Linkage disequilibrium and historical effective population size in the Thoroughbred horse

Many genomic methodologies rely on the presence and extent of linkage disequilibrium (LD) between markers and genetic variants underlying traits of interest, but the extent of LD in the horse has yet to be comprehensively characterized. In this study, we evaluate the extent and decay of LD in a sample of 817 Thoroughbreds. Horses were genotyped for over 50,000 single nucleotide polymorphism (SNP) markers across the genome, with 34,848 autosomal SNPs used in the final analysis. Linkage disequilibrium, as measured by the squared correlation coefficient (r(2)), was found to be relatively high between closely linked markers (>0.6 at 5 kb) and to extend over long distances, with average r(2) maintained above non-syntenic levels for single nucleotide polymorphisms (SNPs) up to 20 Mb apart. Using formulae which relate expected LD to effective population size (N(e)), and assuming a constant actual population size, N(e) was estimated to be 100 in our population. Values of historical N(e), calculated assuming linear population growth, suggested a decrease in N(e) since the distant past, reaching a minimum twenty generations ago, followed by a subsequent increase until the present time. The qualitative trends observed in N(e) can be rationalized by current knowledge of the history of the Thoroughbred breed, and inbreeding statistics obtained from published pedigree analyses are in agreement with observed values of N(e). Given the high LD observed and the small estimated N(e), genomic methodologies such as genomic selection could feasibly be applied to this population using the existing SNP marker set.

The identification of SNPs with indeterminate positions using the Equine SNP50 BeadChip

We have used linkage disequilibrium (LD) to identify single nucleotide polymorphisms (SNPs) on the Illumina Equine SNP50 BeadChip, which may be incorrectly positioned on the genome map. A total of 1201 Thoroughbred horses were genotyped using the Illumina Equine SNP50 BeadChip. LD was evaluated in a pairwise fashion between all autosomal SNPs, both within and across chromosomes. Filters were then applied to the data, firstly to identify SNPs that may have been mapped to the wrong chromosome and secondly to identify SNPs that may have been incorrectly positioned within chromosomes. We identified a single SNP on ECA28, which showed low LD with neighbouring SNPs but considerable LD with a group of SNPs on ECA10. Furthermore, a cluster of SNPs on ECA5 showed unusually low LD with surrounding SNPs. A total of 39 SNPs met the criteria for unusual within-chromosome LD. The results of this study indicate that some SNPs may be misplaced. This finding is significant, as misplaced SNPs may lead to difficulties in the application of genomic methods, such as homozygosity mapping, for which SNP order is important.

Genome sequence, comparative analysis, and population genetics of the domestic horse

We report a high-quality draft sequence of the genome of the horse (Equus caballus). The genome is relatively repetitive but has little segmental duplication. Chromosomes appear to have undergone few historical rearrangements: 53% of equine chromosomes show conserved synteny to a single human chromosome. Equine chromosome 11 is shown to have an evolutionary new centromere devoid of centromeric satellite DNA, suggesting that centromeric function may arise before satellite repeat accumulation. Linkage disequilibrium, showing the influences of early domestication of large herds of female horses, is intermediate in length between dog and human, and there is long-range haplotype sharing among breeds.

Evolution of sensory complexity recorded in a myxobacterial genome

Myxobacteria are single-celled, but social, eubacterial predators. Upon starvation they build multicellular fruiting bodies using a developmental program that progressively changes the pattern of cell movement and the repertoire of genes expressed. Development terminates with spore differentiation and is coordinated by both diffusible and cell-bound signals. The growth and development of Myxococcus xanthus is regulated by the integration of multiple signals from outside the cells with physiological signals from within. A collection of M. xanthus cells behaves, in many respects, like a multicellular organism. For these reasons M. xanthus offers unparalleled access to a regulatory network that controls development and that organizes cell movement on surfaces. The genome of M. xanthus is large (9.14 Mb), considerably larger than the other sequenced delta-proteobacteria. We suggest that gene duplication and divergence were major contributors to genomic expansion from its progenitor. More than 1,500 duplications specific to the myxobacterial lineage were identified, representing >15% of the total genes. Genes were not duplicated at random; rather, genes for cell-cell signaling, small molecule sensing, and integrative transcription control were amplified selectively. Families of genes encoding the production of secondary metabolites are overrepresented in the genome but may have been received by horizontal gene transfer and are likely to be important for predation.

