Radiation hybrid mapping: a somatic cell genetic method for constructing high-resolution maps of mammalian chromosomes

Humanising and dehumanising pigs in genomic and transplantation research

Biologists who work on the pig (Sus scrofa) take advantage of its similarity to humans by constructing the inferential and material means to traffic data, information and knowledge across the species barrier. Their research has been funded due to its perceived value for agriculture and medicine. Improving selective breeding practices, for instance, has been a driver of genomics research. The pig is also an animal model for biomedical research and practice, and is proposed as a source of organs for cross-species transplantation: xenotransplantation. Genomics research has informed transplantation biology, which has itself motivated developments in genomics. Both have generated models of correspondences between the genomes of pigs and humans. Concerning genomics, I detail how researchers traverse species boundaries to develop representations of the pig genome, alongside ensuring that such representations are sufficiently porcine. In transplantation biology, the representations of the genomes of humans and pigs are used to detect and investigate immunologically-pertinent differences between the two species. These key differences can then be removed, to 'humanise' donor pigs so that they can become a safe and effective source of organs. In both of these endeavours, there is a tension between practices that 'humanise' the pig (or representations thereof) through using resources from human genomics, and the need to 'dehumanise' the pig to maintain distinctions for legal, ethical and scientific reasons. This paper assesses the ways in which this tension has been managed, observing the differences between its realisations across comparative pig genomics and transplantation biology, and considering the consequences of this.

Genetic studies of human-chimpanzee divergence using stem cell fusions

Complete genome sequencing has identified millions of DNA changes that differ between humans and chimpanzees. Although a subset of these changes likely underlies important phenotypic differences between humans and chimpanzees, it is currently difficult to distinguish causal from incidental changes and to map specific phenotypes to particular genome locations. To facilitate further genetic study of human-chimpanzee divergence, we have generated human and chimpanzee autotetraploids and allotetraploids by fusing induced pluripotent stem cells (iPSCs) of each species. The resulting tetraploid iPSCs can be stably maintained and retain the ability to differentiate along ectoderm, mesoderm, and endoderm lineages. RNA sequencing identifies thousands of genes whose expression differs between humans and chimpanzees when assessed in single-species diploid or autotetraploid iPSCs. Analysis of gene expression patterns in interspecific allotetraploid iPSCs shows that human-chimpanzee expression differences arise from substantial contributions of both cis-acting changes linked to the genes themselves and trans-acting changes elsewhere in the genome. To enable further genetic mapping of species differences, we tested chemical treatments for stimulating genome-wide mitotic recombination between human and chimpanzee chromosomes, and CRISPR methods for inducing species-specific changes on particular chromosomes in allotetraploid cells. We successfully generated derivative cells with nested deletions or interspecific recombination on the X chromosome. These studies confirm an important role for the X chromosome in trans regulation of expression differences between species and illustrate the potential of this system for more detailed cis and trans mapping of the molecular basis of human and chimpanzee evolution.

Cost-Effective Mapping of Genetic Interactions in Mammalian Cells

Comprehensive maps of genetic interactions in mammalian cells are daunting to construct because of the large number of potential interactions, ~ 2 × 10⁸ for protein coding genes. We previously used co-inheritance of distant genes from published radiation hybrid (RH) datasets to identify genetic interactions. However, it was necessary to combine six legacy datasets from four species to obtain adequate statistical power. Mapping resolution was also limited by the low density PCR genotyping. Here, we employ shallow sequencing of nascent human RH clones as an economical approach to constructing interaction maps. In this initial study, 15 clones were analyzed, enabling construction of a network with 225 genes and 2,359 interactions (FDR < 0.05). Despite its small size, the network showed significant overlap with the previous RH network and with a protein-protein interaction network. Consumables were ≲$50 per clone, showing that affordable, high quality genetic interaction maps are feasible in mammalian cells.

Pooled analysis of radiation hybrids identifies loci for growth and drug action in mammalian cells

Genetic screens in mammalian cells commonly focus on loss-of-function approaches. To evaluate the phenotypic consequences of extra gene copies, we used bulk segregant analysis (BSA) of radiation hybrid (RH) cells. We constructed six pools of RH cells, each consisting of ∼2500 independent clones, and placed the pools under selection in media with or without paclitaxel. Low pass sequencing identified 859 growth loci, 38 paclitaxel loci, 62 interaction loci, and three loci for mitochondrial abundance at genome-wide significance. Resolution was measured as ∼30 kb, close to single-gene. Divergent properties were displayed by the RH-BSA growth genes compared to those from loss-of-function screens, refuting the balance hypothesis. In addition, enhanced retention of human centromeres in the RH pools suggests a new approach to functional dissection of these chromosomal elements. Pooled analysis of RH cells showed high power and resolution and should be a useful addition to the mammalian genetic toolkit.

New Approaches for Genome Assembly and Scaffolding

Affordable, high-throughput DNA sequencing has accelerated the pace of genome assembly over the past decade. Genome assemblies from high-throughput, short-read sequencing, however, are often not as contiguous as the first generation of genome assemblies. Whereas early genome assembly projects were often aided by clone maps or other mapping data, many current assembly projects forego these scaffolding data and only assemble genomes into smaller segments. Recently, new technologies have been invented that allow chromosome-scale assembly at a lower cost and faster speed than traditional methods. Here, we give an overview of the problem of chromosome-scale assembly and traditional methods for tackling this problem. We then review new technologies for chromosome-scale assembly and recent genome projects that used these technologies to create highly contiguous genome assemblies at low cost.

Weird Animals, Sex, and Genome Evolution

Making my career in Australia exposed me to the tyranny of distance, but it gave me opportunities to study our unique native fauna. Distantly related animal species present genetic variation that we can use to explore the most fundamental biological structures and processes. I have compared chromosomes and genomes of kangaroos and platypus, tiger snakes and emus, devils (Tasmanian) and dragons (lizards). I particularly love the challenges posed by sex chromosomes, which, apart from determining sex, provide stunning examples of epigenetic control and break all the evolutionary rules that we currently understand. Here I describe some of those amazing animals and the insights on genome structure, function, and evolution they have afforded us. I also describe my sometimes-random walk in science and the factors and people who influenced my direction. Being a woman in science is still not easy, and I hope others will find encouragement and empathy in my story.

