A BAC pooling strategy combined with PCR-based screenings in a large, highly repetitive genome enables integration of the maize genetic and physical maps

BAC-FISH Based Physical Map of Endangered Catfish Clarias magur for Chromosome Cataloguing and Gene Isolation through Positional Cloning

Construction of a physical chromosome map of a species is important for positional cloning, targeted marker development, fine mapping of genes, selection of candidate genes for molecular breeding, as well as understanding the genome organization. The genomic libraries in the form of bacterial artificial chromosome (BAC) clones are also a very useful resource for physical mapping and identification and isolation of full-length genes and the related cis acting elements. Some BAC-FISH based studies reported in the past were gene based physical chromosome maps of Clarias magur (magur) to understand the genome organization of the species and to establish the relationships with other species in respect to genes' organization and evolution in the past. In the present study, we generated end sequences of the BAC clones and analyzed those end sequences within the scaffolds of the draft genome of magur to identify and map the genes bioinformatically for each clone. A total of 36 clones mostly possessing genes were identified and used in probe construction and their subsequent hybridization on the metaphase chromosomes of magur. This study successfully mapped all 36 specific clones on 16 chromosome pairs, out of 25 pairs of magur chromosomes. These clones are now recognized as chromosome-specific makers, which are an aid in individual chromosome identification and fine assembly of the genome sequence, and will ultimately help in developing anchored genes' map on the chromosomes of C. magur for understanding their organization, inheritance of important fishery traits and evolution of magur with respect to channel catfish, zebrafish and other species.

Up-regulation of GhTT2-3A in cotton fibres during secondary wall thickening results in brown fibres with improved quality

Brown cotton fibres are the most widely used naturally coloured raw materials for the eco-friendly textile industry. Previous studies have indicated that brown fibre pigments belong to proanthocyanidins (PAs) or their derivatives, and fibre coloration is negatively associated with cotton productivity and fibre quality. To date, the molecular basis controlling the biosynthesis and accumulation of brown pigments in cotton fibres is largely unknown. In this study, based on expressional and transgenic analyses of cotton homologs of ArabidopsisPA regulator TRANSPARENT TESTA 2 (TT2) and fine-mapping of the cotton dark-brown fibre gene (Lc1), we show that a TT2 homolog, GhTT2-3A, controls PA biosynthesis and brown pigmentation in cotton fibres. We observed that GhTT2-3A activated GhbHLH130D, a homolog of ArabidopsisTT8, which in turn synergistically acted with GhTT2-3A to activate downstream PA structural genes and PA synthesis and accumulation in cotton fibres. Furthermore, the up-regulation of GhTT2-3A in fibres at the secondary wall-thickening stage resulted in brown mature fibres, and fibre quality and lint percentage were comparable to that of the white-fibre control. The findings of this study reveal the regulatory mechanism controlling brown pigmentation in cotton fibres and demonstrate a promising biotechnological strategy to break the negative linkage between coloration and fibre quality and/or productivity.

Effective BAC clone anchoring with genotyping-by-sequencing and Diversity Arrays Technology in a large genome cereal rye

Identification of bacterial artificial chromosome (BAC) clones containing specific sequences is a prerequisite for many applications, such as physical map anchoring or gene cloning. Existing BAC library screening strategies are either low-throughput or require a considerable initial input of resources for platform establishment. We describe a high-throughput, reliable, and cost-effective BAC library screening approach deploying genotyping platforms which are independent from the availability of sequence information: a genotyping-by-sequencing (GBS) method DArTSeq and the microarray-based Diversity Arrays Technology (DArT). The performance of these methods was tested in a very large and complex rye genome. The DArTseq approach delivered superior results: a several fold higher efficiency of addressing genetic markers to BAC clones and anchoring of BAC clones to genetic map and also a higher reliability. Considering the sequence independence of the platform, the DArTseq-based library screening can be proposed as an attractive method to speed up genomics research in resource poor species.

Construction and characterization of three wheat bacterial artificial chromosome libraries

We have constructed three bacterial artificial chromosome (BAC) libraries of wheat cultivar Triticum aestivum Wangshuibai, germplasms T. monococcum TA2026 and TA2033. A total of 1,233,792,170,880 and 263,040 clones were picked and arrayed in 384-well plates. On the basis of genome sizes of 16.8 Gb for hexaploid wheat and 5.6 Gb for diploid wheat, the three libraries represented 9.05-, 2.60-, and 3.71-fold coverage of the haploid genomes, respectively. An improved descending pooling system for BAC libraries screening was established. This improved strategy can save 80% of the time and 68% of polymerase chain reaction (PCR) with the same successful rate as the universal 6D pooling strategy.