Canine RPGRIP1 mutation establishes cone–rod dystrophy in miniature longhaired dachshunds as a homologue of human Leber congenital amaurosis

Cone-rod dystrophy 1 (cord1) is a recessive condition that occurs naturally in miniature longhaired dachshunds (MLHDs). We mapped the cord1 locus to a region of canine chromosome CFA15 that is syntenic with a region of human chromosome 14 (HSA14q11.2) containing the retinitis pigmentosa GTPase regulator-interacting protein 1 (RPGRIP1) gene. Mutations in RPGRIP1 have been shown to cause Leber congenital amaurosis, a group of retinal dystrophies that represent the most common genetic causes of congenital visual impairment in infants and children. Using the newly available canine genome sequence we sequenced RPGRIP1 in affected and carrier MLHDs and identified a 44-nucleotide insertion in exon 2 that alters the reading frame and introduces a premature stop codon. All affected and carrier dogs within an extended inbred pedigree were homozygous and heterozygous, respectively, for the mutation. We conclude the mutation is responsible for cord1 and demonstrate that this canine disease is a valuable model for exploring disease mechanisms and potential therapies for human Leber congenital amaurosis.

The DNA sequence and biological annotation of human chromosome 1

The reference sequence for each human chromosome provides the framework for understanding genome function, variation and evolution. Here we report the finished sequence and biological annotation of human chromosome 1. Chromosome 1 is gene-dense, with 3,141 genes and 991 pseudogenes, and many coding sequences overlap. Rearrangements and mutations of chromosome 1 are prevalent in cancer and many other diseases. Patterns of sequence variation reveal signals of recent selection in specific genes that may contribute to human fitness, and also in regions where no function is evident. Fine-scale recombination occurs in hotspots of varying intensity along the sequence, and is enriched near genes. These and other studies of human biology and disease encoded within chromosome 1 are made possible with the highly accurate annotated sequence, as part of the completed set of chromosome sequences that comprise the reference human genome.

The DNA sequence and comparative analysis of human chromosome 20

The finished sequence of human chromosome 20 comprises 59,187,298 base pairs (bp) and represents 99.4% of the euchromatic DNA. A single contig of 26 megabases (Mb) spans the entire short arm, and five contigs separated by gaps totalling 320 kb span the long arm of this metacentric chromosome. An additional 234,339 bp of sequence has been determined within the pericentromeric region of the long arm. We annotated 727 genes and 168 pseudogenes in the sequence. About 64% of these genes have a 5' and a 3' untranslated region and a complete open reading frame. Comparative analysis of the sequence of chromosome 20 to whole-genome shotgun-sequence data of two other vertebrates, the mouse Mus musculus and the puffer fish Tetraodon nigroviridis, provides an independent measure of the efficiency of gene annotation, and indicates that this analysis may account for more than 95% of all coding exons and almost all genes.

Genome sequence of the plant pathogen and biotechnology agent Agrobacterium tumefaciens C58

Agrobacterium tumefaciens is a plant pathogen capable of transferring a defined segment of DNA to a host plant, generating a gall tumor. Replacing the transferred tumor-inducing genes with exogenous DNA allows the introduction of any desired gene into the plant. Thus, A. tumefaciens has been critical for the development of modern plant genetics and agricultural biotechnology. Here we describe the genome of A. tumefaciens strain C58, which has an unusual structure consisting of one circular and one linear chromosome. We discuss genome architecture and evolution and additional genes potentially involved in virulence and metabolic parasitism of host plants.

Analysis of vertebrate SCL loci identifies conserved enhancers

The SCL gene encodes a highly conserved bHLH transcription factor with a pivotal role in hemopoiesis and vasculogenesis. We have sequenced and analyzed 320 kb of genomic DNA composing the SCL loci from human, mouse, and chicken. Long-range sequence comparisons demonstrated multiple peaks of human/mouse homology, a subset of which corresponded precisely with known SCL enhancers. Comparisons between mammalian and chicken sequences identified some, but not all, SCL enhancers. Moreover, one peak of human/mouse homology (+23 region), which did not correspond to a known enhancer, showed significant homology to an analogous region of the chicken SCL locus. A transgenic Xenopus reporter assay was established and demonstrated that the +23 region contained a new neural enhancer. This combination of long-range comparative sequence analysis with a high-throughput transgenic bioassay provides a powerful strategy for identifying and characterizing developmentally important enhancers.