Whole-Genome Restriction Mapping by "Subhaploid"-Based RAD Sequencing: An Efficient and Flexible Approach for Physical Mapping and Genome Scaffolding

Assembly of complex genomes using short reads remains a major challenge, which usually yields highly fragmented assemblies. Generation of ultradense linkage maps is promising for anchoring such assemblies, but traditional linkage mapping methods are hindered by the infrequency and unevenness of meiotic recombination that limit attainable map resolution. Here we develop a sequencing-based "in vitro" linkage mapping approach (called RadMap), where chromosome breakage and segregation are realized by generating hundreds of "subhaploid" fosmid/bacterial-artificial-chromosome clone pools, and by restriction site-associated DNA sequencing of these clone pools to produce an ultradense whole-genome restriction map to facilitate genome scaffolding. A bootstrap-based minimum spanning tree algorithm is developed for grouping and ordering of genome-wide markers and is implemented in a user-friendly, integrated software package (AMMO). We perform extensive analyses to validate the power and accuracy of our approach in the model plant Arabidopsis thaliana and human. We also demonstrate the utility of RadMap for enhancing the contiguity of a variety of whole-genome shotgun assemblies generated using either short Illumina reads (300 bp) or long PacBio reads (6-14 kb), with up to 15-fold improvement of N50 (∼816 kb-3.7 Mb) and high scaffolding accuracy (98.1-98.5%). RadMap outperforms BioNano and Hi-C when input assembly is highly fragmented (contig N50 = 54 kb). RadMap can capture wide-range contiguity information and provide an efficient and flexible tool for high-resolution physical mapping and scaffolding of highly fragmented assemblies.

A High Resolution Radiation Hybrid Map of Wheat Chromosome 4A

Bread wheat has a large and complex allohexaploid genome with low recombination level at chromosome centromeric and peri-centromeric regions. This significantly hampers ordering of markers, contigs of physical maps and sequence scaffolds and impedes obtaining of high-quality reference genome sequence. Here we report on the construction of high-density and high-resolution radiation hybrid (RH) map of chromosome 4A supported by high-density chromosome deletion map. A total of 119 endosperm-based RH lines of two RH panels and 15 chromosome deletion bin lines were genotyped with 90K iSelect single nucleotide polymorphism (SNP) array. A total of 2316 and 2695 markers were successfully mapped to the 4A RH and deletion maps, respectively. The chromosome deletion map was ordered in 19 bins and allowed precise identification of centromeric region and verification of the RH panel reliability. The 4A-specific RH map comprises 1080 mapping bins and spans 6550.9 cR with a resolution of 0.13 Mb/cR. Significantly higher mapping resolution in the centromeric region was observed as compared to recombination maps. Relatively even distribution of deletion frequency along the chromosome in the RH panel was observed and putative functional centromere was delimited within a region characterized by two SNP markers.

A whole-genome, radiation hybrid mapping resource of hexaploid wheat

Generating a contiguous, ordered reference sequence of a complex genome such as hexaploid wheat (2n = 6x = 42; approximately 17 GB) is a challenging task due to its large, highly repetitive, and allopolyploid genome. In wheat, ordering of whole-genome or hierarchical shotgun sequencing contigs is primarily based on recombination and comparative genomics-based approaches. However, comparative genomics approaches are limited to syntenic inference and recombination is suppressed within the pericentromeric regions of wheat chromosomes, thus, precise ordering of physical maps and sequenced contigs across the whole-genome using these approaches is nearly impossible. We developed a whole-genome radiation hybrid (WGRH) resource and tested it by genotyping a set of 115 randomly selected lines on a high-density single nucleotide polymorphism (SNP) array. At the whole-genome level, 26 299 SNP markers were mapped on the RH panel and provided an average mapping resolution of approximately 248 Kb/cR1500 with a total map length of 6866 cR1500 . The 7296 unique mapping bins provided a five- to eight-fold higher resolution than genetic maps used in similar studies. Most strikingly, the RH map had uniform bin resolution across the entire chromosome(s), including pericentromeric regions. Our research provides a valuable and low-cost resource for anchoring and ordering sequenced BAC and next generation sequencing (NGS) contigs. The WGRH developed for reference wheat line Chinese Spring (CS-WGRH), will be useful for anchoring and ordering sequenced BAC and NGS based contigs for assembling a high-quality, reference sequence of hexaploid wheat. Additionally, this study provides an excellent model for developing similar resources for other polyploid species.

A new physical mapping approach refines the sex-determining gene positions on the Silene latifolia Y-chromosome

Sex chromosomes are particularly interesting regions of the genome for both molecular genetics and evolutionary studies; yet, for most species, we lack basic information, such as the gene order along the chromosome. Because they lack recombination, Y-linked genes cannot be mapped genetically, leaving physical mapping as the only option for establishing the extent of synteny and homology with the X chromosome. Here, we developed a novel and general method for deletion mapping of non-recombining regions by solving “the travelling salesman problem”, and evaluate its accuracy using simulated datasets. Unlike the existing radiation hybrid approach, this method allows us to combine deletion mutants from different experiments and sources. We applied our method to a set of newly generated deletion mutants in the dioecious plant Silene latifolia and refined the locations of the sex-determining loci on its Y chromosome map.

Formation of Extrachromosomal Circular DNA from Long Terminal Repeats of Retrotransposons in Saccharomyces cerevisiae

Extrachromosomal circular DNA (eccDNA) derived from chromosomal Ty retrotransposons in yeast can be generated in multiple ways. Ty eccDNA can arise from the circularization of extrachromosomal linear DNA during the transpositional life cycle of retrotransposons, or from circularization of genomic Ty DNA. Circularization may happen through nonhomologous end-joining (NHEJ) of long terminal repeats (LTRs) flanking Ty elements, by Ty autointegration, or by LTR–LTR recombination. By performing an in-depth investigation of sequence reads stemming from Ty eccDNAs obtained from populations of Saccharomyces cerevisiae S288c, we find that eccDNAs predominantly correspond to full-length Ty1 elements. Analyses of sequence junctions reveal no signs of NHEJ or autointegration events. We detect recombination junctions that are consistent with yeast Ty eccDNAs being generated through recombination events within the genome. This opens the possibility that retrotransposable elements could move around in the genome without an RNA intermediate directly through DNA circularization.

