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author:("Mudge, joanna")
1.  Comparative genome analysis of non-toxigenic non-O1 versus toxigenic O1 Vibrio cholerae 
Genomics discovery  2014;2(1):1-15.
Pathogenic strains of Vibrio cholerae are responsible for endemic and pandemic outbreaks of the disease cholera. The complete toxigenic mechanisms underlying virulence in Vibrio strains are poorly understood. The hypothesis of this work was that virulent versus non-virulent strains of V. cholerae harbor distinctive genomic elements that encode virulence. The purpose of this study was to elucidate genomic differences between the O1 serotypes and non-O1 V. cholerae PS15, a non-toxigenic strain, in order to identify novel genes potentially responsible for virulence. In this study, we compared the whole genome of the non-O1 PS15 strain to the whole genomes of toxigenic serotypes at the phylogenetic level, and found that the PS15 genome was distantly related to those of toxigenic V. cholerae. Thus we focused on a detailed gene comparison between PS15 and the distantly related O1 V. cholerae N16961. Based on sequence alignment we tentatively assigned chromosome numbers 1 and 2 to elements within the genome of non-O1 V. cholerae PS15. Further, we found that PS15 and O1 V. cholerae N16961 shared 98% identity and 766 genes, but of the genes present in N16961 that were missing in the non-O1 V. cholerae PS15 genome, 56 were predicted to encode not only for virulence–related genes (colonization, antimicrobial resistance, and regulation of persister cells) but also genes involved in the metabolic biosynthesis of lipids, nucleosides and sulfur compounds. Additionally, we found 113 genes unique to PS15 that were predicted to encode other properties related to virulence, disease, defense, membrane transport, and DNA metabolism. Here, we identified distinctive and novel genomic elements between O1 and non-O1 V. cholerae genomes as potential virulence factors and, thus, targets for future therapeutics. Modulation of such novel targets may eventually enhance eradication efforts of endemic and pandemic disease cholera in afflicted nations.
PMCID: PMC4338557  PMID: 25722857
Vibrio cholerae; O1; non-O1; serogroup; cholera; cholera toxin; virulence; genome comparison
2.  Comparisons of De Novo Transcriptome Assemblers in Diploid and Polyploid Species Using Peanut (Arachis spp.) RNA-Seq Data 
PLoS ONE  2014;9(12):e115055.
The narrow genetic base and limited genetic information on Arachis species have hindered the process of marker-assisted selection of peanut cultivars. However, recent developments in sequencing technologies have expanded opportunities to exploit genetic resources, and at lower cost. To use the genetic information for Arachis species available at the transcriptome level, it is important to have a good quality reference transcriptome. The available Tifrunner 454 FLEX transcriptome sequences have an assembly with 37,000 contigs and low N50 values of 500-751bp. Therefore, we generated de novo transcriptome assemblies, with about 38 million reads in the tetraploid cultivar OLin, and 16 million reads in each of the diploids, A. duranensis K38901 and A. ipaënsis KGBSPSc30076 using three different de novo assemblers, Trinity, SOAPdenovo-Trans and TransAByss. All these assemblers can use single kmer analysis, and the latter two also permit multiple kmer analysis. Assemblies generated for all three samples had N50 values ranging from 1278–1641 bp in Arachis hypogaea (AABB), 1401–1492 bp in Arachis duranensis (AA), and 1107–1342 bp in Arachis ipaënsis (BB). Comparison with legume ESTs and protein databases suggests that assemblies generated had more than 40% full length transcripts with good continuity. Also, on mapping the raw reads to each of the assemblies generated, Trinity had a high success rate in assembling sequences compared to both TransAByss and SOAPdenovo-Trans. De novo assembly of OLin had a greater number of contigs (67,098) and longer contig length (N50 = 1,641) compared to the Tifrunner TSA. Despite having shorter read length (2×50) than the Tifrunner 454FLEX TSA, de novo assembly of OLin proved superior in comparison. Assemblies generated to represent different genome combinations may serve as a valuable resource for the peanut research community.
PMCID: PMC4281230  PMID: 25551607
3.  Draft Genome Sequences of Vancomycin-Susceptible Staphylococcus aureus Related to Heterogeneous Vancomycin-Intermediate S. aureus 
Genome Announcements  2014;2(5):e01033-14.
We report the draft genome sequences of three vancomycin-susceptible methicillin-resistant Staphylococcus aureus strains. S. aureus strain MV8 is a sequence type 8 (ST-8) staphylococcal cassette chromosome mec element type IV (SCCmec IV) derivative, while the other two strains (S. aureus MM25 and MM61) are ST-5 SCCmec II strains. MM61 is also closely related to the heterogeneous vancomycin-intermediate S. aureus strain MM66.
PMCID: PMC4192394  PMID: 25301662
4.  Draft Genomes of Heterogeneous Vancomycin-Intermediate Staphylococcus aureus Strain MM66 and MM66 Derivatives with Altered Vancomycin Resistance Levels 
Genome Announcements  2014;2(4):e00688-14.
