Old can be new again: HAPPY whole genome sequencing, mapping and assembly

During the last three decades, both genome mapping and sequencing methods have advanced significantly to provide a foundation for scientists to understand genome structures and functions in many species. Generally speaking, genome mapping relies on genome sequencing to provide basic materials, such as DNA probes and markers for their localizations, thus constructing the maps. On the other hand, genome sequencing often requires a high-resolution map as a skeleton for whole genome assembly. However, both genome mapping and sequencing have never come together in one pipeline. After reviewing mapping and next-generation sequencing methods, we would like to share our thoughts with the genome community on how to combine the HAPPY mapping technique with the new-generation sequencing, thus integrating two systems into one pipeline, called HAPPY pipeline. The pipeline starts with preparation of a HAPPY panel, followed by multiple displacement amplification for producing a relatively large quantity of DNA. Instead of conventional marker genotyping, the amplified panel DNA samples are subject to new-generation sequencing with barcode method, which allows us to determine the presence/absence of a sequence contig as a traditional marker in the HAPPY panel. Statistical analysis will then be performed to infer how close or how far away from each other these contigs are within a genome and order the whole genome sequence assembly as well. We believe that such a universal approach will play an important role in genome sequencing, mapping, and assembly of many species; thus advancing genome science and its applications in biomedicine and agriculture.     

Model genomes: the benefits of analysing homologous human and mouse sequences.

The human genome initiative has provided the motivating force for launching sequencing projects suitable for testing various DNA-sequencing strategies, as well as motivating the development of mapping and sequencing technologies. In addition to projects targeting selected regions of the human genome, other projects are based on model organisms such as yeast, nematode and mouse. The sequencing of homologous regions of human and mouse genomes is a new approach to genome analysis, and is providing insights into gene evolution, function and regulation which could not be determined so easily from the analysis of just one species.     

Alternative approaches to genome mapping and sequencing

The main methods used for large-scale mapping of the human and other genomes are reviewed. These methods comprise two procedures of random mapping/sequencing and an approach using linking and jumping libraries. Importantly, no method used up to now has proved efficient in comparative genome analysis. A new method is presented basing on slalom libraries. These libraries provide 10-100 times higher efficiency and may be used for mapping and sequencing whole genomes by small research groups.     

Alternative Approaches to Genome Mapping and Sequencing

The main methods used for large-scale mapping of the human and other genomes are reviewed. These methods comprise two procedures of random mapping/sequencing and an approach using linking and jumping libraries. Importantly, no method used up to now has proved efficient in comparative genome analysis. A new method is presented basing on slalom libraries. These libraries provide 10–100 times higher efficiency and may be used for mapping and sequencing whole genomes by small research groups.     

基于三代测序技术的微生物组学研究进展

微生物在人类生活中无处不在, 过去人们对微生物的认识仅停留在单菌培养和定性研究上, 而测序技术的发展极大地促进了微生物组学的研究。越来越多的证据表明: 人体共生微生物、特别是肠道微生物与人类健康息息相关。 二代测序技术凭借其高通量、高准确率和低成本的特点, 成为微生物组学研究中的主流测序技术。但是随着研究的深入, 二代测序技术的短读长(< 450 bp)增加了后续数据分析和基因组拼接难度, 也限制了该技术在未来研究中的应用。在此背景下, 第三代测序技术应运而生。第三代测序技术又称单分子测序, 能够直接对单个DNA分子进行实时测序, 而不需要经过PCR扩增。第三代测序技术的平均读长在2-10 kb左右, 最高可以达到2.2 Mb, 实现了长序列的高通量测序。凭借其超长的测序读长、无GC偏好性等优势, 三代测序技术为微生物基因组全长测序, 组装完整可靠的基因组提供了新的方法。本文在描述三代测序的技术特点和原理的基础上, 重点介绍了三代测序技术在微生物16S/18S rRNA基因测序、单菌的基因组组装以及宏基因组中的研究应用和进展。     

Human DNA polymorphisms and methods of analysis.

The current predominant method of analyzing base substitution polymorphisms, RFLP analysis, is likely to be gradually supplanted by methods based on PCR because of the improved sensitivity and genotyping rate. The most promising PCR methods for analysis appear to be allele-specific PCR and single-stranded conformational analysis. The single-stranded conformation approach has already been applied to the scanning of cystic fibrosis exons for new mutations. Linkage mapping projects that cover large segments of the human genome will probably rely, in the coming years, primarily on tandem repeat polymorphisms, particularly microsatellite polymorphisms. Microsatellite polymorphisms have at least a fourfold advantage over base substitution RFLPs because they are twice as informative and can be typed at at least twice the rate. The facioscapulohumeral muscular dystrophy gene was recently mapped in just 6 weeks using microsatellite polymorphisms. Because of the informativeness handicap, it will be difficult for base substitution polymorphisms to overtake tandem repeat markers for large-scale linkage mapping. Methods that allow base substitution polymorphisms to be typed at two or three times the rate of microsatellite markers would have to be developed. Most of the other applications of DNA polymorphisms described in the introduction are also increasingly likely to rely on highly informative tandem repeat markers in the future. Methods for analysis will probably be based on PCR. It is easy to envisage, for example, an automated method for large-scale DNA fingerprinting of individuals based upon a standard set of highly informative, dependable microsatellite polymorphisms. Methods for analyzing base substitution polymorphisms will continue to be important for the diagnostic detection of disease-gene alleles.(ABSTRACT TRUNCATED AT 250 WORDS)     

