A global assembly of cotton ESTs

Systematic analysis of the pectin methylesterase gene family in Nicotiana tabacum and reveal their multiple roles in plant development and abiotic stresses

The pectin methylesterases (PMEs) play multiple roles in regulating plant development and responses to various stresses. In our study, a total of 121 PME genes were identified in the tobacco genome, which were clustered into two groups based on phylogenetic analysis together with Arabidopsis members. The investigations of gene structure and conserved motif indicated that exon/intron and motif organizations were relatively conserved in each group. Additionally, several stress-related elements were identified in the promoter region of these genes. The survey of duplication events revealed that segmental duplications were critical to the expansion of the PME gene family in tobacco. The expression profiles analysis revealed that these genes were expressed in various tissues and could be induced by diverse abiotic stresses. Notably, NtPME029 and NtPME043, were identified as homologues with AtPME3 and AtPME31, respectively. Furthermore, NtPME029 was highly expressed in roots and the over-expression of the NtPME029 gene could promote the development of roots. While NtPME043 could be induced by salt and ABA treatments, and the over-expression of the NtPME043 gene could significantly enhance the salt-stress tolerance in tobacco. Overall, these findings may shed light on the biological and functional characterization of NtPME genes in tobacco.

Transcriptional Landscape of Cotton Fiber Development and Its Alliance With Fiber-Associated Traits

Cotton fiber development is still an intriguing question to understand fiber commitment and development. At different fiber developmental stages, many genes change their expression pattern and have a pivotal role in fiber quality and yield. Recently, numerous studies have been conducted for transcriptional regulation of fiber, and raw data were deposited to the public repository for comprehensive integrative analysis. Here, we remapped > 380 cotton RNAseq data with uniform mapping strategies that span ∼400 fold coverage to the genome. We identified stage-specific features related to fiber cell commitment, initiation, elongation, and Secondary Cell Wall (SCW) synthesis and their putative cis-regulatory elements for the specific regulation in fiber development. We also mined Exclusively Expressed Transcripts (EETs) that were positively selected during cotton fiber evolution and domestication. Furthermore, the expression of EETs was validated in 100 cotton genotypes through the nCounter assay and correlated with different fiber-related traits. Thus, our data mining study reveals several important features related to cotton fiber development and improvement, which were consolidated in the "CottonExpress-omics" database.

Insights into the Evolution of the New World Diploid Cottons (Gossypium, Subgenus Houzingenia) Based on Genome Sequencing

We employed phylogenomic methods to study molecular evolutionary processes and phylogeny in the geographically widely dispersed New World diploid cottons (Gossypium, subg. Houzingenia). Whole genome resequencing data (average of 33× genomic coverage) were generated to reassess the phylogenetic history of the subgenus and provide a temporal framework for its diversification. Phylogenetic analyses indicate that the subgenus likely originated following transoceanic dispersal from Africa about 6.6 Ma, but that nearly all of the biodiversity evolved following rapid diversification in the mid-Pleistocene (0.5-2.0 Ma), with multiple long-distance dispersals required to account for range expansion to Arizona, the Galapagos Islands, and Peru. Comparative analyses of cpDNAversus nuclear data indicate that this history was accompanied by several clear cases of interspecific introgression. Repetitive DNAs contribute roughly half of the total 880 Mb genome, but most transposable element families are relatively old and stable among species. In the genic fraction, pairwise synonymous mutation rates average 1% per Myr, with nonsynonymous changes being about seven times less frequent. Over 1.1 million indels were detected and phylogenetically polarized, revealing a 2-fold bias toward deletions over small insertions. We suggest that this genome down-sizing bias counteracts genome size growth by TE amplification and insertions, and helps explain the relatively small genomes that are restricted to this subgenus. Compared with the rate of nucleotide substitution, the rate of indel occurrence is much lower averaging about 17 nucleotide substitutions per indel event.

Insights Into Upland Cotton (Gossypium hirsutum L.) Genetic Recombination Based on 3 High-Density Single-Nucleotide Polymorphism and a Consensus Map Developed Independently With Common Parents