Radiation Hybrid Map of Barley Chromosome 3H

Assembly of the barley (Hordeum vulgare L.) genome is complicated by its large size (5.1 Gb) and proportion of repetitive elements (84%). This process is facilitated by high resolution maps for aligning bacterial artificial chromosome (BAC) contigs along chromosomes. Available genetic maps, however, do not provide accurate information on the physical position of a large portion of the genome located in recombination-poor regions. Radiation hybrid (RH) mapping is an alternative approach, which is based on radiation-induced deletions along the length of chromosomes. In this study, the first RH map for barley chromosome 3H was developed. In total, 373 in vivo RH lines were generated by irradiating wheat (Triticum aestivum L.)-barley chromosome 3H addition lines and crossing them to a normal wheat cultivar. Each RH informative line (containing deletions) had, on average, three deletions. The induced deletion size varied from 36.58 Kb to 576.00 Mb, with an average length of 52.42 Mb. This initial chromosome 3H radiation hybrid (3H-RH) map had a 9.53× higher resolution than an analogous genetic map, reaching a maximum of >262.40× resolution in regions around the centromere. The final RH map was 3066.1 cR in length, with a 0.76 Mb resolution. It was estimated that the map resolution can be improved to an average of 30.34 Kb by saturating the 3H-RH map with molecular markers. The generated RH panel enabled alignment of BAC and sequenced contigs as small as 1.50 Kb in size. The high resolution and the coverage of poor-recombination regions make RH maps an ideal resource for barley genome assembly, as well as other genetic studies.

The Genome 10K Project: a way forward

The Genome 10K Project was established in 2009 by a consortium of biologists and genome scientists determined to facilitate the sequencing and analysis of the complete genomes of 10,000 vertebrate species. Since then the number of selected and initiated species has risen from ∼26 to 277 sequenced or ongoing with funding, an approximately tenfold increase in five years. Here we summarize the advances and commitments that have occurred by mid-2014 and outline the achievements and present challenges of reaching the 10,000-species goal. We summarize the status of known vertebrate genome projects, recommend standards for pronouncing a genome as sequenced or completed, and provide our present and future vision of the landscape of Genome 10K. The endeavor is ambitious, bold, expensive, and uncertain, but together the Genome 10K Consortium of Scientists and the worldwide genomics community are moving toward their goal of delivering to the coming generation the gift of genome empowerment for many vertebrate species.

Radiation hybrid QTL mapping of Tdes2 involved in the first meiotic division of wheat

Since the dawn of wheat cytogenetics, chromosome 3B has been known to harbor a gene(s) that, when removed, causes chromosome desynapsis and gametic sterility. The lack of natural genetic diversity for this gene(s) has prevented any attempt to fine map and further characterize it. Here, gamma radiation treatment was used to create artificial diversity for this locus. A total of 696 radiation hybrid lines were genotyped with a custom mini array of 140 DArT markers, selected to evenly span the whole 3B chromosome. The resulting map spanned 2,852 centi Ray with a calculated resolution of 0.384 Mb. Phenotyping for the occurrence of meiotic desynapsis was conducted by measuring the level of gametic sterility as seeds produced per spikelet and pollen viability at booting. Composite interval mapping revealed a single QTL with LOD of 16.2 and r (2) of 25.6 % between markers wmc326 and wPt-8983 on the long arm of chromosome 3B. By independent analysis, the location of the QTL was confirmed to be within the deletion bin 3BL7-0.63-1.00 and to correspond to a single gene located ~1.4 Mb away from wPt-8983. The meiotic behavior of lines lacking this gene was characterized cytogenetically to reveal striking similarities with mutants for the dy locus, located on the syntenic chromosome 3 of maize. This represents the first example to date of employing radiation hybrids for QTL analysis. The success achieved by this approach provides an ideal starting point for the final cloning of this interesting gene involved in meiosis of cereals.

Endosperm tolerance of paternal aneuploidy allows radiation hybrid mapping of the wheat D-genome and a measure of γ ray-induced chromosome breaks

Physical mapping and genome sequencing are underway for the ≈17 Gb wheat genome. Physical mapping methods independent of meiotic recombination, such as radiation hybrid (RH) mapping, will aid precise anchoring of BAC contigs in the large regions of suppressed recombination in Triticeae genomes. Reports of endosperm development following pollination with irradiated pollen at dosages that cause embryo abortion prompted us to investigate endosperm as a potential source of RH mapping germplasm. Here, we report a novel approach to construct RH based physical maps of all seven D-genome chromosomes of the hexaploid wheat ‘Chinese Spring’, simultaneously. An 81-member subset of endosperm samples derived from 20-Gy irradiated pollen was genotyped for deletions, and 737 markers were mapped on seven D-genome chromosomes. Analysis of well-defined regions of six chromosomes suggested a map resolution of ∼830 kb could be achieved; this estimate was validated with assays of markers from a sequenced contig. We estimate that the panel contains ∼6,000 deletion bins for D-genome chromosomes and will require ∼18,000 markers for high resolution mapping. Map-based deletion estimates revealed a majority of 1–20 Mb interstitial deletions suggesting mutagenic repair of double-strand breaks in pollen provides a useful resource for RH mapping and map based cloning studies.

First steps: bovine genomics in historical perspective

The vision of Morris Soller was instrumental in launching the field of bovine genomics. This study is a review of the early years of bovine gene mapping leading up to the sequencing and assembly of the bovine genome in 2009. A historical perspective of parasexual, linkage and physical mapping is provided with a focus on the contribution of these maps to the eventual assignment and orientation of genes and sequence to cattle chromosomes.