The draft genomes of heterogeneous vancomycin-intermediate Staphylococcus aureus (hVISA) strain MM66 and MM66 isolates demonstrating altered vancomycin resistance levels were produced in an effort to provide information on mutations contributing to the vancomycin resistance levels observed in these strains.
PMCID: PMC4110754  PMID: 25013145
5.  The transcriptome landscape of early maize meiosis 
BMC Plant Biology  2014;14:118.
A major step in the higher plant life cycle is the decision to leave the mitotic cell cycle and begin the progression through the meiotic cell cycle that leads to the formation of gametes. The molecular mechanisms that regulate this transition and early meiosis remain largely unknown. To gain insight into gene expression features during the initiation of meiotic recombination, we profiled early prophase I meiocytes from maize (Zea mays) using capillary collection to isolate meiocytes, followed by RNA-seq.
We detected ~2,000 genes as preferentially expressed during early meiotic prophase, most of them uncharacterized. Functional analysis uncovered the importance of several cellular processes in early meiosis. Processes significantly enriched in isolated meiocytes included proteolysis, protein targeting, chromatin modification and the regulation of redox homeostasis. The most significantly up-regulated processes in meiocytes were processes involved in carbohydrate metabolism. Consistent with this, many mitochondrial genes were up-regulated in meiocytes, including nuclear- and mitochondrial-encoded genes. The data were validated with real-time PCR and in situ hybridization and also used to generate a candidate maize homologue list of known meiotic genes from Arabidopsis.
Taken together, we present a high-resolution analysis of the transcriptome landscape in early meiosis of an important crop plant, providing support for choosing genes for detailed characterization of recombination initiation and regulation of early meiosis. Our data also reveal an important connection between meiotic processes and altered/increased energy production.
PMCID: PMC4032173  PMID: 24885405
Maize; Meiosis; Meiocytes; Mitochondria; RNA-seq; Transcriptome
6.  Improved Hybrid Genome Assemblies of Two Strains of Bacteroides xylanisolvens, SD_CC_1b and SD_CC_2a, Obtained Using Illumina and 454 Sequencing Technologies 
Genome Announcements  2014;2(2):e00237-14.
Bacteroides xlyanisolvens strains (SD_CC_1b, SD_CC_2a) isolated from human feces were grown on crystalline cellulose. Cellulolytic properties are not common in Bacteroides species. Here, we report improved genome sequences of both of the B. xlyanisolvens strains.
PMCID: PMC3974937  PMID: 24699955
7.  Sequencing-based large-scale genomics approaches with small numbers of isolated maize meiocytes 
High-throughput sequencing has become the large-scale approach of choice to study global gene expression and the distribution of specific chromatin marks and features. However, the limited availability of large amounts of purified cells made it very challenging to apply sequencing-based techniques in plant meiosis research in the past. In this paper, we describe a method to isolate meiocytes from maize anthers and detailed protocols to successfully perform RNA-seq, smRNA-seq, H3K4me3-ChIP-seq, and DNA bisulfite conversion sequencing with 5000–30,000 isolated maize male meiotic cells. These methods can be adjusted for other flowering plant species as well.
PMCID: PMC3933774  PMID: 24611068
meiocytes; meiosis; Illumina sequencing; RNA-seq; ChIP-seq; DNA methylation; small RNA; maize
8.  A transcriptomic approach to elucidate the physiological significance of human cytochrome P450 2S1 in bronchial epithelial cells 
BMC Genomics  2013;14:833.
Cytochrome P450 2S1 (CYP2S1) is an orphan P450 with an unknown biological function. Data from our laboratory and others suggest that CYP2S1 may have an important physiological role in modulating the synthesis and metabolism of bioactive lipids including prostaglandins and retinoids. CYP2S1 expression is elevated in multiple epithelial-derived cancers as well as in the chronic hyperproliferative disease psoriasis. Whether CYP2S1 expression in proliferative disease is protective, detrimental, or neutral to disease progression remains to be determined. Two human bronchial epithelial cells (BEAS-2B) were constructed to represent chronic depletion of CYP2S1 using short-hairpin RNA (shRNA) silencing directed toward the 3’UTR (759) and exon 3 (984) of the CYP2S1 gene and compared with a non-targeting shRNA control (SCRAM). Both CYP2S1 mRNA and protein were depleted by approximately 75% in stable cell lines derived from both targeted shRNA constructs (759 and 984). To elucidate the biological significance of CYP2S1, we analyzed transcriptome alterations in response to CYP2S1 depletion in human lung cells.
RNA-sequencing (RNA-seq) analysis was performed to compare the transcriptome of the control (SCRAM) and the CYP2S1-depleted (759) BEAS-2B cell lines. Transcriptomes of the replicates from the two cell lines were found to be distinct populations as determined using Principal Component Analysis and hierarchical clustering. Approximately 1000 genes were differentially expressed in response to CYP2S1 depletion. Consistent with our previous phenotypes, DAVID analysis revealed altered regulation in key pathways implicated in cell proliferation and migration. Transcriptomic profiles were also consistent with the metabolism of proposed endogenous substrates. Pathway analysis also revealed significant expression changes within mTOR signaling, a critical pathway in cell growth. To determine whether these changes manifest as altered cell size, cell diameter and volume were calculated, revealing that CYP2S1 depletion promotes cell growth in BEAS-2B cells.