Consolidation of the genetic and cytogenetic maps of turbot (Scophthalmus maximus) using FISH with BAC clones

Bacterial artificial chromosomes (BAC) have been widely used for fluorescence in situ hybridization (FISH) mapping of chromosome landmarks in different organisms, including a few in teleosts. In this study, we used BAC-FISH to consolidate the previous genetic and cytogenetic maps of the turbot (Scophthalmus maximus), a commercially important pleuronectiform. The maps consisted of 24 linkage groups (LGs) but only 22 chromosomes. All turbot LGs were assigned to specific chromosomes using BAC probes obtained from a turbot 5× genomic BAC library. It consisted of 46,080 clones with inserts of at least 100 kb and <5 % empty vectors. These BAC probes contained gene-derived or anonymous markers, most of them linked to quantitative trait loci (QTL) related to productive traits. BAC clones were mapped by FISH to unique marker-specific chromosomal positions, which showed a notable concordance with previous genetic mapping data. The two metacentric pairs were cytogenetically assigned to LG2 and LG16, and the nucleolar organizer region (NOR)-bearing pair was assigned to LG15. Double-color FISH assays enabled the consolidation of the turbot genetic map into 22 linkage groups by merging LG8 with LG18 and LG21 with LG24. In this work, a first-generation probe panel of BAC clones anchored to the turbot linkage and cytogenetical map was developed. It is a useful tool for chromosome traceability in turbot, but also relevant in the context of pleuronectiform karyotypes, which often show small hardly identifiable chromosomes. This panel will also be valuable for further integrative genomics of turbot within Pleuronectiformes and teleosts, especially for fine QTL mapping for aquaculture traits, comparative genomics, and whole-genome assembly.

The barley Frost resistance-H2 locus

Frost resistance-H2 (Fr-H2) is a major QTL affecting freezing tolerance in barley, yet its molecular basis is still not clearly understood. To gain a better insight into the structural characterization of the locus, a high-resolution linkage map developed from the Nure × Tremois cross was initially implemented to map 13 loci which divided the 0.602 cM total genetic distance into ten recombination segments. A PCR-based screening was then applied to identify positive bacterial artificial chromosome (BAC) clones from two genomic libraries of the reference genotype Morex. Twenty-six overlapping BACs from the integrated physical-genetic map were 454 sequenced. Reads assembled in contigs were subsequently ordered, aligned and manually curated in 42 scaffolds. In a total of 1.47 Mbp, 58 protein-coding sequences were identified, 33 of which classified according to similarity with sequences in public databases. As three complete barley C-repeat Binding Factors (HvCBF) genes were newly identified, the locus contained13 full-length HvCBFs, four Related to AP2 Triticeae (RAPT) genes, and at least five CBF pseudogenes. The final overall assembly of Fr-H2 includes more than 90 % of target region: all genes were identified along the locus, and a general survey of Repetitive Elements obtained. We believe that this gold-standard sequence for the Morex Fr-H2 will be a useful genomic tool for structural and evolutionary comparisons with Fr-H2 in winter-hardy cultivars along with Fr-2 of other Triticeae crops.

Global transcriptome analysis and identification of a CONSTANS-like gene family in the orchid Erycina pusilla

The high chromosome numbers, polyploid genomes, and long juvenile phases of most ornamental orchid species render functional genomics difficult and limit the discovery of genes influencing horticultural traits. The orchid Erycina pusilla has a low chromosome number (2n = 12) and flowers in vitro within 1 year, making it a standout candidate for use as a model orchid. However, transcriptomic and genomic information from E. pusilla remains limited. In this study, next-generation sequencing (NGS) technology was used to identify 90,668 unigenes by de novo assembly. These unigenes were annotated functionally and analyzed with regard to their gene ontology (GO), clusters of orthologous groups (COG), and KEGG pathways. To validate the discovery methods, a homolog of CONSTANS (CO), one of the key genes in the flowering pathway, was further analyzed. The Arabidopsis CO-Like (COL) amino acid sequences were used to screen for homologs in the E. pusilla transcriptome database. Specific primers to the homologous unigenes were then used to isolate BAC clones, which were sequenced to identify 12 E. pusilla CO-like (EpCOL) full-length genes. Based on sequence homology, domain structure, and phylogenetic analysis, these EpCOL genes were divided into four groups. Four EpCOLs fused with GFP were localized in the nucleus. Some EpCOL genes were regulated by light. These results demonstrate that nascent E. pusilla resources (transcriptome and BAC library) can be used to investigate the E. pusilla photoperiod-dependent flowering genes. In future, this strategy can be applied to other biological processes, marketable traits, and molecular breeding in this model orchid.