Tdr2, a new zebrafish transposon of the Tc1 family

We describe here Tdr2, a new class of Tc1-like transposons in zebrafish. Tdr2 was identified from the genomic sequence of a zebrafish PAC (P1 artificial chromosome) clone, and fragments of Tdr2 were found in several zebrafish EST (expressed sequence tag) sequences. Predicted translation of the Tdr2 transposase gene showed that it was most closely related to Caenorhabditis elegans Tc3A, suggesting an ancient origin of the Tdr2 transposon. Tdr2 spans 1. 1kb and is flanked by inverted repeats of approx. 100bp. The 5' repeat is itself composed of an inverted repeat, raising the possibility of the formation of a cruciform DNA structure. Tdr2 transposons may facilitate the development of novel transposon-based tools for the genetic analysis of zebrafish.

Identification and characterization of the human homologue (RAI2) of a mouse retinoic acid-induced gene in Xp22

We have identified a novel human gene during studies of a 1.3-Mb region of Xp22 between DXS418 and DXS999. A PAC contig spanning the region was constructed, sequenced, and analyzed by gene and exon prediction programs and by homology searches. Further investigation of predicted exons from PAC clone 389A20 led to the identification of a single-exon gene, designated RAI2 (retinoic acid-induced 2). RAI2 mapped 28 kb centromeric to marker DXS7996, between DXS7996 and DXS7997, and was transcribed from centromere to telomere. Northern blot analysis and reverse transcription-polymerase chain reaction analysis revealed expression of a 2.5-kb transcript in four fetal tissues (brain, lung, kidney, and heart) and eight adult tissues (heart, brain, placenta, lung, skeletal muscle, kidney, pancreas, and retina) but not in fetal or adult liver. The 530-amino-acid protein (57 kDa predicted mass) displays 94% homology with a mouse retinoic acid-induced gene product and contains a novel proline-rich (39%) domain of 68 amino acids. Retinoic acid is involved in vertebrate anteroposterior axis formation and cellular differentiation and has been shown to modulate gene expression controlling early embryonal development, suggesting a developmental role for RAI2. RAI2 remains a candidate gene for diseases mapping to the Xp22 region.

Host response to EBV infection in X-linked lymphoproliferative disease results from mutations in an SH2-domain encoding gene

X-linked lymphoproliferative syndrome (XLP or Duncan disease) is characterized by extreme sensitivity to Epstein-Barr virus (EBV), resulting in a complex phenotype manifested by severe or fatal infectious mononucleosis, acquired hypogammaglobulinemia and malignant lymphoma. We have identified a gene, SH2D1A, that is mutated in XLP patients and encodes a novel protein composed of a single SH2 domain. SH2D1A is expressed in many tissues involved in the immune system. The identification of SH2D1A will allow the determination of its mechanism of action as a possible regulator of the EBV-induced immune response.

Characterization of SCML1, a new gene in Xp22, with homology to developmental polycomb genes

Using exon trapping, we have identified a new human gene in Xp22 encoding a 3-kb mRNA. Expression of this RNA is detectable in a range of tissues but is most pronounced in skeletal muscle and heart. The gene, designated "sex comb on midleg-like-1" (SCML1), maps 14 kb centromeric of marker DXS418, between DXS418 and DXS7994, and is transcribed from telomere to centromere. SCML1 spans 18 kb of genomic DNA, consists of six exons, and has a 624-bp open reading frame. The predicted 27-kDa SCML1 protein contains two domains that each have a high homology to two Drosophila transcriptional repressors of the polycomb group (PcG) genes and their homologues in mouse and human. PcG genes are known to be involved in the regulation of homeotic genes, and the mammalian homologues of the PcG genes repress the expression of Hox genes. SCML1 appears to be a new human member of this gene group and may play an important role in the control of embryonal development.