DNA repair and crossing over favor similar chromosome regions as discovered in radiation hybrid of Triticum

BACKGROUND

The uneven distribution of recombination across the length of chromosomes results in inaccurate estimates of genetic to physical distances. In wheat (Triticum aestivum L.) chromosome 3B, it has been estimated that 90% of the cross over events occur in distal sub-telomeric regions representing 40% of the chromosome. Radiation hybrid (RH) mapping which does not rely on recombination is a strategy to map genomes and has been widely employed in animal species and more recently in some plants. RH maps have been proposed to provide i) higher and ii) more uniform resolution than genetic maps, and iii) to be independent of the distribution patterns observed for meiotic recombination. An in vivo RH panel was generated for mapping chromosome 3B of wheat in an attempt to provide a complete scaffold for this ~1 Gb segment of the genome and compare the resolution to previous genetic maps.

RESULTS

A high density RH map with 541 marker loci anchored to chromosome 3B spanning a total distance of 1871.9 cR was generated. Detailed comparisons with a genetic map of similar quality confirmed that i) the overall resolution of the RH map was 10.5 fold higher and ii) six fold more uniform. A significant interaction (r = 0.879 at p = 0.01) was observed between the DNA repair mechanism and the distribution of crossing-over events. This observation could be explained by accepting the possibility that the DNA repair mechanism in somatic cells is affected by the chromatin state in a way similar to the effect that chromatin state has on recombination frequencies in gametic cells.

CONCLUSIONS

The RH data presented here support for the first time in vivo the hypothesis of non-casual interaction between recombination hot-spots and DNA repair. Further, two major hypotheses are presented on how chromatin compactness could affect the DNA repair mechanism. Since the initial RH application 37 years ago, we were able to show for the first time that the iii) third hypothesis of RH mapping might not be entirely correct.

DNA repair and crossing over favor similar chromosome regions as discovered in radiation hybrid of Triticum

BACKGROUND

The uneven distribution of recombination across the length of chromosomes results in inaccurate estimates of genetic to physical distances. In wheat (Triticum aestivum L.) chromosome 3B, it has been estimated that 90% of the cross over events occur in distal sub-telomeric regions representing 40% of the chromosome. Radiation hybrid (RH) mapping which does not rely on recombination is a strategy to map genomes and has been widely employed in animal species and more recently in some plants. RH maps have been proposed to provide i) higher and ii) more uniform resolution than genetic maps, and iii) to be independent of the distribution patterns observed for meiotic recombination. An in vivo RH panel was generated for mapping chromosome 3B of wheat in an attempt to provide a complete scaffold for this ~1 Gb segment of the genome and compare the resolution to previous genetic maps.

RESULTS

A high density RH map with 541 marker loci anchored to chromosome 3B spanning a total distance of 1871.9 cR was generated. Detailed comparisons with a genetic map of similar quality confirmed that i) the overall resolution of the RH map was 10.5 fold higher and ii) six fold more uniform. A significant interaction (r = 0.879 at p = 0.01) was observed between the DNA repair mechanism and the distribution of crossing-over events. This observation could be explained by accepting the possibility that the DNA repair mechanism in somatic cells is affected by the chromatin state in a way similar to the effect that chromatin state has on recombination frequencies in gametic cells.

CONCLUSIONS

The RH data presented here support for the first time in vivo the hypothesis of non-casual interaction between recombination hot-spots and DNA repair. Further, two major hypotheses are presented on how chromatin compactness could affect the DNA repair mechanism. Since the initial RH application 37 years ago, we were able to show for the first time that the iii) third hypothesis of RH mapping might not be entirely correct.

Construction of whole genome radiation hybrid panels and map of chromosome 5A of wheat using asymmetric somatic hybridization

To explore the feasibility of constructing a whole genome radiation hybrid (WGRH) map in plant species with large genomes, asymmetric somatic hybridization between wheat (Triticum aestivum L.) and Bupleurum scorzonerifolium Willd. was performed. The protoplasts of wheat were irradiated with ultraviolet light (UV) and gamma-ray and rescued by protoplast fusion using B. scorzonerifolium as the recipient. Assessment of SSR markers showed that the radiation hybrids have the average marker retention frequency of 15.5%. Two RH panels (RHPWI and RHPWII) that contained 92 and 184 radiation hybrids, respectively, were developed and used for mapping of 68 SSR markers in chromosome 5A of wheat. A total of 1557 and 2034 breaks were detected in each panel. The RH map of chromosome 5A based on RHPWII was constructed. The distance of the comprehensive map was 2103 cR and the approximate resolution was estimated to be ∼501.6 kb/break. The RH panels evaluated in this study enabled us to order the ESTs in a single deletion bin or in the multiple bins cross the chromosome. These results demonstrated that RH mapping via protoplast fusion is feasible at the whole genome level for mapping purposes in wheat and the potential value of this mapping approach for the plant species with large genomes.

A high-resolution 15,000(Rad) radiation hybrid panel for the domestic cat

The current genetic and recombination maps of the cat have fewer than 3,000 markers and a resolution limit greater than 1 Mb. To complement the first-generation domestic cat maps, support higher resolution mapping studies, and aid genome assembly in specific areas as well as in the whole genome, a 15,000(Rad) radiation hybrid (RH) panel for the domestic cat was generated. Fibroblasts from the female Abyssinian cat that was used to generate the cat genomic sequence were fused to a Chinese hamster cell line (A23), producing 150 hybrid lines. The clones were initially characterized using 39 short tandem repeats (STRs) and 1,536 SNP markers. The utility of whole-genome amplification in preserving and extending RH panel DNA was also tested using 10 STR markers; no significant difference in retention was observed. The resolution of the 15,000(Rad) RH panel was established by constructing framework maps across 10 different 1-Mb regions on different feline chromosomes. In these regions, 2-point analysis was used to estimate RH distances, which compared favorably with the estimation of physical distances. The study demonstrates that the 15,000(Rad) RH panel constitutes a powerful tool for constructing high-resolution maps, having an average resolution of 40.1 kb per marker across the ten 1-Mb regions. In addition, the RH panel will complement existing genomic resources for the domestic cat, aid in the accurate re-assemblies of the forthcoming cat genomic sequence, and support cross-species genomic comparisons.