Construction and characterization of a BAC library from the Coffea arabica genotype Timor Hybrid CIFC 832/2

Most of the world's coffee production originates from Coffea arabica, an allotetraploid species with low genetic diversity and for which few genomic resources are available. Genomic libraries with large DNA fragment inserts are useful tools for the study of plant genomes, including the production of physical maps, integration studies of physical and genetic maps, genome structure analysis and gene isolation by positional cloning. Here, we report the construction and characterization of a Bacterial Artificial Chromosome (BAC) library from C. arabica Timor Hybrid CIFC 832/2, a parental genotype for several modern coffee cultivars. The BAC library consists of 56,832 clones with an average insert size of 118 kb, which represents a dihaploid genome coverage of five to sixfold. The content of organellar DNA was estimated at 1.04 and 0.5 % for chloroplast and mitochondrial DNA, respectively. The BAC library was screened for the NADPH-dependent mannose-6-phosphate reductase gene (CaM6PR) with markers positioned on four linkage groups of a partial C. arabica genetic map. A mixed approach using PCR and membrane hybridization of BAC pools allowed for the discovery of nine BAC clones with the CaM6PR gene and 53 BAC clones that were anchored to the genetic map with simple sequence repeat markers. This library will be a useful tool for future studies on comparative genomics and the identification of genes and regulatory elements controlling major traits in this economically important crop species.

Identification and characterization of a homologue to the Arabidopsis INDEHISCENT gene in common Bean

Reduction in pod shattering represents a key component of the domestication syndrome in common bean (Phaseolus vulgaris) and makes this domesticate dependent upon the farmer for seed dispersal. Attempts to elucidate the genetic control of this process have led to the identification of a major gene (St) linked to the presence/absence of pod suture fibers affecting pod shattering. Although St has been placed on the common bean genetic map, the sequence and the specific functions of this gene remain unknown. The purpose of this study was to identify a candidate gene for St. In Arabidopsis thaliana, INDEHISCENT gene (IND) is the primary factory required for silique shattering. A sequence homologous to IND was successfully amplified in P. vulgaris and placed on the common bean map using two recombinant inbred populations (BAT93 × Jalo EEP558; Midas × G12873). Although PvIND maps near the St locus, the lack of complete cosegregation between PvIND and St and the lack of polymorphisms at the PvIND locus correlated with the dehiscent/indehiscent phenotype suggests that PvIND may not be directly involved in pod shattering and may not be the gene underlying the St locus. However, PvIND may be closely linked to an as yet unidentified regulatory element at the St locus. Alternatively, a more precise phenotyping method taking into account quantitative trait variation needs to be developed to more accurately map the St locus.

Second-generation genetic linkage map of catfish and its integration with the BAC-based physical map

Construction of high-density genetic linkage maps is crucially important for quantitative trait loci (QTL) studies, and they are more useful when integrated with physical maps. Such integrated maps are valuable genome resources for fine mapping of QTL, comparative genomics, and accurate and efficient whole-genome assembly. Previously, we established both linkage maps and a physical map for channel catfish, Ictalurus punctatus, the dominant aquaculture species in the United States. Here we added 2030 BAC end sequence (BES)-derived microsatellites from 1481 physical map contigs, as well as markers from singleton BES, ESTs, anonymous microsatellites, and SNPs, to construct a second-generation linkage map. Average marker density across the 29 linkage groups reached 1.4 cM/marker. The increased marker density highlighted variations in recombination rates within and among catfish chromosomes. This work effectively anchored 44.8% of the catfish BAC physical map contigs, covering ∼52.8% of the genome. The genome size was estimated to be 2546 cM on the linkage map, and the calculated physical distance per centimorgan was 393 Kb. This integrated map should enable comparative studies with teleost model species as well as provide a framework for ordering and assembling whole-genome scaffolds.

Distribution of genes and repetitive elements in the Diabrotica virgifera virgifera genome estimated using BAC sequencing

Feeding damage caused by the western corn rootworm, Diabrotica virgifera virgifera, is destructive to corn plants in North America and Europe where control remains challenging due to evolution of resistance to chemical and transgenic toxins. A BAC library, DvvBAC1, containing 109,486 clones with 104 ± 34.5 kb inserts was created, which has an ~4.56X genome coverage based upon a 2.58 Gb (2.80 pg) flow cytometry-estimated haploid genome size. Paired end sequencing of 1037 BAC inserts produced 1.17 Mb of data (~0.05% genome coverage) and indicated ~9.4 and 16.0% of reads encode, respectively, endogenous genes and transposable elements (TEs). Sequencing genes within BAC full inserts demonstrated that TE densities are high within intergenic and intron regions and contribute to the increased gene size. Comparison of homologous genome regions cloned within different BAC clones indicated that TE movement may cause haplotype variation within the inbred strain. The data presented here indicate that the D. virgifera virgifera genome is large in size and contains a high proportion of repetitive sequence. These BAC sequencing methods that are applicable for characterization of genomes prior to sequencing may likely be valuable resources for genome annotation as well as scaffolding.

Utilization of super BAC pools and Fluidigm access array platform for high-throughput BAC clone identification: proof of concept

Bacterial artificial chromosome (BAC) libraries are critical for identifying full-length genomic sequences, correlating genetic and physical maps, and comparative genomics. Here we describe the utilization of the Fluidigm access array genotyping system in conjunction with KASPar genotyping technology to identify individual BAC clones corresponding to specific single-nucleotide polymorphisms (SNPs) from an Amplicon Express seven-plate super pooled Amaranthus hypochondriacus BAC library. Ninety-six SNP loci, spanning the length of A. hypochondriacus linkage groups 1, 2, and 15, were simultaneously tested for clone identification from four BAC super pools, corresponding to 28 384-well plates, using a single Fluidigm integrated fluidic chip (IFC). Forty-six percent of the SNPs were associated with a single unambiguous identified BAC clone. PCR amplification and next-generation sequencing of individual BAC clones confirmed the IFC clone identification. Utilization of the Fluidigm Dynamic array platform allowed for the simultaneous PCR screening of 10,752 BAC pools for 96 SNP tag sites in less than three hours at a cost of ~$0.05 per reaction.

