and publication of the results.

Genotypes of chicken major histocompatibility complex B locus associated with regression of Rous sarcoma virus J-strain tumors

The chicken MHC-B locus affects the response to several strains of Rous sarcoma virus (RSV). We evaluated the association between haplotypes of the MHC-B locus and responses to the J strain of RSV by using an F(2) experimental resource family constructed with tumor-regressive (White Leghorn) and tumor-progressive (Rhode Island Red) chickens. The MHC-B haplotypes were determined by genotyping of the microsatellite marker LEI0258 and MHC-B locus class I alpha chain 2 (BF2). Two haplotypes in the resource family, one associated with tumor regression and one with progression, were defined by these 2 markers. To discriminate more precisely the regressive haplotype in this family, we further developed 35 SNP markers at the MHC-B locus. Information on the haplotypes revealed here should be useful for identifying chickens with regression and progression phenotypes of J-strain RSV-induced tumors.

Genomic structure of the whole D-J-C clusters and the upstream region coding V segments of the TRB locus in pig

In the vertebrate immune system, T cells play a central role in host defense against microbial or viral infection. Previous studies suggested that at least two sets of TRBD-J-C clusters are harbored in the porcine genome. In this study, we determined 212,193 bp of a continuous porcine genomic sequence covering the entire TRBC region. EPHB6, TRPV6, TRY, and ten TRBV genes were conserved in the vicinity of the TRBD-J-C clusters. Interestingly, three TRBD-J-C clusters were identified in this sequence; each TRBD-J-C cluster consisted of one TRBD and seven TRBJ segments, with one TRBC region composed of four exons. The distribution of repetitive sequences and phylogenetic analysis indicated that the TRBD-J-C cluster, located at the center of the three clusters identified, had a structure combined with the others. Most of the TRBJ segments were available in public databases, suggesting that all three TRBD-J-C clusters are functional in pigs.