These data suggest that pathway analysis of sequence-based gene expression is a powerful method to identify pathways and phenotypic alterations in response to changes in orphan enzyme expression. Our results suggest a novel role for CYP2S1-mediated metabolism in modulating BEAS-2B cell size. These findings warrant further studies on CYP2S1 regulated pathways to elucidate potential substrates of CYP2S1.
PMCID: PMC3884200  PMID: 24279958
CYP2S1; BEAS-2B; Retinoic acid; Arachidonic acid; RNA Seq; Orphan; shRNA; PGE2
9.  Carrier Testing for Severe Childhood Recessive Diseases by Next-Generation Sequencing 
Science translational medicine  2011;3(65):65ra4.
Of 7028 disorders with suspected Mendelian inheritance, 1139 are recessive and have an established molecular basis. Although individually uncommon, Mendelian diseases collectively account for ~20% of infant mortality and ~10% of pediatric hospitalizations. Preconception screening, together with genetic counseling of carriers, has resulted in remarkable declines in the incidence of several severe recessive diseases including Tay-Sachs disease and cystic fibrosis. However, extension of preconception screening to most severe disease genes has hitherto been impractical. Here, we report a preconception carrier screen for 448 severe recessive childhood diseases. Rather than costly, complete sequencing of the human genome, 7717 regions from 437 target genes were enriched by hybrid capture or microdroplet polymerase chain reaction, sequenced by next-generation sequencing (NGS) to a depth of up to 2.7 gigabases, and assessed with stringent bioinformatic filters. At a resultant 160× average target coverage, 93% of nucleotides had at least 20× coverage, and mutation detection/genotyping had ~95% sensitivity and ~100% specificity for substitution, insertion/deletion, splicing, and gross deletion mutations and single-nucleotide polymorphisms. In 104 unrelated DNA samples, the average genomic carrier burden for severe pediatric recessive mutations was 2.8 and ranged from 0 to 7. The distribution of mutations among sequenced samples appeared random. Twenty-seven percent of mutations cited in the literature were found to be common polymorphisms or misannotated, underscoring the need for better mutation databases as part of a comprehensive carrier testing strategy. Given the magnitude of carrier burden and the lower cost of testing compared to treating these conditions, carrier screening by NGS made available to the general population may be an economical way to reduce the incidence of and ameliorate suffering associated with severe recessive childhood disorders.
PMCID: PMC3740116  PMID: 21228398
10.  Correction: Transcriptome Analysis of Cytokinin Response in Tomato Leaves 
PLoS ONE  2013;8(8):10.1371/annotation/58de6bf0-996f-418c-8970-13bbae3ddc20.
PMCID: PMC3739842
11.  Candidate Genes and Genetic Architecture of Symbiotic and Agronomic Traits Revealed by Whole-Genome, Sequence-Based Association Genetics in Medicago truncatula 
PLoS ONE  2013;8(5):e65688.
Genome-wide association study (GWAS) has revolutionized the search for the genetic basis of complex traits. To date, GWAS have generally relied on relatively sparse sampling of nucleotide diversity, which is likely to bias results by preferentially sampling high-frequency SNPs not in complete linkage disequilibrium (LD) with causative SNPs. To avoid these limitations we conducted GWAS with >6 million SNPs identified by sequencing the genomes of 226 accessions of the model legume Medicago truncatula. We used these data to identify candidate genes and the genetic architecture underlying phenotypic variation in plant height, trichome density, flowering time, and nodulation. The characteristics of candidate SNPs differed among traits, with candidates for flowering time and trichome density in distinct clusters of high linkage disequilibrium (LD) and the minor allele frequencies (MAF) of candidates underlying variation in flowering time and height significantly greater than MAF of candidates underlying variation in other traits. Candidate SNPs tagged several characterized genes including nodulation related genes SERK2, MtnodGRP3, MtMMPL1, NFP, CaML3, MtnodGRP3A and flowering time gene MtFD as well as uncharacterized genes that become candidates for further molecular characterization. By comparing sequence-based candidates to candidates identified by in silico 250K SNP arrays, we provide an empirical example of how reliance on even high-density reduced representation genomic makers can bias GWAS results. Depending on the trait, only 30–70% of the top 20 in silico array candidates were within 1 kb of sequence-based candidates. Moreover, the sequence-based candidates tagged by array candidates were heavily biased towards common variants; these comparisons underscore the need for caution when interpreting results from GWAS conducted with sparsely covered genomes.
PMCID: PMC3669257  PMID: 23741505
12.  Comparative genomics of the core and accessory genomes of 48 Sinorhizobium strains comprising five genospecies 
Genome Biology  2013;14(2):R17.
The sinorhizobia are amongst the most well studied members of nitrogen-fixing root nodule bacteria and contribute substantial amounts of fixed nitrogen to the biosphere. While the alfalfa symbiont Sinorhizobium meliloti RM 1021 was one of the first rhizobial strains to be completely sequenced, little information is available about the genomes of this large and diverse species group.