Advances in BAC-based physical mapping and map integration strategies in plants

In the advent of next-generation sequencing (NGS) platforms, map-based sequencing strategy has been recently suppressed being too expensive and laborious. The detailed studies on NGS drafts alone indicated these assemblies remain far from gold standard reference quality, especially when applied on complex genomes. In this context the conventional BAC-based physical mapping has been identified as an important intermediate layer in current hybrid sequencing strategy. BAC-based physical map construction and its integration with high-density genetic maps have benefited from NGS and high-throughput array platforms. This paper addresses the current advancements of BAC-based physical mapping and high-throughput map integration strategies to obtain densely anchored well-ordered physical maps. The resulted maps are of immediate utility while providing a template to harness the maximum benefits of the current NGS platforms.

Integration of molecular biology tools for identifying promoters and genes abundantly expressed in flowers of Oncidium Gower Ramsey

BACKGROUND

Orchids comprise one of the largest families of flowering plants and generate commercially important flowers. However, model plants, such as Arabidopsis thaliana do not contain all plant genes, and agronomic and horticulturally important genera and species must be individually studied.

RESULTS

Several molecular biology tools were used to isolate flower-specific gene promoters from Oncidium 'Gower Ramsey' (Onc. GR). A cDNA library of reproductive tissues was used to construct a microarray in order to compare gene expression in flowers and leaves. Five genes were highly expressed in flower tissues, and the subcellular locations of the corresponding proteins were identified using lip transient transformation with fluorescent protein-fusion constructs. BAC clones of the 5 genes, together with 7 previously published flower- and reproductive growth-specific genes in Onc. GR, were identified for cloning of their promoter regions. Interestingly, 3 of the 5 novel flower-abundant genes were putative trypsin inhibitor (TI) genes (OnTI1, OnTI2 and OnTI3), which were tandemly duplicated in the same BAC clone. Their promoters were identified using transient GUS reporter gene transformation and stable A. thaliana transformation analyses.

CONCLUSIONS

By combining cDNA microarray, BAC library, and bombardment assay techniques, we successfully identified flower-directed orchid genes and promoters.

A new implementation of high-throughput five-dimensional clone pooling strategy for BAC library screening

Background

A five-dimensional (5-D) clone pooling strategy for screening of bacterial artificial chromosome (BAC) clones with molecular markers utilizing highly-parallel Illumina GoldenGate assays and PCR facilitates high-throughput BAC clone and BAC contig anchoring on a genetic map. However, this strategy occasionally needs manual PCR to deconvolute pools and identify truly positive clones.

Results

A new implementation is reported here for our previously reported clone pooling strategy. Row and column pools of BAC clones are divided into sub-pools with 1~2× genome coverage. All BAC pools are screened with Illumina's GoldenGate assay and the BAC pools are deconvoluted to identify individual positive clones. Putative positive BAC clones are then further analyzed to find positive clones on the basis of them being neighbours in a contig. An exhaustive search or brute force algorithm was designed for this deconvolution and integrated into a newly developed software tool, FPCBrowser, for analyzing clone pooling data. This algorithm was used with empirical data for 55 Illumina GoldenGate SNP assays detecting SNP markers mapped on Aegilops tauschii chromosome 2D and Ae. tauschii contig maps. Clones in single contigs were successfully assigned to 48 (87%) specific SNP markers on the map with 91% precision.

Conclusion

A new implementation of 5-D BAC clone pooling strategy employing both GoldenGate assay screening and assembled BAC contigs is shown here to be a high-throughput, low cost, rapid, and feasible approach to screening BAC libraries and anchoring BAC clones and contigs on genetic maps. The software FPCBrowser with the integrated clone deconvolution algorithm has been developed and is downloadable at http://avena.pw.usda.gov/wheatD/fpcbrowser.shtml.

A Comparative BAC map for the gilthead sea bream (Sparus aurata L.)

This study presents the first comparative BAC map of the gilthead sea bream (Sparus aurata), a highly valuated marine aquaculture fish species in the Mediterranean. High-throughput end sequencing of a BAC library yielded 92,468 reads (60.6 Mbp). Comparative mapping was achieved by anchoring BAC end sequences to the three-spined stickleback (Gasterosteus aculeatus) genome. BACs that were consistently ordered along the stickleback chromosomes accounted for 14,265 clones. A fraction of 5,249 BACs constituted a minimal tiling path that covers 73.5% of the stickleback chromosomes and 70.2% of the genes that have been annotated. The N50 size of 1,485 “BACtigs” consisting of redundant BACs is 337,253 bp. The largest BACtig covers 2.15 Mbp in the stickleback genome. According to the insert size distribution of mapped BACs the sea bream genome is 1.71-fold larger than the stickleback genome. These results represent a valuable tool to researchers in the field and may support future projects to elucidate the whole sea bream genome.