A new approach to genome mapping and sequencing: slalom libraries

We describe here an efficient strategy for simultaneous genome mapping and sequencing. The approach is based on physically oriented, overlapping restriction fragment libraries called slalom libraries. Slalom libraries combine features of general genomic, jumping and linking libraries. Slalom libraries can be adapted to different applications and two main types of slalom libraries are described in detail. This approach was used to map and sequence (with ~46% coverage) two human P1-derived artificial chromosome (PAC) clones, each of ~100 kb. This model experiment demonstrates the feasibility of the approach and shows that the efficiency (cost-effectiveness and speed) of existing mapping/sequencing methods could be improved at least 5–10-fold. Furthermore, since the efficiency of contig assembly in the slalom approach is virtually independent of length of sequence reads, even short sequences produced by rapid, high throughput sequencing techniques would suffice to complete a physical map and a sequence scan of a small genome.     

Whole-genome experimental identification of insertion/deletion polymorphisms of interspersed repeats by a new general approach

A new experimental technique for genome-wide detection of integration sites of polymorphic retroelements (REs) is described. The technique allows one to reveal the absence of a retroelement in an individual genome provided that this retroelement is present in at least one of several other genomes under comparison. Since quite a number of genomes are compared simultaneously, the search for polymorphic REs insertions is very efficient. The technique includes two whole-genome selective PCR amplifications of sequences flanking REs: one for a particular genome and another one for a mixture of ten different genomes. A subsequent subtractive hybridization of the obtained amplicons with DNA of a particular genome as driver results in isolation of polymorphic insertions. The technique was successfully applied for identification of 41 new polymorphic human AluYa5/Ya8 insertions. Among them, 18 individual Alu elements first sequenced in this work were not found in the available human genome databases. This result suggests that significant part of polymorphic REs were not identified during genome sequencing and remain to be detected and characterized. The proposed method does not depend on preliminary knowledge of evolutionary history of retroelements and can be applied for identification of insertion/deletion polymorphic markers in genomes of different species.     

RAD-seq技术在基因组研究中的现状及展望

Restriction-site associated DNA sequencing(RAD-seq)技术是在二代测序基础上发展起来的一项基于全基因组酶切位点的简化基因组测序技术。该方法技术流程简单, 不受有无参考基因组的限制, 可大大简化基因组的复杂性, 减少实验费用, 通过一次测序就可以获得数以万计的多态性标记。目前, RAD-seq技术已成功应用于超高密度遗传图谱的构建、重要性状的精细定位、辅助基因组序列组装、群体基因组学以及系统发生学等基因组研究热点领域。文章主要介绍了RAD-seq的技术原理、技术发展及其在基因组研究中的广泛应用。鉴于RAD-seq方法的独特性, 该技术必将在复杂基因组研究领域具有广泛的应用前景。     

New opportunities revealed by biotechnological explorations of extremophiles

Over the past few decades the extremes at which life thrives has continued to challenge our understanding of biochemistry, biology and evolution. As more new extremophiles are brought into laboratory culture, they have provided a multitude of potential applications for biotechnology. More recently, innovative culturing approaches, environmental genome sequencing and whole genome sequencing have provided new opportunities for the biotechnological exploration of extremophiles.     

Single-molecule detection as an approach to rapid DNA sequencing.

Current sequencing technologies are insufficient to cope with large-scale projects such as sequencing the human genome and genomes of model organisms. In addition, as genetic lesions associated with specific human diseases are identified, DNA sequencing will be used increasingly for clinical applications. Thus, new approaches are needed to combine high-throughput with accuracy for both research and diagnostic purposes. A novel technology based on detection of individual fluorescent nucleotides in a flowing sample stream is under development.     