Here we report the draft assembly and annotation of 48 strains of Sinorhizobium comprising five genospecies. While S. meliloti and S. medicae are taxonomically related, they displayed different nodulation patterns on diverse Medicago host plants, and have differences in gene content, including those involved in conjugation and organic sulfur utilization. Genes involved in Nod factor and polysaccharide biosynthesis, denitrification and type III, IV, and VI secretion systems also vary within and between species. Symbiotic phenotyping and mutational analyses indicated that some type IV secretion genes are symbiosis-related and involved in nitrogen fixation efficiency. Moreover, there is a correlation between the presence of type IV secretion systems, heme biosynthesis and microaerobic denitrification genes, and symbiotic efficiency.
Our results suggest that each Sinorhizobium strain uses a slightly different strategy to obtain maximum compatibility with a host plant. This large genome data set provides useful information to better understand the functional features of five Sinorhizobium species, especially compatibility in legume-Sinorhizobium interactions. The diversity of genes present in the accessory genomes of members of this genus indicates that each bacterium has adopted slightly different strategies to interact with diverse plant genera and soil environments.
PMCID: PMC4053727  PMID: 23425606
13.  Genome Sequence of Non-O1 Vibrio cholerae PS15 
Genome Announcements  2013;1(1):e00227-12.
The draft genome sequence of a non-O1 Vibrio cholerae strain, PS15, organized into 3,512 open reading frames within a 3.9-Mb genome, was determined. The PS15 genome sequence will allow for the study of the evolution of virulence and environmental adaptation in V. cholerae.
PMCID: PMC3569316  PMID: 23409261
14.  Transcriptome profiling of cytokinin and auxin regulation in tomato root 
Journal of Experimental Botany  2013;64(2):695-704.
Tomato is a model and economically important crop plant with little information available about gene expression in roots. Currently, there have only been a few studies that examine hormonal responses in tomato roots and none at a genome-wide level. This study examined the transcriptome atlas of tomato root regions (root tip, lateral roots, and whole roots) and the transcriptional regulation of each root region in response to the plant hormones cytokinin and auxin using Illumina RNA sequencing. More than 165 million 1×54 base pair reads were mapped onto the Solanum lycopersicum reference genome and differential expression patterns in each root region in response to each hormone were assessed. Many novel cytokinin- and auxin-induced and -repressed genes were identified as significantly differentially expressed and the expression levels of several were confirmed by qPCR. A number of these regulated genes represent tomato orthologues of cytokinin- or auxin-regulated genes identified in other species, including CKXs, type-A RRs, Aux/IAAs, and ARFs. Additionally, the data confirm some of the hormone regulation studies for recently examined genes in tomato such as SlIAAs and SlGH3s. Moreover, genes expressed abundantly in each root region were identified which provide a spatial distribution of many classes of genes, including plant defence, secondary metabolite production, and general metabolism across the root. Overall this study presents the first global expression patterns of hormone-regulated transcripts in tomato roots, which will be functionally relevant for future studies directed towards tomato root growth and development.
PMCID: PMC3542057  PMID: 23307920
Auxin; cytokinin; lateral root; RNA sequencing; root; root tip; tomato.
15.  Transcriptome Analysis of Cytokinin Response in Tomato Leaves 
PLoS ONE  2013;8(1):e55090.
Tomato is one of the most economically and agriculturally important Solanaceous species and vegetable crops, serving as a model for examination of fruit biology and compound leaf development. Cytokinin is a plant hormone linked to the control of leaf development and is known to regulate a wide range of genes including many transcription factors. Currently there is little known of the leaf transcriptome in tomato and how it might be regulated by cytokinin. We employ high throughput mRNA sequencing technology and bioinformatic methodologies to robustly analyze cytokinin regulated tomato leaf transcriptomes. Leaf samples of two ages, 13d and 35d were treated with cytokinin or the solvent vehicle control dimethyl sulfoxide (DMSO) for 2 h or 24 h, after which RNA was extracted for sequencing. To confirm the accuracy of RNA sequencing results, we performed qPCR analysis of select transcripts identified as cytokinin regulated by the RNA sequencing approach. The resulting data provide the first hormone transcriptome analysis of leaves in tomato. Specifically we identified several previously untested tomato orthologs of cytokinin-related genes as well as numerous novel cytokinin-regulated transcripts in tomato leaves. Principal component analysis of the data indicates that length of cytokinin treatment and plant age are the major factors responsible for changes in transcripts observed in this study. Two hour cytokinin treatment showed a more robust transcript response indicated by both greater fold change of induced transcripts and the induction of twice as many cytokinin-related genes involved in signaling, metabolism, and transport in young vs. older leaves. This difference in transcriptome response in younger vs. older leaves was also found to a lesser extent with an extended (24 h) cytokinin treatment. Overall data presented here provides a solid foundation for future study of cytokinin and cytokinin regulated genes involved in compound leaf development or other developmental processes in tomato.