Sex chromosome evolution in amniotes: applications for bacterial artificial chromosome libraries

Variability among sex chromosome pairs in amniotes denotes a dynamic history. Since amniotes diverged from a common ancestor, their sex chromosome pairs and, more broadly, sex-determining mechanisms have changed reversibly and frequently. These changes have been studied and characterized through the use of many tools and experimental approaches but perhaps most effectively through applications for bacterial artificial chromosome (BAC) libraries. Individual BAC clones carry 100–200 kb of sequence from one individual of a target species that can be isolated by screening, mapped onto karyotypes, and sequenced. With these techniques, researchers have identified differences and similarities in sex chromosome content and organization across amniotes and have addressed hypotheses regarding the frequency and direction of past changes. Here, we review studies of sex chromosome evolution in amniotes and the ways in which the field of research has been affected by the advent of BAC libraries.

A computer simulator for assessing different challenges and strategies of de novo sequence assembly

This study presents a new computer program for assessing the effects of different factors and sequencing strategies on de novo sequence assembly. The program uses reads from actual sequencing studies or from simulations with a reference genome that may also be real or simulated. The simulated reads can be created with our read simulator. They can be of differing length and coverage, consist of paired reads with varying distance, and include sequencing errors such as color space miscalls to imitate SOLiD data. The simulated or real reads are mapped to their reference genome and our assembly simulator is then used to obtain optimal assemblies that are limited only by the distribution of repeats. By way of this mapping, the assembly simulator determines which contigs are theoretically possible, or conversely (and perhaps more importantly), which are not. We illustrate the application and utility of our new simulation tools with several experiments that test the effects of genome complexity (repeats), read length and coverage, word size in De Bruijn graph assembly, and alternative sequencing strategies (e.g., BAC pooling) on sequence assemblies. These experiments highlight just some of the uses of our simulators in the experimental design of sequencing projects and in the further development of assembly algorithms.

Generation of a BAC-based physical map of the melon genome

BACKGROUND

Cucumis melo (melon) belongs to the Cucurbitaceae family, whose economic importance among horticulture crops is second only to Solanaceae. Melon has high intra-specific genetic variation, morphologic diversity and a small genome size (450 Mb), which make this species suitable for a great variety of molecular and genetic studies that can lead to the development of tools for breeding varieties of the species. A number of genetic and genomic resources have already been developed, such as several genetic maps and BAC genomic libraries. These tools are essential for the construction of a physical map, a valuable resource for map-based cloning, comparative genomics and assembly of whole genome sequencing data. However, no physical map of any Cucurbitaceae has yet been developed. A project has recently been started to sequence the complete melon genome following a whole-genome shotgun strategy, which makes use of massive sequencing data. A BAC-based melon physical map will be a useful tool to help assemble and refine the draft genome data that is being produced.

RESULTS

A melon physical map was constructed using a 5.7 x BAC library and a genetic map previously developed in our laboratories. High-information-content fingerprinting (HICF) was carried out on 23,040 BAC clones, digesting with five restriction enzymes and SNaPshot labeling, followed by contig assembly with FPC software. The physical map has 1,355 contigs and 441 singletons, with an estimated physical length of 407 Mb (0.9 x coverage of the genome) and the longest contig being 3.2 Mb. The anchoring of 845 BAC clones to 178 genetic markers (100 RFLPs, 76 SNPs and 2 SSRs) also allowed the genetic positioning of 183 physical map contigs/singletons, representing 55 Mb (12%) of the melon genome, to individual chromosomal loci. The melon FPC database is available for download at http://melonomics.upv.es/static/files/public/physical_map/.

CONCLUSIONS

Here we report the construction of the first physical map of a Cucurbitaceae species described so far. The physical map was integrated with the genetic map so that a number of physical contigs, representing 12% of the melon genome, could be anchored to known genetic positions. The data presented is already helping to improve the quality of the melon genomic sequence available as a result of a project currently being carried out in Spain, adopting a whole genome shotgun approach based on 454 sequencing data.

A simple, high throughput method to locate single copy sequences from Bacterial Artificial Chromosome (BAC) libraries using High Resolution Melt analysis

BACKGROUND

The high-throughput anchoring of genetic markers into contigs is required for many ongoing physical mapping projects. Multidimentional BAC pooling strategies for PCR-based screening of large insert libraries is a widely used alternative to high density filter hybridisation of bacterial colonies. To date, concerns over reliability have led most if not all groups engaged in high throughput physical mapping projects to favour BAC DNA isolation prior to amplification by conventional PCR.