Current trends in mapping human genes

The human is estimated to have at least 50,000 expressed genes (gene loci). Some information is available concerning about 5000 of these gene loci and about 1900 have been mapped, i.e., assigned to specific chromosomes (and in most instances particular chromosome regions). Progress has been achieved by a combination of physical mapping (e.g., study of somatic cell hybrids and chromosomal in situ hybridization) and genetic mapping (e.g., genetic linkage studies). New methods for both physical and genetic mapping are expanding the armamentarium. The usefulness of the mapping information is already evident; the spin-off from the Human Genome Project (HGP) begins immediately. The complete nucleotide sequence is the ultimate map of the human genome. Sequencing, although already under way for limited segments of the genome, will await further progress in gene mapping, and in particular creation of contig maps for each chromosome. Meanwhile the technology of sequencing and sequence information handling will be developed. It is argued that the HGP is a new form of coordinated, interdisciplinary science; that its primary objective must be seen as the creation of a tool for biomedical research--a source book that will be the basis of study of variation and function for a long time; that the impact on scientist training will be salutary by relieving graduate students of useless drudgery and by training scientists competent in both molecular genetics and computational science; and that the funding of the HGP will have an insignificant negative effect on science funding generally, and indeed may have a beneficial effect through economy of scale and a focusing of attention on the excitement of biology and medical science.     

Current status and future possibilities of molecular genetics techniques in <Emphasis Type="Italic">Brassica napus</Emphasis>

As PCR methods have improved over the last 15 years, there has been an upsurge in the number of new DNA marker tools, which has allowed the generation of high-density molecular maps for all the key Brassica crop types. Biotechnology and molecular plant breeding have emerged as a significant tool for molecular understanding that led to a significant crop improvement in the Brassica napus species. Brassica napus possess a very complicated polyploidy-based genomics. The quantitative trait locus (QTL) is not sufficient to develop effective markers for trait introgression. In the coming years, the molecular marker techniques will be more effective to determine the whole genome impairing desired traits. Available genetic markers using the single-nucleotide sequence (SNP) technique and high-throughput sequencing are effective in determining the maps and genome polymorphisms amongst candidate genes and allele interactions. High-throughput sequencing and gene mapping techniques are involved in discovering new alleles and gene pairs, serving as a bridge between the gene map and genome evaluation. The decreasing cost for DNA sequencing will help in discovering full genome sequences with less resources and time. This review describes (1) the current use of integrated approaches, such as molecular marker technologies, to determine genome arrangements and interspecific outcomes combined with cost-effective genomes to increase the efficiency in prognostic breeding efforts. (2) It also focused on functional genomics, proteomics and field-based breeding practices to achieve insight into the genetics underlying both simple and complex traits in canola.     

Automated sequencing of complete mitochondrial genomes from laser-capture microdissected samples

Mitochondrial DNA mutations have been related to both aging and a variety of diseases such as cancer. Due to the relatively small size of the genome (16 kb) and with the use of automated DNA sequencing, the entire genome can be sequenced from clinical specimens in days. We present a reliable approach to complete mitochondrial genome sequencing from laser-capture microdissected human clinical cancer specimens that overcome the inherent limitations of relatively small tissue samples and partial DNA degradation, which are unavoidable when laser-capture microdissection is used to attain pure populations of cells from heterogeneous tissues obtained from surgical procedures. The acquisition of sufficient template combined with a standard set of 18 pairs of PCR primers allows for the efficient amplification of the genome. Subsequent single-stranded amplification is performed using 36 sequencing primers, and samples are run on an ABI PRISM 3100 Genetic Analyzer. The use of this procedure should allow even investigators with little experience sequencing from clinical specimens success in complete mitochondrial genome sequencing.     

PCR amplification of chromosome-specific alpha satellite DNA: definition of centromeric STS markers and polymorphic analysis.

Alpha satellite DNA is a tandemly repetitive DNA family found at the centromere of every human chromosome. Chromosome-specific subsets have been isolated for over half the chromosomes and have prove useful as markers for both genetic and physical mapping. We have developed specific oligonucleotide primer sets for polymerase chain reaction (PCR) amplification of alpha satellite DNA from chromosomes 3, 7, 13/21, 17, X, and Y. For each set of primers, PCR products amplified from human genomic DNA are specific for the centromere of the target chromosome(s), as shown by somatic cell hybrid mapping and by fluorescence in situ hybridization. These six subsets represent several evolutionarily related alpha satellite subfamilies, suggesting that specific primer pairs can be designed for most or all chromosomal subsets in the genome. The PCR products from chromosome 17 directly reveal the polymorphic nature of this subset, and a new DraI polymorphism is described. The PCR products from chromosome 13 are also polymorphic, allowing in informative cases genetic analysis of this centromeric subset distinguished from the highly homologous chromosome 21 subset. These primer sets should allow placement of individual centromeres on the proposed STS map of the human genome and may be useful for somatic cell hybrid characterization and for making in situ probes. In addition, the ability to amplify chromosome-specific repetitive DNA families directly will contribute to the structural and functional analysis of these abundant classes of DNA.     