PMCID: PMC3555872  PMID: 23372818
16.  Genome sequencing and mapping reveal loss of heterozygosity as a mechanism for rapid adaptation in the vegetable pathogen Phytophthora capsici 
The oomycete vegetable pathogen Phytophthora capsici has shown remarkable adaptation to fungicides and new hosts. Like other members of this destructive genus, P. capsici has an explosive epidemiology, rapidly producing massive numbers of asexual spores on infected hosts. In addition, P. capsici can remain dormant for years as sexually-recombined oospores, making it difficult to produce crops at infested sites, and allowing outcrossing populations to maintain significant genetic variation. Genome sequencing, development of a high-density genetic map, and integrative genomic/genetic characterization of P. capsici field isolates and intercross progeny revealed significant mitotic loss of heterozygosity (LOH) and higher levels of SNVs than those reported for humans, plants, and P. infestans. LOH was detected in clonally propagated field isolates and sexual progeny, cumulatively affecting >30% of the genome. LOH altered genotypes for more than 11,000 single nucleotide variant (SNV) sites and showed a strong association with changes in mating type and pathogenicity. Overall, it appears that LOH may provide a rapid mechanism for fixing alleles and may be an important component of adaptability for P. capsici.
PMCID: PMC3551261  PMID: 22712506
17.  Analysis of the Transcriptomes Downstream of Eyeless and the Hedgehog, Decapentaplegic and Notch Signaling Pathways in Drosophila melanogaster 
PLoS ONE  2012;7(8):e44583.
Tissue-specific transcription factors are thought to cooperate with signaling pathways to promote patterned tissue specification, in part by co-regulating transcription. The Drosophila melanogaster Pax6 homolog Eyeless forms a complex, incompletely understood regulatory network with the Hedgehog, Decapentaplegic and Notch signaling pathways to control eye-specific gene expression. We report a combinatorial approach, including mRNAseq and microarray analyses, to identify targets co-regulated by Eyeless and Hedgehog, Decapentaplegic or Notch. Multiple analyses suggest that the transcriptomes resulting from co-misexpression of Eyeless+signaling factors provide a more complete picture of eye development compared to previous efforts involving Eyeless alone: (1) Principal components analysis and two-way hierarchical clustering revealed that the Eyeless+signaling factor transcriptomes are closer to the eye control transcriptome than when Eyeless is misexpressed alone; (2) more genes are upregulated at least three-fold in response to Eyeless+signaling factors compared to Eyeless alone; (3) based on gene ontology analysis, the genes upregulated in response to Eyeless+signaling factors had a greater diversity of functions compared to Eyeless alone. Through a secondary screen that utilized RNA interference, we show that the predicted gene CG4721 has a role in eye development. CG4721 encodes a neprilysin family metalloprotease that is highly up-regulated in response to Eyeless+Notch, confirming the validity of our approach. Given the similarity between D. melanogaster and vertebrate eye development, the large number of novel genes identified as potential targets of Ey+signaling factors will provide novel insights to our understanding of eye development in D. melanogaster and humans.
PMCID: PMC3432130  PMID: 22952997
18.  Population Genomics of the Facultatively Mutualistic Bacteria Sinorhizobium meliloti and S. medicae 
PLoS Genetics  2012;8(8):e1002868.
The symbiosis between rhizobial bacteria and legume plants has served as a model for investigating the genetics of nitrogen fixation and the evolution of facultative mutualism. We used deep sequence coverage (>100×) to characterize genomic diversity at the nucleotide level among 12 Sinorhizobium medicae and 32 S. meliloti strains. Although these species are closely related and share host plants, based on the ratio of shared polymorphisms to fixed differences we found that horizontal gene transfer (HGT) between these species was confined almost exclusively to plasmid genes. Three multi-genic regions that show the strongest evidence of HGT harbor genes directly involved in establishing or maintaining the mutualism with host plants. In both species, nucleotide diversity is 1.5–2.5 times greater on the plasmids than chromosomes. Interestingly, nucleotide diversity in S. meliloti but not S. medicae is highly structured along the chromosome – with mean diversity (θπ) on one half of the chromosome five times greater than mean diversity on the other half. Based on the ratio of plasmid to chromosome diversity, this appears to be due to severely reduced diversity on the chromosome half with less diversity, which is consistent with extensive hitchhiking along with a selective sweep. Frequency-spectrum based tests identified 82 genes with a signature of adaptive evolution in one species or another but none of the genes were identified in both species. Based upon available functional information, several genes identified as targets of selection are likely to alter the symbiosis with the host plant, making them attractive targets for further functional characterization.