The complexity of finding multiple solutions to betweenness and quartet compatibility

We show that two important problems that have applications in computational biology are ASP-complete, which implies that, given a solution to a problem, it is NP-complete to decide if another solution exists. We show first that a variation of BETWEENNESS, which is the underlying problem of questions related to radiation hybrid mapping, is ASP-complete. Subsequently, we use that result to show that QUARTET COMPATIBILITY, a fundamental problem in phylogenetics that asks whether a set of quartets can be represented by a parent tree, is also ASP-complete. The latter result shows that Steel’s QUARTET CHALLENGE, which asks whether a solution to QUARTET COMPATIBILITY is unique, is coNP-complete.

Effects of genome-wide copy number variation on expression in mammalian cells

Background

There is only a limited understanding of the relation between copy number and expression for mammalian genes. We fine mapped cis and trans regulatory loci due to copy number change for essentially all genes using a human-hamster radiation hybrid (RH) panel. These loci are called copy number expression quantitative trait loci (ceQTLs).

Results

Unexpected findings from a previous study of a mouse-hamster RH panel were replicated. These findings included decreased expression as a result of increased copy number for 30% of genes and an attenuated relationship between expression and copy number on the X chromosome suggesting an Xist independent form of dosage compensation. In a separate glioblastoma dataset, we found conservation of genes in which dosage was negatively correlated with gene expression. These genes were enriched in signaling and receptor activities. The observation of attenuated X-linked gene expression in response to increased gene number was also replicated in the glioblastoma dataset. Of 523 gene deserts of size > 600 kb in the human RH panel, 325 contained trans ceQTLs with -log₁₀ P > 4.1. Recently discovered genes, ultra conserved regions, noncoding RNAs and microRNAs explained only a small fraction of the results, suggesting a substantial portion of gene deserts harbor as yet unidentified functional elements.

Conclusion

Radiation hybrids are a useful tool for high resolution mapping of cis and trans loci capable of affecting gene expression due to copy number change. Analysis of two independent radiation hybrid panels show agreement in their findings and may serve as a discovery source for novel regulatory loci in noncoding regions of the genome.

Status of the cattle genome map

During the last 30 years, the cattle genome map has been expanded from 4 genes linked on chromosome X to over 22,000 genes identified in the cattle genome sequence assembly. This progress has been achieved due to numerous projects on linkage and physical mapping of the cattle genome driven by its agricultural and scientific significance. Indeed, the high-resolution mapping and functional analysis of the genome led to the discovery of major quantitative trait loci (QTL) regions and several quantitative trait nucleotides (QTNs), as well as some disease genes in the cow population. In addition, a comparison of the cattle genome to the genomes of other mammals has revealed its unique features gained during the speciation and adaptation. With the development of non-expensive sequencing techniques, the analysis of the cattle genome will shift towards the identification of differences between breeds or individuals within breeds that account for the unique features of each breed. This approach holds promise for the development of effective tools for the marker assistant selection and disease diagnostics in cattle.

Enhancing radiation hybrid mapping through whole genome amplification

Radiation hybrid (RH) mapping is limited by the inherent genomic instability of RH clones entailing both, limited DNA sample amounts and genomic heterogeneity of the clones. Here the instability of RH clones is quantified and the suitability of the multiple strand displacement whole genome amplification method (WGA) for radiation hybrid mapping is assessed. To quantify the instability of RH clones, eleven clones of a 10,000(Rad) rhesus macaque radiation hybrid panel were passaged ten times and analyzed by interspersed repeat sequence specific quantitative PCR and by genotyping of 46 macaque chromosome 5 STS markers. The quantitative PCR data indicate an average loss of 55% of the donor DNA over 10 passages. Over the same period, a dropout of 46.2% of the STS markers was observed. These data indicate a genome wide half-life of the donor DNA of 8.7 passages and of 10.6 passages for the chromosome 5 markers. The genotyping data of the genomic RH DNA were compared to three sets of WGA experiments: 1) single wgaDNA amplifications, 2) six WGA replicates, and 3) re-amplification of wga DNA. The assays demonstrated concordance rates of 97.6%, 98% and 99.3%, respectively, and indicated the marker specificity of some repeated WGA dropouts. The study confirms that WGA is suitable for RH mapping studies should enable the accurate analysis of almost an infinite numbers of markers. WGA will allow the analysis of earliest RH clone passages, thus limiting their heterogeneity and RH mapping artifacts.

A Poisson model for random multigraphs

MOTIVATION

Biological networks are often modeled by random graphs. A better modeling vehicle is a multigraph where each pair of nodes is connected by a Poisson number of edges. In the current model, the mean number of edges equals the product of two propensities, one for each node. In this context it is possible to construct a simple and effective algorithm for rapid maximum likelihood estimation of all propensities. Given estimated propensities, it is then possible to test statistically for functionally connected nodes that show an excess of observed edges over expected edges. The model extends readily to directed multigraphs. Here, propensities are replaced by outgoing and incoming propensities.

RESULTS

The theory is applied to real data on neuronal connections, interacting genes in radiation hybrids, interacting proteins in a literature curated database, and letter and word pairs in seven Shaskespearean plays.

AVAILABILITY

All data used are fully available online from their respective sites. Source code and software is available from http://code.google.com/p/poisson-multigraph/.

An integrated RH map of porcine chromosome 10

BACKGROUND

Whole genome radiation hybrid (WG-RH) maps serve as "scaffolds" to significantly improve the orientation of small bacterial artificial chromosome (BAC) contigs, order genes within the contigs and assist assembly of a sequence-ready map for virtually any species. Here, we report the construction of a porcine: human comparative map for pig (Sus scrofa) chromosome 10 (SSC10) using the IMNpRH2(12,000-rad) porcine WG-RH panel, integrated with the IMpRH(7000-rad) WG-RH, genetic and BAC fingerprinted contig (FPC) maps.

RESULTS

Map vectors from the IMNpRH2(12,000-rad) and IMpRH(7,000-rad) panels were merged to construct parallel framework (FW) maps, within which FW markers common to both panels have an identical order. This strategy reduced map discrepancies between the two panels and significantly improved map accuracy. A total of 216 markers, including 50 microsatellites (MSs), 97 genes and ESTs, and 69 BAC end sequences (BESs), were ordered within two linkage groups at two point (2 pt) LOD score of 8. One linkage group covers SSC10p with accumulated map distances of 738.2 cR(7,000) and 1814.5 cR(12,000), respectively. The second group covers SSC10q at map distances of 1336.9 cR(7,000) and 3353.6 cR(12,000), yielding an overall average map resolution of 16.4 kb/cR(12,000) or 393.5 kb per marker on SSC10. This represents an approximately 2.5-fold increase in map resolution over the IMpRH(7,000-rad) panel. Based on 127 porcine markers that have homologous sequences in the human genome, a detailed comparative map between SSC10 and human (Homo sapiens) chromosome (HSA) 1, 9 and 10 was built.