The pufferfish SLP-1 gene, a new member of the SCL/TAL-1 family of transcription factors

The SCL/TAL-1 gene encodes a basic helix-loop-helix (bHLH) transcription factor essential for the development of all hemopoietic lineages and also acts as a T-cell oncogene. Four related genes have been described in mammals (LYL-1, TAL-2, NSCL1, and NSCL2), all of which exhibit a high degree of sequence similarity to SCL/TAL-1 in the bHLH domain and two of which (LYL-1 and TAL-2) have also been implicated in the pathogenesis of T-cell acute lymphoblastic leukemia. In this study we describe the identification and characterization of a pufferfish gene termed SLP-1, which represents a new member of this gene family. The genomic structure and sequence of SLP-1 suggests that it forms a subfamily with SCL/TAL-1 and LYL-1 and is most closely related to SCL/TAL-1. However, unlike SCL/TAL-1, SLP-1 is widely expressed. Sequence analysis of a whole cosmid containing SLP-1 shows that SLP-1 is flanked upstream by a zinc finger gene and a fork-head-domain gene and downstream by a heme-oxygenase and a RING finger gene.

Regions of human chromosome 2 (2q32-q35) and mouse chromosome 1 show synteny with the pufferfish genome (Fugu rubripes)

We have isolated and sequenced a cosmid clone from the compact genome of the Japanese pufferfish (Fugu rubripes) containing portions of three genes that have the same order as in human. The gene order is microtubule-associated protein (MAP-2), myosin light chain (MYL-1), and carbamoyl phosphate synthetase (CPS III). The intron-exon organization of Fugu CPS III is identical with that of rat CPS I, although the equivalent genomic fragments of rat and Fugu CPS span 87.9 and 21 kb, respectively. This is the first report of a piscine CPS III genomic structure and predicts a close evolutionary link between CPS III and CPS I. The 8-kb intergenic region between MYL-1 and CPS gave no clear areas of transcription factor-binding sites by pairwise comparison with shark or rat CPS promoter regions. However, there was a match with the rat myosin light chain 2 (MLC-2) gene promoter and a MyoD transcription factor-binding site 874 bp upstream of the MYL-1 gene.

Comparative analysis of the polycystic kidney disease 1 (PKD1) gene reveals an integral membrane glycoprotein with multiple evolutionary conserved domains

PKD1 is the major locus of the common genetic disorder autosomal dominant polycystic kidney disease (ADPKD). Analysis of the predicted protein sequence of the human PKD1 gene, polycystin, shows a large molecule with a unique arrangement of extracellular domains and multiple putative transmembrane regions. The precise function of polycystin remains unclear with a paucity of mutations to define key structural and functional domains. To refine the structure of this protein we have cloned the genomic region encoding the Fugu PKD1 gene. Fugu PKD1 spans 36 kb of genomic DNA and has greater complexity with 54 exons compared with 46 in man. Comparative analysis of the predicted protein sequences shows a lower level of homology than in similar studies with identity of 40 and 59% similarity. However key structural motifs including leucine rich repeats (LRR), a C-type lectin and LDL-A like domains and 16 PKD repeats are maintained. A region of homology with the sea urchin REJ protein was also confirmed in Fugu but found to extend over 1000 amino acids. Several highly conserved intra- and extra-cellular regions, with no known sequence homologies, that are likely to be of functional importance were detected. The likely structure of the membrane associated region has been refined with similarity to the PKD2 protein and voltage gated Ca2+ and Na+ channels highlighted over part of this area. The overall protein structure has therefore been clarified and this comparative analysis derived structure will form the basis for the functional study of polycystin and its individual domains.

Comparative analysis and genomic structure of the tuberous sclerosis 2 (TSC2) gene in human and pufferfish

Germ-line mutations of the TSC2 tumour suppressor gene have been identified in humans with tuberous sclerosis and in the Eker rat. Tuberin, the human TSC2 gene product, has a small region of homology with rap1GAP and stimulates rap1 GTPase activity in vitro, suggesting that one of its cellular roles is to function as a GTPase activating protein (GAP). We have undertaken a comparative analysis of the TSC2 gene in human and the pufferfish, Fugu rubripes. In addition to the GAP domain, three other regions of the proteins are highly conserved (peptide sequence similarity > 80%). These regions are likely to represent further functional domains. To facilitate analysis of mutations within these domains we have determined the genomic structure of the human TSC2 gene. It comprises 41 exons, including exon 31 which was absent from the originally described spliceoform of the human TSC2 transcript and was identified following exon prediction from Fugu genomic sequence. These findings support the proposal of the Fugu genome as a tool for human gene analysis.