An Anchored Framework BAC Map of Mouse Chromosome 11 Assembled Using Multiplex Oligonucleotide Hybridization

Despite abundant library resources for many organisms, physical mapping of these organisms has been seriously limited due to lack of efficient library screening techniques. We have developed a highly efficient strategy for large-scale screening of genomic libraries based on multiplex oligonucleotide hybridization on high-density genomic filters. We have applied this strategy to generate a bacterial artificial chromosome (BAC) anchored map of mouse chromosome 11. Using the MIT mouse SSLP data, 320 pairs of oligonucleotide probes were designed with an “overgo” computer program that selects new primer sequences that avoid the microsatellite repeat. BACs identified by these probes are automatically anchored to the chromosome. Ninety-two percent of the probes identified positive clones from a 5.9-fold coverage mouse BAC library with an average of 7 positive clones per marker. An average of 4.2 clones was confirmed for 204 markers by PCR. Our data show that a large number of clones can be efficiently isolated from a large genomic library using this strategy with minimal effort. This strategy will have wide application for large-scale mapping and sequencing of human and other large genomes.     

Multiplex restriction amplicon sequencing: a novel next‐generation sequencing‐based marker platform for high‐throughput genotyping

To enable rapid selection of traits in marker‐assisted breeding, markers must be technically simple, low‐cost, high‐throughput and randomly distributed in a genome. We developed such a technology, designated as Multiplex Restriction Amplicon Sequencing (MRASeq), which reduces genome complexity by polymerase chain reaction (PCR) amplification of amplicons flanked by restriction sites. The first PCR primers contain restriction site sequences at 3’‐ends, preceded by 6‐10 bases of specific or degenerate nucleotide sequences and then by a unique M13‐tail sequence which serves as a binding site for a second PCR that adds sequencing primers and barcodes to allow sample multiplexing for sequencing. The sequences of restriction sites and adjacent nucleotides can be altered to suit different species. Physical mapping of MRASeq SNPs from a biparental population of allohexaploid wheat (Triticum aestivum L.) showed a random distribution of SNPs across the genome. MRASeq generated thousands of SNPs from a wheat biparental population and natural populations of wheat and barley (Hordeum vulgare L.). This novel, next‐generation sequencing‐based genotyping platform can be used for linkage mapping to screen quantitative trait loci (QTL), background selection in breeding and many other genetics and breeding applications of various species.     

Bacterial Genomes: Habitat Specificity and Uncharted Organisms

The capability and speed in generating genomic data have increased profoundly since the release of the draft human genome in 2000. Additionally, sequencing costs have continued to plummet as the next generation of highly efficient sequencing technologies (next-generation sequencing) became available and commercial facilities promote market competition. However, new challenges have emerged as researchers attempt to efficiently process the massive amounts of sequence data being generated. First, the described genome sequences are unequally distributed among the branches of bacterial life and, second, bacterial pan-genomes are often not considered when setting aims for sequencing projects. Here, we propose that scientists should be concerned with attaining an improved equal representation of most of the bacterial tree of life organisms, at the genomic level. Moreover, they should take into account the natural variation that is often observed within bacterial species and the role of the often changing surrounding environment and natural selection pressures, which is central to bacterial speciation and genome evolution. Not only will such efforts contribute to our overall understanding of the microbial diversity extant in ecosystems as well as the structuring of the extant genomes, but they will also facilitate the development of better methods for (meta)genome annotation.     

Generation of Radiation-Induced Deletion Complexes in the Mouse Genome Using Embryonic Stem Cells

As the genetic and physical mapping stage of the Human Genome Project nears completion, the focus is shifting toward the development of technologies for high-throughput analysis of gene function. Whereas DNA sequencing will enable the assignment of presumed function to a large number of genes in mice and humans, it is clear that the great majority of genes will have to be evaluatedin vivoto accurately assess their role in a complex organism. While gene targeting in mouse embryonic stem (ES) cells is the current method of choice for the characterization of gene function in mice, it remains relatively labor intensive and lacks the throughput required for analysis of genome function on a large scale. Alternative methods of efficient mutagenesis will clearly be required for this task. Chromosomal deletions are powerful tools in the genetic analysis of complex genomes, enabling the systematic identification and localization of functional units along defined chromosomal regions. Not only are deletions useful for the identification of genetic functions, but they serve as mapping reagents for existing mutations or traits. While their use has been an essential tool inDrosophilagenetics, classical mutagenesis in mice has been logistically impractical for generating deletions. We have previously described an efficient method for generating radiation-induced deletion complexes at defined regions in the genome using ES cells. In this article, we detail the methodological aspects of this technology and describe the applications of chromosomal deletions for characterizing gene function in ways that make optimal use of the information generated by the first stage of the Genome Project.