CONCLUSION

This initial comparative RH map of SSC10 refines the syntenic regions between SSC10 and HSA1, 9 and 10. It integrates the IMNpRH2(12,000-rad) and IMpRH(7,000-rad), genetic and BAC FPC maps and provides a scaffold to close potential gaps between contigs prior to genome sequencing and assembly. This map is also useful in fine mapping of QTLs on SSC10.

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.

Application of dissociation curve analysis to radiation hybrid panel marker scoring: generation of a map of river buffalo (B. bubalis) chromosome 20

Background

Fluorescence of dyes bound to double-stranded PCR products has been utilized extensively in various real-time quantitative PCR applications, including post-amplification dissociation curve analysis, or differentiation of amplicon length or sequence composition. Despite the current era of whole-genome sequencing, mapping tools such as radiation hybrid DNA panels remain useful aids for sequence assembly, focused resequencing efforts, and for building physical maps of species that have not yet been sequenced. For placement of specific, individual genes or markers on a map, low-throughput methods remain commonplace. Typically, PCR amplification of DNA from each panel cell line is followed by gel electrophoresis and scoring of each clone for the presence or absence of PCR product. To improve sensitivity and efficiency of radiation hybrid panel analysis in comparison to gel-based methods, we adapted fluorescence-based real-time PCR and dissociation curve analysis for use as a novel scoring method.

Results

As proof of principle for this dissociation curve method, we generated new maps of river buffalo (Bubalus bubalis) chromosome 20 by both dissociation curve analysis and conventional marker scoring. We also obtained sequence data to augment dissociation curve results. Few genes have been previously mapped to buffalo chromosome 20, and sequence detail is limited, so 65 markers were screened from the orthologous chromosome of domestic cattle. Thirty bovine markers (46%) were suitable as cross-species markers for dissociation curve analysis in the buffalo radiation hybrid panel under a standard protocol, compared to 25 markers suitable for conventional typing. Computational analysis placed 27 markers on a chromosome map generated by the new method, while the gel-based approach produced only 20 mapped markers. Among 19 markers common to both maps, the marker order on the map was maintained perfectly.

Conclusion

Dissociation curve analysis is reliable and efficient for radiation hybrid panel scoring, and is more sensitive and robust than conventional gel-based typing methods. Several markers could be scored only by the new method, and ambiguous scores were reduced. PCR-based dissociation curve analysis decreases both time and resources needed for construction of radiation hybrid panel marker maps and represents a significant improvement over gel-based methods in any species.

Application of radiation hybrid in gene mapping

Radiation hybrid (RH) mapping technique was exploited to determine chromosome locations of 26 human novel full length cDNAs recently cloned. All these cDNA clones were isolated from human cord blood CD (+) (34) cells and may be related to regulation of hematopoiesis. 23 genes were successfully mapped to chromosomal positions, while RH analyses were not possible in the remaining 3 cases. RH technique is indeed a powerful tool for mapping novel cDNA sequences due to its rapidity, precision, convenience and reproducibility.

Mapping and expression analysis of chicken NDRG1 and NDRG3 genes

N-myc downstream-regulated genes 1 and 3 (NDRG1 and NDRG3) are members of the alpha/beta hydrolase superfamily. Phylogenetic analysis of the family demonstrated that human NDRG1 and 3 belong to a subfamily. The mapping and gene expression patterns of these genes represent one step toward further investigation into their possible roles in the chicken (Gallus gallus). To map these genes in the chicken chromosome, a 6000 rads chicken-hamster radiation hybrid panel (ChickRH6) was used. Primers were designed according to the published human sequences for amplification of those two genes. We compared the corresponding human mRNA sequences with the predicted coding sequences of the chicken NDRG1 and 3 genes and found that the assembled contigs shared a high percentage of similarity with the human genes. PCR of samples from ChickRH6 revealed that the locations of NDRG1 and 3 are linked to the markers MYC (58 cRs away, LOD score 4.52) and SEQ0265 (10 cRs away, LOD score 17.81), respectively. This result adds two new markers to the chicken RH map, and it reinforces that the RH technique is indeed a powerful tool for mapping genes due to its rapidity, precision, convenience, and reproducibility. In addition, we detected the gene expression and distribution of chicken NDRG1 and 3 in seven tissues, including heart, liver, spleen, lung, muscle, brain, and thymus, by RT-PCR, and found that NDRG1 is relatively ubiquitously expressed in all the tested tissues and highly expressed in heart and liver, whereas NDRG3 is high in heart, muscle, and brain.

A high-resolution comparative radiation hybrid map of ovine chromosomal regions that are homologous to human chromosome 6 (HSA6)

In this study, we constructed high-resolution radiation hybrid (RH) and comparative maps of ovine chromosomes or chromosomal segments that are homologous to human chromosome 6 (HSA6). A total of 251 markers were successfully genotyped across the recently developed USUoRH5000 whole-genome panel; 208 of these markers were assigned to five RH linkage groups distributed on three ovine chromosomes (OAR8, 9 and 20). The RH maps have good correspondence with previous chromosome painting data, although a small centromeric region on OAR9 that is homologous to HSA6 had not been previously detected using human chromosome paints on ovine chromosomal spreads. High percentages of the ovine markers were identified as orthologues in the bovine (86.3%), dog (85.8%), horse (69.3%) and human (88.7%) genomes. These maps contribute to investigations in mammalian chromosome evolution and the search for economic trait loci in sheep.

Overview of linkage analysis in complex traits

The goal of this unit is to instruct the reader on the general approaches now available for mapping complex disease. The term complex genetic disease refers to a trait phenotype with an unknown mode of inheritance. The goal of this unit is to instruct the reader on the general approaches now available for mapping complex disease.