Author Summary
Facultative mutualisms are relationships between two species that can live independently, but derive benefits when living together with their mutualistic partners. The facultative mutualism between rhizobial bacteria and legume plants contributes approximately half of all biologically fixed nitrogen, an essential plant nutrient, and is an important source of nitrogen to both natural and agricultural ecosystems. We resequenced the genomes of 44 strains of two closely related species of the genus Sinorhizobium that form facultative mutualisms with the model legme Medicago truncatula. These data provide one of the most complete examinations of genomic diversity segregating within microbial species that are not causative agents of human illness. Our analyses reveal that horizontal gene transfer, a common source of new genes in microbial species, disproportionately affects genes with direct roles in the rhizobia-plant symbiosis. Analyses of nucleotide diversity segregating within each species suggests that strong selection, along with genetic hitchhiking has sharply reduced diversity along an entire chromosome half in S. meliloti. Despite the two species' ecological similarity, we did not find evidence for selection acting on the same genetic targets. In addition to providing insight into the evolutionary history of rhizobial, this study shows the feasibility and potential power of applying population genomic analyses to microbial species.
PMCID: PMC3410850  PMID: 22876202
19.  Characterization of a set of novel meiotically-active promoters in Arabidopsis 
BMC Plant Biology  2012;12:104.
Homologous recombination, together with selection, laid the foundation for traditional plant breeding. The recombination process that takes place during meiotic cell division is crucial for the creation of novel variations of highly desired traits by breeders. Gaining control over this process is important for molecular breeding to achieve more precise, large-scale and quicker plant improvement. As conventional ubiquitous promoters are neither tissue-specific nor efficient in driving gene expression in meiocytes, promoters with high meiotic activities are potential candidates for manipulating the recombination process. So far, only a few meiotically-active promoters have been reported. Recently developed techniques to profile the transcriptome landscape of isolated meiocytes provided the means to discover promoters from genes that are actively expressed in meiosis.
In a screen for meiotically-active promoters, we examined ten promoter sequences that are associated with novel meiotic candidate genes. Each promoter was tested by expressing a GFP reporter gene in Arabidopsis. Characterization of regulatory regions revealed that these meiotically-active promoters possessed conserved motifs and motif arrangement. Some of the promoters unite optimal properties which are invaluable for meiosis-directed studies such as delivering specific gene expression in early meiosis I and/or meiosis II. Furthermore, the examination of homologs of the corresponding genes within green plants points to a great potential of applying the information from Arabidopsis to other species, especially crop plants.
We identified ten novel meiotically-active promoters; which, along with their homologs, are prime candidates to specifically drive gene expression during meiosis in plants and can thus provide important tools for meiosis study and crop breeding.
PMCID: PMC3462685  PMID: 22776406
Meiosis; Homologous recombination; Promoter; GFP; cis-regulatory elements; Plant molecular breeding
20.  The Medicago Genome Provides Insight into the Evolution of Rhizobial Symbioses 
Young, Nevin D. | Debellé, Frédéric | Oldroyd, Giles E. D. | Geurts, Rene | Cannon, Steven B. | Udvardi, Michael K. | Benedito, Vagner A. | Mayer, Klaus F. X. | Gouzy, Jérôme | Schoof, Heiko | Van de Peer, Yves | Proost, Sebastian | Cook, Douglas R. | Meyers, Blake C. | Spannagl, Manuel | Cheung, Foo | De Mita, Stéphane | Krishnakumar, Vivek | Gundlach, Heidrun | Zhou, Shiguo | Mudge, Joann | Bharti, Arvind K. | Murray, Jeremy D. | Naoumkina, Marina A. | Rosen, Benjamin | Silverstein, Kevin A. T. | Tang, Haibao | Rombauts, Stephane | Zhao, Patrick X. | Zhou, Peng | Barbe, Valérie | Bardou, Philippe | Bechner, Michael | Bellec, Arnaud | Berger, Anne | Bergès, Hélène | Bidwell, Shelby | Bisseling, Ton | Choisne, Nathalie | Couloux, Arnaud | Denny, Roxanne | Deshpande, Shweta | Dai, Xinbin | Doyle, Jeff | Dudez, Anne-Marie | Farmer, Andrew D. | Fouteau, Stéphanie | Franken, Carolien | Gibelin, Chrystel | Gish, John | Goldstein, Steven | González, Alvaro J. | Green, Pamela J. | Hallab, Asis | Hartog, Marijke | Hua, Axin | Humphray, Sean | Jeong, Dong-Hoon | Jing, Yi | Jöcker, Anika | Kenton, Steve M. | Kim, Dong-Jin | Klee, Kathrin | Lai, Hongshing | Lang, Chunting | Lin, Shaoping | Macmil, Simone L | Magdelenat, Ghislaine | Matthews, Lucy | McCorrison, Jamison | Monaghan, Erin L. | Mun, Jeong-Hwan | Najar, Fares Z. | Nicholson, Christine | Noirot, Céline | O’Bleness, Majesta | Paule, Charles R. | Poulain, Julie | Prion, Florent | Qin, Baifang | Qu, Chunmei | Retzel, Ernest F. | Riddle, Claire | Sallet, Erika | Samain, Sylvie | Samson, Nicolas | Sanders, Iryna | Saurat, Olivier | Scarpelli, Claude | Schiex, Thomas | Segurens, Béatrice | Severin, Andrew J. | Sherrier, D. Janine | Shi, Ruihua | Sims, Sarah | Singer, Susan R. | Sinharoy, Senjuti | Sterck, Lieven | Viollet, Agnès | Wang, Bing-Bing | Wang, Keqin | Wang, Mingyi | Wang, Xiaohong | Warfsmann, Jens | Weissenbach, Jean | White, Doug D. | White, Jim D. | Wiley, Graham B. | Wincker, Patrick | Xing, Yanbo | Yang, Limei | Yao, Ziyun | Ying, Fu | Zhai, Jixian | Zhou, Liping | Zuber, Antoine | Dénarié, Jean | Dixon, Richard A. | May, Gregory D. | Schwartz, David C. | Rogers, Jane | Quétier, Francis | Town, Christopher D. | Roe, Bruce A.