RESULTS

Here, we report the first combined use of Multiplex Tandem PCR (MT-PCR) and High Resolution Melt (HRM) analysis on bacterial stocks of BAC library superpools as a means of rapidly anchoring markers to BAC colonies and thereby to integrate genetic and physical maps. We exemplify the approach using a BAC library of the model plant Arabidopsis thaliana. Super pools of twenty five 384-well plates and two-dimension matrix pools of the BAC library were prepared for marker screening. The entire procedure only requires around 3 h to anchor one marker.

CONCLUSIONS

A pre-amplification step during MT-PCR allows high multiplexing and increases the sensitivity and reliability of subsequent HRM discrimination. This simple gel-free protocol is more reliable, faster and far less costly than conventional PCR screening. The option to screen in parallel 3 genetic markers in one MT-PCR-HRM reaction using templates from directly pooled bacterial stocks of BAC-containing bacteria further reduces time for anchoring markers in physical maps of species with large genomes.

Construction of a BAC library and a physical map of a major QTL for CBB resistance of common bean (Phaseolus vulgaris L.)

A major quantitative trait loci (QTL) conditioning common bacterial blight (CBB) resistance in common bean (Phaseolus vulgaris L.) lines HR45 and HR67 was derived from XAN159, a resistant line obtained from an interspecific cross between common bean lines and the tepary bean (P. acutifolius L.) line PI319443. This source of CBB resistance is widely used in bean breeding. Several other CBB resistance QTL have been identified but none of them have been physically mapped. Four molecular markers tightly linked to this QTL have been identified suitable for marker assisted selection and physical mapping of the resistance gene. A bacterial artificial chromosome (BAC) library was constructed from high molecular weight DNA of HR45 and is composed of 33,024 clones. The size of individual BAC clone inserts ranges from 30 kb to 280 kb with an average size of 107 kb. The library is estimated to represent approximately sixfold genome coverage. The BAC library was screened as BAC pools using four PCR-based molecular markers. Two to seven BAC clones were identified by each marker. Two clones were found to have both markers PV-tttc001 and STS183. One preliminary contig was assembled based on DNA finger printing of those positive BAC clones. The minimum tiling path of the contig contains 6 BAC clones spanning an estimated size of 750 kb covering the QTL region.

Rapid identification of the three homoeologues of the wheat dwarfing gene Rht using a novel PCR-based screen of three-dimensional BAC pools

A high-throughput two-step PCR strategy for the identification of selected genes from a BAC library derived from hexaploid wheat (16,974 Mbp) is described. The screen is based on the pooling of DNA from BAC clones into 675 "superpools" arrayed in a three-dimensional configuration. Each BAC clone is represented in three superpools to allow the identification of candidate 384-well plates of clones after the first round of PCR; identification is facilitated by an associated Perl script. A second round of PCR detects the specific BAC clone within the candidate plate that corresponds to the gene of interest. Thus, a single copy of the target gene can be identified from the library of over 700,000 clones (approximately 5 genome equivalents) by assaying only three 384-well plates. The pooling strategy was validated by screening the library with primers specific for the reduced height (Rht-1a) gene. Using relatively stringent selection criteria, 13 Rht-containing clones were identified from 17 candidate plates, and sequence analysis of the amplified products showed that all three Rht homoeologues were represented. Furthermore, the method confirmed the estimated coverage of the BAC library. Thus, this methodology allows the rapid and cost-effective identification of genes, and their homoeologues, from large-insert libraries of complex genomes such as hexaploid wheat.

Feasibility of physical map construction from fingerprinted bacterial artificial chromosome libraries of polyploid plant species

BACKGROUND

The presence of closely related genomes in polyploid species makes the assembly of total genomic sequence from shotgun sequence reads produced by the current sequencing platforms exceedingly difficult, if not impossible. Genomes of polyploid species could be sequenced following the ordered-clone sequencing approach employing contigs of bacterial artificial chromosome (BAC) clones and BAC-based physical maps. Although BAC contigs can currently be constructed for virtually any diploid organism with the SNaPshot high-information-content-fingerprinting (HICF) technology, it is currently unknown if this is also true for polyploid species. It is possible that BAC clones from orthologous regions of homoeologous chromosomes would share numerous restriction fragments and be therefore included into common contigs. Because of this and other concerns, physical mapping utilizing the SNaPshot HICF of BAC libraries of polyploid species has not been pursued and the possibility of doing so has not been assessed. The sole exception has been in common wheat, an allohexaploid in which it is possible to construct single-chromosome or single-chromosome-arm BAC libraries from DNA of flow-sorted chromosomes and bypass the obstacles created by polyploidy.

RESULTS

The potential of the SNaPshot HICF technology for physical mapping of polyploid plants utilizing global BAC libraries was evaluated by assembling contigs of fingerprinted clones in an in silico merged BAC library composed of single-chromosome libraries of two wheat homoeologous chromosome arms, 3AS and 3DS, and complete chromosome 3B. Because the chromosome arm origin of each clone was known, it was possible to estimate the fidelity of contig assembly. On average 97.78% or more clones, depending on the library, were from a single chromosome arm. A large portion of the remaining clones was shown to be library contamination from other chromosomes, a feature that is unavoidable during the construction of single-chromosome BAC libraries.