Radiation hybrid (RH) and HAPPY mapping in plants

Radiation hybrid (RH) and HAPPY mapping are two technologies used in animal systems that have attracted the attention of the plant genetics community because they bridge the resolution gap between meiotic and BAC-based physical mapping that would facilitate the analysis of plant species lacking substantial genomics resources. Research has shown that the essence of these approaches can be applied and that a variety of strategies can be used to produce mapping panels. Mapping panels composed of live plants, protoplast fusion cultures, and sub-genomic DNA samples have been described. The resolution achievable by RH mapping panels involving live-plant derivatives of a monosomic maize (Zea mays) chromosome 9 addition in allohexaploid oat (Avena sativa), a monosomic chromosome 1D addition in allotetraploid durum wheat (Triticum turgidum), and interspecific hybrids between two tetraploid cotton species (G. hirsutum and G. barbadense), has been estimated to range from 0.6 to 6 Mb. On the other hand, a more comprehensive evaluation of one panel from durum wheat suggests that a higher mapping resolution (approximately 200 kb) is possible. In cases involving RH mapping panels based on barley (Hordeum vulgare)-tobacco (Nicotiana tabacum) protoplast fusions or a HAPPY mapping panel based on genomic DNA from Arabidopsis thaliana, the potential mapping resolution appears to be higher (50 to 200 kb). Despite these encouraging results, the application of either RH or HAPPY mapping in plants is still in the experimental phase and additional work is clearly needed before these methods are more routinely utilized.

Overview of somatic cell hybrid mapping

This unit provides an introduction to the various types of hybrid panels in use and reviews the status of commercially available panels.

Statistical analysis of radiation hybrid data

This unit opens with a comparison of radiation hybrid and somatic cell hybrid analyses. Definitions, assumptions, and a mathematical model for radiation hybrid analysis are presented. In addition, parametric and nonparametric methods of data analysis are presented.

Construction and assay of radiation hybrids

This unit describes the irradiation and fusion of cells to produce radiation hybrids and the harvesting and expansion of RH colonies. A support protocol describes amplification of human chromosome fragments by PCR and analysis of those fragments by agarose gel electrophoresis.

High resolution radiation hybrid maps of bovine chromosomes 19 and 29: comparison with the bovine genome sequence assembly

Background

High resolution radiation hybrid (RH) maps can facilitate genome sequence assembly by correctly ordering genes and genetic markers along chromosomes. The objective of the present study was to generate high resolution RH maps of bovine chromosomes 19 (BTA19) and 29 (BTA29), and compare them with the current 7.1X bovine genome sequence assembly (bovine build 3.1). We have chosen BTA19 and 29 as candidate chromosomes for mapping, since many Quantitative Trait Loci (QTL) for the traits of carcass merit and residual feed intake have been identified on these chromosomes.

Results

We have constructed high resolution maps of BTA19 and BTA29 consisting of 555 and 253 Single Nucleotide Polymorphism (SNP) markers respectively using a 12,000 rad whole genome RH panel. With these markers, the RH map of BTA19 and BTA29 extended to 4591.4 cR and 2884.1 cR in length respectively. When aligned with the current bovine build 3.1, the order of markers on the RH map for BTA19 and 29 showed inconsistencies with respect to the genome assembly. Maps of both the chromosomes show that there is a significant internal rearrangement of the markers involving displacement, inversion and flips within the scaffolds with some scaffolds being misplaced in the genome assembly. We also constructed cattle-human comparative maps of these chromosomes which showed an overall agreement with the comparative maps published previously. However, minor discrepancies in the orientation of few homologous synteny blocks were observed.

Conclusion

The high resolution maps of BTA19 (average 1 locus/139 kb) and BTA29 (average 1 locus/208 kb) presented in this study suggest that by the incorporation of RH mapping information, the current bovine genome sequence assembly can be significantly improved. Furthermore, these maps can serve as a potential resource for fine mapping QTL and identification of causative mutations underlying QTL for economically important traits.

Integrating physical and genetic maps: from genomes to interaction networks

Physical and genetic mapping data have become as important to network biology as they once were to the Human Genome Project. Integrating physical and genetic networks currently faces several challenges: increasing the coverage of each type of network; establishing methods to assemble individual interaction measurements into contiguous pathway models; and annotating these pathways with detailed functional information. A particular challenge involves reconciling the wide variety of interaction types that are currently available. For this purpose, recent studies have sought to classify genetic and physical interactions along several complementary dimensions, such as ordered versus unordered, alleviating versus aggravating, and first versus second degree.

Analysis of remote asymmetric somatic hybrids between common wheat and Arabidopsis thaliana

Callus-derived protoplasts of common wheat (Triticum aestivum L. cv. Hesheng 3) irradiated with ultraviolet light were fused by using the PEG method with cell suspension-derived protoplasts of Arabidopsis thaliana. Regenerated calli and green plants resembling that of wheat were obtained. The hybrid nature of putative calli and plants were confirmed by isozyme, random amplified polymorphic DNA and genomic in situ hybridization (GISH) analyses. GISH results indicated that 1 approximately 3 small chromosome fragments of A. thaliana were found introgression into the terminals of wheat chromosomes, forming highly asymmetric hybrids. Cytoplasmic genome tests did not show any cytoplasmic genetic materials from A. thaliana. However, variations from the normal wheat cytoplasmic genome were found, indicating recombination or rearrangement occurred during the process of somatic hybridization. The chromosome elimination in the asymmetric somatic hybridization of remote phylogenetic relationship was discussed. A miniature inverted-repeat transposable element related sequence was found by chance in the hybrids which might accompany and impact the process of somatic hybridization.