Nature  2011;480(7378):520-524.
Legumes (Fabaceae or Leguminosae) are unique among cultivated plants for their ability to carry out endosymbiotic nitrogen fixation with rhizobial bacteria, a process that takes place in a specialized structure known as the nodule. Legumes belong to one of the two main groups of eurosids, the Fabidae, which includes most species capable of endosymbiotic nitrogen fixation 1. Legumes comprise several evolutionary lineages derived from a common ancestor 60 million years ago (Mya). Papilionoids are the largest clade, dating nearly to the origin of legumes and containing most cultivated species 2. Medicago truncatula (Mt) is a long-established model for the study of legume biology. Here we describe the draft sequence of the Mt euchromatin based on a recently completed BAC-assembly supplemented with Illumina-shotgun sequence, together capturing ~94% of all Mt genes. A whole-genome duplication (WGD) approximately 58 Mya played a major role in shaping the Mt genome and thereby contributed to the evolution of endosymbiotic nitrogen fixation. Subsequent to the WGD, the Mt genome experienced higher levels of rearrangement than two other sequenced legumes, Glycine max (Gm) and Lotus japonicus (Lj). Mt is a close relative of alfalfa (M. sativa), a widely cultivated crop with limited genomics tools and complex autotetraploid genetics. As such, the Mt genome sequence provides significant opportunities to expand alfalfa’s genomic toolbox.
PMCID: PMC3272368  PMID: 22089132
21.  Meiosis-specific gene discovery in plants: RNA-Seq applied to isolated Arabidopsis male meiocytes 
BMC Plant Biology  2010;10:280.
Meiosis is a critical process in the reproduction and life cycle of flowering plants in which homologous chromosomes pair, synapse, recombine and segregate. Understanding meiosis will not only advance our knowledge of the mechanisms of genetic recombination, but also has substantial applications in crop improvement. Despite the tremendous progress in the past decade in other model organisms (e.g., Saccharomyces cerevisiae and Drosophila melanogaster), the global identification of meiotic genes in flowering plants has remained a challenge due to the lack of efficient methods to collect pure meiocytes for analyzing the temporal and spatial gene expression patterns during meiosis, and for the sensitive identification and quantitation of novel genes.
A high-throughput approach to identify meiosis-specific genes by combining isolated meiocytes, RNA-Seq, bioinformatic and statistical analysis pipelines was developed. By analyzing the studied genes that have a meiosis function, a pipeline for identifying meiosis-specific genes has been defined. More than 1,000 genes that are specifically or preferentially expressed in meiocytes have been identified as candidate meiosis-specific genes. A group of 55 genes that have mitochondrial genome origins and a significant number of transposable element (TE) genes (1,036) were also found to have up-regulated expression levels in meiocytes.
These findings advance our understanding of meiotic genes, gene expression and regulation, especially the transcript profiles of MGI genes and TE genes, and provide a framework for functional analysis of genes in meiosis.
PMCID: PMC3018465  PMID: 21167045
22.  Genome, epigenome and RNA sequences of monozygotic twins discordant for multiple sclerosis 
Nature  2010;464(7293):1351-1356.
Monozygotic (MZ) or “identical” twins have been widely studied to dissect the relative contributions of genetics and environment in human diseases. In multiple sclerosis (MS), an autoimmune demyelinating disease and common cause of neurodegeneration and disability in young adults, disease discordance in MZ twins has been interpreted to indicate environmental importance in its pathogenesis1–8. However, genetic and epigenetic differences between MZ twins have been described, challenging the accepted experimental paradigm in disambiguating effects of nature and nurture.9–12 Here, we report the genome sequences of one MS-discordant MZ twin pair and messenger RNA (mRNA) transcriptome and epigenome sequences of CD4+ lymphocytes from three MS-discordant, MZ twin pairs. No reproducible differences were detected between co-twins among ~3.6 million single nucleotide polymorphisms (SNPs) or ~0.2 million insertion-deletion polymorphisms (indels). Nor were any reproducible differences observed between siblings of the three twin pairs in HLA haplotypes, confirmed MS-susceptibility SNPs, copy number variations, mRNA and genomic SNP and indel genotypes, or expression of ~19,000 genes in CD4+ T cells. Only two to 176 differences in methylation of ~2 million CpG dinucleotides were detected between siblings of the three twin pairs, in contrast to ~800 methylation differences between T cells of unrelated individuals and several thousand differences between tissues or normal and cancerous tissues. In the first systematic effort to estimate sequence variation among MZ co-twins, we did not find evidence for genetic, epigenetic or transcriptome differences that explained disease discordance. These are the first female, twin and autoimmune disease individual genome sequences reported.