CONCLUSIONS

The negligibly low level of incorporation of clones from homoeologous chromosome arms into a contig during contig assembly suggested that it is feasible to construct contigs and physical maps using global BAC libraries of wheat and almost certainly also of other plant polyploid species with genome sizes comparable to that of wheat. Because of the high purity of the resulting assembled contigs, they can be directly used for genome sequencing. It is currently unknown but possible that equally good BAC contigs can be also constructed for polyploid species containing smaller, more gene-rich genomes.

Repetitive genome elements in a European corn borer, Ostrinia nubilalis, bacterial artificial chromosome library were indicated by bacterial artificial chromosome end sequencing and development of sequence tag site markers: implications for lepidopteran genomic research

The European corn borer, Ostrinia nubilalis, is a serious pest of food, fiber, and biofuel crops in Europe, North America, and Asia and a model system for insect olfaction and speciation. A bacterial artificial chromosome library constructed for O. nubilalis contains 36 864 clones with an estimated average insert size of >or=120 kb and genome coverage of 8.8-fold. Screening OnB1 clones comprising approximately 2.76 genome equivalents determined the physical position of 24 sequence tag site markers, including markers linked to ecologically important and Bacillus thuringiensis toxin resistance traits. OnB1 bacterial artificial chromosome end sequence reads (GenBank dbGSS accessions ET217010 to ET217273) showed homology to annotated genes or expressed sequence tags and identified repetitive genome elements, O. nubilalis miniature subterminal inverted repeat transposable elements (OnMITE01 and OnMITE02), and ezi-like long interspersed nuclear elements. Mobility of OnMITE01 was demonstrated by the presence or absence in O. nubilalis of introns at two different loci. A (GTCT)n tetranucleotide repeat at the 5' ends of OnMITE01 and OnMITE02 are evidence for transposon-mediated movement of lepidopteran microsatellite loci. The number of repetitive elements in lepidopteran genomes will affect genome assembly and marker development. Single-locus sequence tag site markers described here have downstream application for integration within linkage maps and comparative genomic studies.

A novel manual pooling system for preparing three-dimensional pools of a deep coverage soybean bacterial artificial chromosome library

Compared with hybridization-based techniques, polymerase chain reaction-based screening of large insert libraries has been used widely as it is fast, easy and sensitive. However, various pooling strategies are needed to ensure efficient screening. It is time-consuming and labourious to prepare three-dimensional pools for a deep coverage bacterial artificial chromosome (BAC) library of soybean (1.12 × 10(9)  bp) in the absence of robotic facility. In the present study, we describe a novel manual pooling system for preparing three-dimensional pools of a soybean BAC library. This simple technique enables a single researcher to construct three-dimensional pools for a deep-coverage (12 haploid genome equivalents) BAC library of soybean in less than 2 months without any robotic manipulation. When the prepared three-dimensional pools were screened with 29 polymerase chain reaction-based markers, an average of 9.2 clones per marker were identified. These identified clones will be useful either in quantitative trait loci gene isolation or in synteny study between soybean and other legumes including Lotus japonicus. This efficient pooling system could be applied to any other BAC libraries without the need for robotic manipulation.

Physical and genetic structure of the maize genome reflects its complex evolutionary history

Maize (Zea mays L.) is one of the most important cereal crops and a model for the study of genetics, evolution, and domestication. To better understand maize genome organization and to build a framework for genome sequencing, we constructed a sequence-ready fingerprinted contig-based physical map that covers 93.5% of the genome, of which 86.1% is aligned to the genetic map. The fingerprinted contig map contains 25,908 genic markers that enabled us to align nearly 73% of the anchored maize genome to the rice genome. The distribution pattern of expressed sequence tags correlates to that of recombination. In collinear regions, 1 kb in rice corresponds to an average of 3.2 kb in maize, yet maize has a 6-fold genome size expansion. This can be explained by the fact that most rice regions correspond to two regions in maize as a result of its recent polyploid origin. Inversions account for the majority of chromosome structural variations during subsequent maize diploidization. We also find clear evidence of ancient genome duplication predating the divergence of the progenitors of maize and rice. Reconstructing the paleoethnobotany of the maize genome indicates that the progenitors of modern maize contained ten chromosomes.

As a cash crop and a model biological system, maize is of great public interest. To facilitate maize molecular breeding and its basic biology research, we built a high-resolution physical map with two different fingerprinting methods on the same set of bacterial artificial chromosome clones. The physical map was integrated to a high-density genetic map and further serves as a framework for the maize genome-sequencing project. Comparative genomics showed that the euchromatic regions between rice and maize are very conserved. Physically we delimited these conserved regions and thus detected many genome rearrangements. We defined extensively the duplication blocks within the maize genome. These blocks allowed us to reconstruct the chromosomes of the maize progenitor. We detected that maize genome has experienced two rounds of genome duplications, an ancient one before maize–rice divergence and a recent one after tetraploidization.