Radiation hybrid maps of Medaka chromosomes LG 12, 17, and 22

The Medaka is an excellent genetic system for studies of vertebrate development and disease and environmental and evolutionary biology studies. To facilitate the mapping of markers or the cloning of affected genes in Medaka mutants identified by forward-genetic screens, we have established a panel of whole-genome radiation hybrids (RHs) and RH maps for three Medaka chromosomes. RH mapping is useful, since markers to be mapped need not be polymorphic and one can establish the order of markers that are difficult to resolve by genetic mapping owing to low genetic recombination rates. RHs were generated by fusing the irradiated donor, OLF-136 Medaka cell line, with the host B78 mouse melanoma cells. Of 290 initial RH clones, we selected 93 on the basis of high retention of fragments of the Medaka genome to establish a panel that allows genotyping in the 96-well format. RH maps for linkage groups 12, 17, and 22 were generated using 159 markers. The average retention for the three chromosomes was 19% and the average break point frequency was ∼33 kb/cR. We estimate the potential resolution of the RH panel to be ∼186 kb, which is high enough for integrating RH data with bacterial artificial chromosome clones. Thus, this first RH panel will be a useful tool for mapping mutated genes in Medaka.

Construction of a primary RH panel of Italian ryegrass genome via UV-induced protoplast fusion

Symmetric and asymmetric somatic hybrids were produced via protoplast fusion between common wheat ( TRITICUM AESTIVUM L.) cv. "Jinan 177" and Italian ryegrass ( LOLIUM MULTIFLORUM Lam.). The ryegrass without or with UV irradiation was used as a donor, providing a small amount of chromatin. In these somatic hybrids, most ryegrass chromosomes have been confirmed preferential elimination and the somatic hybrid calli and plants showed wheat-like morphology. Some of the hybrid lines were used for the analysis of distribution and heredity of donor DNA in the hybrid genome and the possibility of establishing a radiation hybrid (RH) panel of the ryegrass in the present experiment. These hybrids, subcultured for two and three years, retained the ryegrass DNA examined by RFLP and GISH analysis, respectively. Distribution of the ryegrass DNA in the wheat genomes of 20 single-cell individuals, randomly selected from hybrid cell lines produced, were analyzed by 21 ryegrass genome specific SSR markers. The average frequencies of molecular marker retention in symmetric hybrid lines (UV 0), as well as asymmetric hybrid lines from UV 30 s and 1 min were 10.88, 15.48 and 33.86, respectively. It was suggested that the UV dose increased the introgression of donor DNA into wheat genome. The ryegrass SSR fragments in most asymmetric hybrid cell lines remained stable over a period of 2 approximately 3 years. This revealed that those asymmetric somatic hybrids are suitable for the introgression of ryegrass DNA into wheat, and for RH panel and RH mapping.

Gene order data from a model amphibian (Ambystoma): new perspectives on vertebrate genome structure and evolution

Background

Because amphibians arise from a branch of the vertebrate evolutionary tree that is juxtaposed between fishes and amniotes, they provide important comparative perspective for reconstructing character changes that have occurred during vertebrate evolution. Here, we report the first comparative study of vertebrate genome structure that includes a representative amphibian. We used 491 transcribed sequences from a salamander (Ambystoma) genetic map and whole genome assemblies for human, mouse, rat, dog, chicken, zebrafish, and the freshwater pufferfish Tetraodon nigroviridis to compare gene orders and rearrangement rates.

Results

Ambystoma has experienced a rate of genome rearrangement that is substantially lower than mammalian species but similar to that of chicken and fish. Overall, we found greater conservation of genome structure between Ambystoma and tetrapod vertebrates, nevertheless, 57% of Ambystoma-fish orthologs are found in conserved syntenies of four or more genes. Comparisons between Ambystoma and amniotes reveal extensive conservation of segmental homology for 57% of the presumptive Ambystoma-amniote orthologs.

Conclusion

Our analyses suggest relatively constant interchromosomal rearrangement rates from the euteleost ancestor to the origin of mammals and illustrate the utility of amphibian mapping data in establishing ancestral amniote and tetrapod gene orders. Comparisons between Ambystoma and amniotes reveal some of the key events that have structured the human genome since diversification of the ancestral amniote lineage.

High-resolution radiation hybrid map of wheat chromosome 1D

Physical mapping methods that do not rely on meiotic recombination are necessary for complex polyploid genomes such as wheat (Triticum aestivum L.). This need is due to the uneven distribution of recombination and significant variation in genetic to physical distance ratios. One method that has proven valuable in a number of nonplant and plant systems is radiation hybrid (RH) mapping. This work presents, for the first time, a high-resolution radiation hybrid map of wheat chromosome 1D (D genome) in a tetraploid durum wheat (T. turgidum L., AB genomes) background. An RH panel of 87 lines was used to map 378 molecular markers, which detected 2312 chromosome breaks. The total map distance ranged from approximately 3,341 cR(35,000) for five major linkage groups to 11,773 cR(35,000) for a comprehensive map. The mapping resolution was estimated to be approximately 199 kb/break and provided the starting point for BAC contig alignment. To date, this is the highest resolution that has been obtained by plant RH mapping and serves as a first step for the development of RH resources in wheat.

Genetic characterization of asymmetric somatic hybrids between Bupleurum scorzonerifolium Willd and Triticum aestivum L.: potential application to the study of the wheat genome

In this paper, we describe how Bupleurum scorzonerifolium/Triticum aestivum asymmetric somatic hybrids can be exploited to study the wheat genome. Protoplasts of B. scorzonerifolium Willd were irradiated with ultraviolet light (UV) and fused with protoplasts of common wheat (T. aestivum L.). All cell clones were similar in appearance to those of B. scorzonerifolium, while the regenerated plantlets were either intermediate or B. scorzonerifolium-like. Genotypic screening using isozymes showed that 39.3% of cell clones formed were hybrid. Some of the hybrid cell clones grew vigorously, and differentiated green leaves, shoots or plantlets. DNA marker analysis of the hybrids demonstrated that wheat DNA was integrated into the nuclear genomes of B. scorzonerifolium and in situ karyotyping cells revealed that a few wheat chromosome fragments had been introgressed into B. scorzonerifolium. The average wheat SSR retention frequency of the RH panel was 20.50%, but was only 6.67% in fusions with a non-irradiated donor. B. scorzonerifolium chromosomes and wheat SSR fragments in most asymmetric hybrid cell lines remained stable over a period of 2.5-3.5 years. We suggest the UV-induced asymmetric somatic hybrids between B. scorzonerifolium Willd and T. aestivum L. have the potential for use in the construction of an RH map of the wheat genome.