High-density linkage maps are vital to supporting the correct placement of scaffolds and gene sequences on chromosomes and fundamental to contemporary organismal research and scientific approaches to genetic improvement, especially in paleopolyploids with exceptionally complex genomes, eg, upland cotton (Gossypium hirsutum L., "2n = 52"). Three independently developed intraspecific upland mapping populations were analyzed to generate 3 high-density genetic linkage single-nucleotide polymorphism (SNP) maps and a consensus map using the CottonSNP63K array. The populations consisted of a previously reported F2, a recombinant inbred line (RIL), and reciprocal RIL population, from "Phytogen 72" and "Stoneville 474" cultivars. The cluster file provided 7417 genotyped SNP markers, resulting in 26 linkage groups corresponding to the 26 chromosomes (c) of the allotetraploid upland cotton (AD)1 arisen from the merging of 2 genomes ("A" Old World and "D" New World). Patterns of chromosome-specific recombination were largely consistent across mapping populations. The high-density genetic consensus map included 7244 SNP markers that spanned 3538 cM and comprised 3824 SNP bins, of which 1783 and 2041 were in the At and Dt subgenomes with 1825 and 1713 cM map lengths, respectively. Subgenome average distances were nearly identical, indicating that subgenomic differences in bin number arose due to the high numbers of SNPs on the Dt subgenome. Examination of expected recombination frequency or crossovers (COs) on the chromosomes within each population of the 2 subgenomes revealed that COs were also not affected by the SNPs or SNP bin number in these subgenomes. Comparative alignment analyses identified historical ancestral At-subgenomic translocations of c02 and c03, as well as of c04 and c05. The consensus map SNP sequences aligned with high congruency to the NBI assembly of Gossypium hirsutum. However, the genomic comparisons revealed evidence of additional unconfirmed possible duplications, inversions and translocations, and unbalance SNP sequence homology or SNP sequence/loci genomic dominance, or homeolog loci bias of the upland tetraploid At and Dt subgenomes. The alignments indicated that 364 SNP-associated previously unintegrated scaffolds can be placed in pseudochromosomes of the NBI G hirsutum assembly. This is the first intraspecific SNP genetic linkage consensus map assembled in G hirsutum with a core of reproducible mendelian SNP markers assayed on different populations and it provides further knowledge of chromosome arrangement of genic and nongenic SNPs. Together, the consensus map and RIL populations provide a synergistically useful platform for localizing and identifying agronomically important loci for improvement of the cotton crop.

Reference Transcriptomes and Detection of Duplicated Copies in Hexaploid and Allododecaploid Spartina Species (Poaceae)

In this study, we report the assembly and annotation of five reference transcriptomes for the European hexaploid Spartina species (S. maritima, S. alterniflora and their homoploid hybrids S. x townsendii and S. x neyrautii) and the allododecaploid invasive species S. anglica These transcriptomes were constructed from various leaf and root cDNA libraries that were sequenced using both Roche-454 and Illumina technologies. Considering the high ploidy levels of the Spartina genomes under study, and considering the absence of diploid reference genome and the need of an appropriate analytical strategy, we developed generic bioinformatics tools to (1) detect different haplotypes of each gene within each species and (2) assign a parental origin to haplotypes detected in the hexaploid hybrids and the neo-allopolyploid. The approach described here allows the detection of putative homeologs from sets of short reads. Synonymous substitution rate (KS) comparisons between haplotypes from the hexaploid species revealed the presence of one KS peak (likely resulting from the tetraploid duplication event). The procedure developed in this study can be applied for future differential gene expression or genomics experiments to study the fate of duplicated genes in the invasive allododecaploid S. anglica.

SNP Marker Discovery in Pima Cotton (Gossypium barbadense L.) Leaf Transcriptomes

The objective of this study was to explore the known narrow genetic diversity and discover single-nucleotide polymorphic (SNP) markers for marker-assisted breeding within Pima cotton (Gossypium barbadense L.) leaf transcriptomes. cDNA from 25-day plants of three diverse cotton genotypes [Pima S6 (PS6), Pima S7 (PS7), and Pima 3-79 (P3-79)] was sequenced on Illumina sequencing platform. A total of 28.9 million reads (average read length of 138 bp) were generated by sequencing cDNA libraries of these three genotypes. The de novo assembly of reads generated transcriptome sets of 26,369 contigs for PS6, 25,870 contigs for PS7, and 24,796 contigs for P3-79. A Pima leaf reference transcriptome was generated consisting of 42,695 contigs. More than 10,000 single-nucleotide polymorphisms (SNPs) were identified between the genotypes, with 100% SNP frequency and a minimum of eight sequencing reads. The most prevalent SNP substitutions were C-T and A-G in these cotton genotypes. The putative SNPs identified can be utilized for characterizing genetic diversity, genotyping, and eventually in Pima cotton breeding through marker-assisted selection.

Homoeologs: What Are They and How Do We Infer Them?

The evolutionary history of nearly all flowering plants includes a polyploidization event. Homologous genes resulting from allopolyploidy are commonly referred to as 'homoeologs', although this term has not always been used precisely or consistently in the literature. With several allopolyploid genome sequencing projects under way, there is a pressing need for computational methods for homoeology inference. Here we review the definition of homoeology in historical and modern contexts and propose a precise and testable definition highlighting the connection between homoeologs and orthologs. In the second part, we survey experimental and computational methods of homoeolog inference, considering the strengths and limitations of each approach. Establishing a precise and evolutionarily meaningful definition of homoeology is essential for understanding the evolutionary consequences of polyploidization.

Identification of Multiple Stress Responsive Genes by Sequencing a Normalized cDNA Library from Sea-Land Cotton (Gossypium barbadense L.)

Background

Plants often face multiple stresses including drought, extreme temperature, salinity, nutrition deficiency and biotic stresses during growth and development. All the stresses result in a series of physiological and metabolic reactions and then generate reversible inhibition of metabolism and growth and can cause seriously irreversible damage, even death. At each stage of cotton growth, environmental stress conditions pose devastating threats to plant growth and development, especially yield and quality. Due to the complex stress conditions and unclear molecular mechanisms of stress response, there is an urgent need to explore the mechanisms of cotton response against abiotic stresses.