Meiotic genes and proteins in cereals

We review the current status of our understanding and knowledge of the genes and proteins controlling meiosis in five major cereals, rye, wheat, barley, rice and maize. For each crop, we describe the genetic and genomic infrastructure available to investigators, before considering the inventory of genes and proteins that have roles to play in this process. Emphasis is given throughout as to how translational genomic and proteomic approaches have enabled us to circumvent some of the intractable features of this important group of plants.

Genetic marker anchoring by six-dimensional pools for development of a soybean physical map

BACKGROUND

Integrated genetic and physical maps are extremely valuable for genomic studies and as important references for assembling whole genome shotgun sequences. Screening of a BAC library using molecular markers is an indispensable procedure for integration of both physical and genetic maps of a genome. Molecular markers provide anchor points for integration of genetic and physical maps and also validate BAC contigs assembled based solely on BAC fingerprints. We employed a six-dimensional BAC pooling strategy and an in silico approach to anchor molecular markers onto the soybean physical map.

RESULTS

A total of 1,470 markers (580 SSRs and 890 STSs) were anchored by PCR on a subset of a Williams 82 BstY I BAC library pooled into 208 pools in six dimensions. This resulted in 7,463 clones (approximately 1x genome equivalent) associated with 1470 markers, of which the majority of clones (6,157, 82.5%) were anchored by one marker and 1106 (17.5%) individual clones contained two or more markers. This contributed to 1184 contigs having anchor points through this 6-D pool screening effort. In parallel, the 21,700 soybean Unigene set from NCBI was used to perform in silico mapping on 80,700 Williams 82 BAC end sequences (BES). This in silico analysis yielded 9,835 positive results anchored by 4152 unigenes that contributed to 1305 contigs and 1624 singletons. Among the 1305 contigs, 305 have not been previously anchored by PCR. Therefore, 1489 (78.8%) of 1893 contigs are anchored with molecular markers. These results are being integrated with BAC fingerprints to assemble the BAC contigs. Ultimately, these efforts will lead to an integrated physical and genetic map resource.

CONCLUSION

We demonstrated that the six-dimensional soybean BAC pools can be efficiently used to anchor markers to soybean BACs despite the complexity of the soybean genome. In addition to anchoring markers, the 6-D pooling method was also effective for targeting BAC clones for investigating gene families and duplicated regions in the genome, as well as for extending physical map contigs.

Physical mapping in large genomes: accelerating anchoring of BAC contigs to genetic maps through in silico analysis

Anchored physical maps represent essential frameworks for map-based cloning, comparative genomics studies, and genome sequencing projects. High throughput anchoring can be achieved by polymerase chain reaction (PCR) screening of bacterial artificial chromosome (BAC) library pools with molecular markers. However, for large genomes such as wheat, the development of high dimension pools and the number of reactions that need to be performed can be extremely large making the screening laborious and costly. To improve the cost efficiency of anchoring in such large genomes, we have developed a new software named Elephant (electronic physical map anchoring tool) that combines BAC contig information generated by FingerPrinted Contig with results of BAC library pools screening to identify BAC addresses with a minimal amount of PCR reactions. Elephant was evaluated during the construction of a physical map of chromosome 3B of hexaploid wheat. Results show that a one dimensional pool screening can be sufficient to anchor a BAC contig while reducing the number of PCR by 384-fold thereby demonstrating that Elephant is an efficient and cost-effective tool to support physical mapping in large genomes.

A transgenomic cytogenetic sorghum (Sorghum propinquum) bacterial artificial chromosome fluorescence in situ hybridization map of maize (Zea mays L.) pachytene chromosome 9, evidence for regions of genome hyperexpansion

A cytogenetic FISH map of maize pachytene-stage chromosome 9 was produced with 32 maize marker-selected sorghum BACs as probes. The genetically mapped markers used are distributed along the linkage maps at an average spacing of 5 cM. Each locus was mapped by means of multicolor direct FISH with a fluorescently labeled probe mix containing a whole-chromosome paint, a single sorghum BAC clone, and the centromeric sequence, CentC. A maize-chromosome-addition line of oat was used for bright unambiguous identification of the maize 9 fiber within pachytene chromosome spreads. The locations of the sorghum BAC-FISH signals were determined, and each new cytogenetic locus was assigned a centiMcClintock position on the short (9S) or long (9L) arm. Nearly all of the markers appeared in the same order on linkage and cytogenetic maps but at different relative positions on the two. The CentC FISH signal was localized between cdo17 (at 9L.03) and tda66 (at 9S.03). Several regions of genome hyperexpansion on maize chromosome 9 were found by comparative analysis of relative marker spacing in maize and sorghum. This transgenomic cytogenetic FISH map creates anchors between various maps of maize and sorghum and creates additional tools and information for understanding the structure and evolution of the maize genome.