PMCID: PMC2862593  PMID: 20428171
23.  C6/36 Aedes albopictus Cells Have a Dysfunctional Antiviral RNA Interference Response 
Mosquitoes rely on RNA interference (RNAi) as their primary defense against viral infections. To this end, the combination of RNAi and invertebrate cell culture systems has become an invaluable tool in studying virus-vector interactions. Nevertheless, a recent study failed to detect an active RNAi response to West Nile virus (WNV) infection in C6/36 (Aedes albopictus) cells, a mosquito cell line frequently used to study arthropod-borne viruses (arboviruses). Therefore, we sought to determine if WNV actively evades the host's RNAi response or if C6/36 cells have a dysfunctional RNAi pathway. C6/36 and Drosophila melanogaster S2 cells were infected with WNV (Flaviviridae), Sindbis virus (SINV, Togaviridae) and La Crosse virus (LACV, Bunyaviridae) and total RNA recovered from cell lysates. Small RNA (sRNA) libraries were constructed and subjected to high-throughput sequencing. In S2 cells, virus-derived small interfering RNAs (viRNAs) from all three viruses were predominantly 21 nt in length, a hallmark of the RNAi pathway. However, in C6/36 cells, viRNAs were primarily 17 nt in length from WNV infected cells and 26–27 nt in length in SINV and LACV infected cells. Furthermore, the origin (positive or negative viral strand) and distribution (position along viral genome) of S2 cell generated viRNA populations was consistent with previously published studies, but the profile of sRNAs isolated from C6/36 cells was altered. In total, these results suggest that C6/36 cells lack a functional antiviral RNAi response. These findings are analogous to the type-I interferon deficiency described in Vero (African green monkey kidney) cells and suggest that C6/36 cells may fail to accurately model mosquito-arbovirus interactions at the molecular level.
Author Summary
Cell culture systems are invaluable tools for studying virus-host interactions. These systems are typically easy to maintain and manipulate; however, they can fail to accurately mimic the host environment encountered by viruses. Therefore, defining the limitations of each system is critical to properly interpreting the results. C6/36 Aedes albopictus cells are commonly used to study arthropod-borne viruses (arboviruses), such as West Nile virus (WNV). Recent evidence suggests that the RNA interference (RNAi) pathway, a critical aspect of the cellular innate antiviral immune response in invertebrates, may not actively target WNV in C6/36 cells. However, it is unknown whether this observation is limited to WNV. Therefore, we examined small RNA populations from C6/36 and Drosophila melanogastor S2 cells infected with WNV, Sindbis virus and La Crosse virus by high-throughput sequencing. We demonstrate that the RNAi pathway actively targets each of the three viruses in S2 cells, but does not in C6/36 cells. These findings suggest that C6/36 cells may fail to accurately model mosquito-arbovirus interactions.
PMCID: PMC2964293  PMID: 21049065
24.  A highly annotated whole-genome sequence of a Korean individual 
Nature  2009;460(7258):1011-1015.
Recent advances in sequencing technologies have initiated an era of personal genome sequences. To date, human genome sequences have been reported for individuals with ancestry in three distinct geographical regions: a Yoruba African, two individuals of north-west European origin, and a person from China1–4. Here we provide a highly annotated, whole-genome sequence for a Korean individual, known as AK1. The genome of AK1 was determined by an exacting, combined approach that included whole-genome shotgun sequencing (27.8× coverage), targeted bacterial artificial chromosome sequencing, and high-resolution comparative genomic hybridization using custom microarrays featuring more than 24 million probes. Alignment to the NCBI reference, a composite of several ethnic clades5,6, disclosed nearly 3.45 million single nucleotide polymorphisms (SNPs), including 10,162 non-synonymous SNPs, and 170,202 deletion or insertion polymorphisms (indels). SNP and indel densities were strongly correlated genome-wide. Applying very conservative criteria yielded highly reliable copy number variants for clinical considerations. Potential medical phenotypes were annotated for non-synonymous SNPs, coding domain indels, and structural variants. The integration of several human whole-genome sequences derived from several ethnic groups will assist in understanding genetic ancestry, migration patterns and population bottlenecks.
PMCID: PMC2860965  PMID: 19587683
25.  Genome-wide association studies: progress and potential for drug discovery and development 
Nature reviews. Drug discovery  2008;7(3):221-230.
Although genetic studies have been critically important for the identification of therapeutic targets in Mendelian disorders, genetic approaches aiming to identify targets for common, complex diseases have traditionally had much more limited success. However, during the past year, a novel genetic approach — genome-wide association (GWA) — has demonstrated its potential to identify common genetic variants associated with complex diseases such as diabetes, inflammatory bowel disease and cancer. Here, we highlight some of these recent successes, and discuss the potential for GWA studies to identify novel therapeutic targets and genetic biomarkers that will be useful for drug discovery, patient selection and stratification in common diseases.
PMCID: PMC2853477  PMID: 18274536

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