The construction and analysis of M13 libraries prepared from YAC DNA

Yeast artificial chromosomes (YACs) provide a powerful way to isolate and map large regions of genomic DNA and their use in genome analysis is now extensive. We modified a series of procedures to produce high quality shotgun libraries from small amounts of YAC DNA. Clones from several different libraries have been sequenced and analyzed for distribution, sequence integrity and degree of contamination from yeast DNA. We describe these procedures and analyses and show that sequencing at about 1-fold coverage, followed by database comparison (survey sequencing) offers a relatively quick method to determine the nature of previously uncharacterized cosmid or YAC clones.

Isolation and characterization of male flower cDNAs from maize

Differential screening of two libraries made from whole, immature maize tassels was used to isolate six cDNAs which show enhanced levels of expression in male flowers. MFS1, MFS2, MFS4, MFS10 and MFS18, which were isolated from a 5 cm tassel library, are expressed throughout tassel growth up until mature pollen is produced in the anthers. MFS14, which was isolated from a 10-12 cm tassel library, has a narrower window of expression associated with microsporogenesis and declines as mature pollen is produced. MFS18 mRNA accumulates in the glumes and in anther walls, paleas and lemmas of mature florets. MFS18 mRNA is particularly associated with the vascular bundle in the glumes and encodes a polypeptide of 12 kDa, rich in glycine, proline and serine that has similarities with other plant structural proteins. In contrast, MFS14 mRNA accumulates in the tapetum and encodes a polypeptide of 13 kDa that is rich in alanine. The MFS14 and MFS18 proteins are basic (isoelectric points of 11.56 and 9.54, respectively) and both have hydrophobic N-termini which display all the characteristics of signal peptides, indicating that these proteins may be secreted.

Mapping nonisotopically labeled DNA probes to human chromosome bands by confocal microscopy

A method for mapping nonisotopically labeled probes to human metaphase chromosomes that can be used with laser scanning confocal microscopy has been developed. Only a limited number of wavelengths are available from the argon ion lasers used in most commercial instruments and therefore a method that allowed the visualization of bands on human chromosomes stained with propidium iodide and, simultaneously, the detection of hybridization signals using FITC-labeled antibodies was developed. The confocal microscope was used to map single-copy probes to chromosome bands and the positions of the probes on the R-banded chromosomes corresponded to map positions previously determined on Hoechst 33258-stained chromosomes (G-banded). A comparison of confocal imaging of single-copy hybridization signals with conventional fluorescence microscopy and high-sensitivity video cameras revealed little difference in sensitivity but greater resolution of chromosome bands with the confocal microscope. The polymerase chain reaction was used to prepare nonisotopically labeled probes for in situ hybridization and to amplify Alu and KpnI family repeats from cloned DNA to be used to suppress hybridization of these repeat sequences so that a cosmid probe could be mapped to a chromosome band.

Fluorescent and radioactive solid phase dideoxy sequencing of pcr products in microtitre plates

In this paper we describe a rapid method for the direct generation of DNA sequencing templates from phage or bacteria. Sequencing of these PCR products can be performed by radioactive and fluorescent methods. The non-radioactive method has been used to sequence a total of approximately 100 kb of human DNA fragments generated by digestion with HpaII and subsequent cloning. The method depends on direct small scale amplification using a biotinylated primer, and the binding of the product to streptavidin coated magnetic beads. All the procedures are carried out in a microtitre plate thus facilitating the handling of large numbers of clones and has potential for automation.

Direct single stranded sequencing from agarose of polymerase chain reaction products

Images

A rapid semi-automated microtiter plate method for analysis and sequencing by PCR from bacterial stocks

Images

The complete nucleotide sequence of the genome of a hepatitis B virus isolated from a naturally infected chimpanzee

The complete nucleotide sequence of a strain of hepatitis B virus, originally isolated from a naturally infected chimpanzee, has been determined. Interesting features of the sequence include the presence of an in-phase stop codon in the 'pre-core' region of the core antigen open reading frame. The sequence shows approximately 10% nucleotide divergence from all of the other hepatitis B virus sequences previously published and the possibility that this divergence is the result of passage through chimpanzees is discussed.

Preparation of hepatitis B polypeptide micelles from human carrier plasma

Water-soluble protein micelles consisting of the 28,000 (gp28) and 23,00 (p23) molecular weight polypeptide complex of hepatitis B surface antigen can be readily prepared from human plasma containing the surface antigen and other markers of infection with hepatitis B virus. The antigenic activity of the polypeptides was preserved throughout the process of solubilisation and reassociation into micelles. Such preparations are therefore eminently suitable as "second generation" hepatitis B vaccines.