Methodology and Principal Findings

A normalized cDNA library was constructed using Gossypium barbadense Hai-7124 treated with different stress conditions (heat, cold, salt, drought, potassium and phosphorus deficit and Verticillium dahliae infection). Random sequencing of this library generated 6,047 high-quality expressed sequence tags (ESTs). The ESTs were clustered and assembled into 3,135 uniESTs, composed of 2,497 contigs and 638 singletons. The blastx results demonstrated 2,746 unigenes showing significant similarity to known genes, 74 uniESTs displaying significant similarity to genes of predicted proteins, and 315 uniESTs remain uncharacterized. Functional classification unveiled the abundance of uniESTs in binding, catalytic activity, and structural molecule activity. Annotations of the uniESTs by the plant transcription factor database (PlantTFDB) and Plant Stress Protein Database (PSPDB) disclosed that transcription factors and stress-related genes were enriched in the current library. The expression of some transcription factors and specific stress-related genes were verified by RT-PCR under various stress conditions.

Conclusions/Significance

Annotation results showed that a huge number of genes respond to stress in our study, such as MYB-related, C2H2, FAR1, bHLH, bZIP, MADS, and mTERF. These results will improve our knowledge of stress tolerance in cotton. In addition, they are also helpful in discovering candidate genes related to stress tolerance. The publicly available ESTs from G. barbadense are a valuable genomic resource that will facilitate further molecular study and breeding of stress-tolerant cotton.

Promoter isolation and characterization of GhAO-like1, a Gossypium hirsutum gene similar to multicopper oxidases that is highly expressed in reproductive organs

Cotton is one of the most economically important cultivated crops. It is the major source of natural fiber for the textile industry and an important target for genetic modification for both biotic stress and herbicide tolerance. Therefore, the characterization of genes and regulatory regions that might be useful for genetic transformation is indispensable. The isolation and characterization of new regulatory regions is of great importance to drive transgene expression in genetically modified crops. One of the major drawbacks in cotton production is pest damage; therefore, the most promising, cost-effective, and sustainable method for pest control is the development of genetically resistant cotton lines. Considering this scenario, our group isolated and characterized the promoter region of a MCO (multicopper oxidase) from Gossypium hirsutum, named GhAO-like1 (ascorbate oxidase-like1). The quantitative expression, together with the in vivo characterization of the promoter region reveals that GhAO-like1 has a flower- and fruit-specific expression pattern. The GUS activity is mainly observed in stamens, as expected considering that the GhAO-like1 regulatory sequence is enriched in cis elements, which have been characterized as a target of reproductive tissue specific transcription factors. Both histological and quantitative analyses in Arabidopsis thaliana have confirmed flower (mainly in stamens) and fruit expression of GhAO-like1. In the present paper, we isolated and characterized both in silico and in vivo the promoter region of the GhAO-like1 gene. The regulatory region of GhAO-like1 might be useful to confer tissue-specific expression in genetically modified plants.

Haplotype Detection from Next-Generation Sequencing in High-Ploidy-Level Species: 45S rDNA Gene Copies in the Hexaploid Spartina maritima

Gene and whole-genome duplications are widespread in plant nuclear genomes, resulting in sequence heterogeneity. Identification of duplicated genes may be particularly challenging in highly redundant genomes, especially when there are no diploid parents as a reference. Here, we developed a pipeline to detect the different copies in the ribosomal RNA gene family in the hexaploid grass Spartina maritima from next-generation sequencing (Roche-454) reads. The heterogeneity of the different domains of the highly repeated 45S unit was explored by identifying single nucleotide polymorphisms (SNPs) and assembling reads based on shared polymorphisms. SNPs were validated using comparisons with Illumina sequence data sets and by cloning and Sanger (re)sequencing. Using this approach, 29 validated polymorphisms and 11 validated haplotypes were reported (out of 34 and 20, respectively, that were initially predicted by our program). The rDNA domains of S. maritima have similar lengths as those found in other Poaceae, apart from the 5′-ETS, which is approximately two-times longer in S. maritima. Sequence homogeneity was encountered in coding regions and both internal transcribed spacers (ITS), whereas high intragenomic variability was detected in the intergenic spacer (IGS) and the external transcribed spacer (ETS). Molecular cytogenetic analysis by fluorescent in situ hybridization (FISH) revealed the presence of one pair of 45S rDNA signals on the chromosomes of S. maritima instead of three expected pairs for a hexaploid genome, indicating loss of duplicated homeologous loci through the diploidization process. The procedure developed here may be used at any ploidy level and using different sequencing technologies.

Analyzing serial cDNA libraries revealed reactive oxygen species and gibberellins signaling pathways in the salt response of Upland cotton (Gossypium hirsutum L.)

By comparing series full-length cDNA libraries stressed and control, the dynamic process of salt stress response in Upland cotton was studied, and reactive oxygen species and gibberellins signaling pathways were proposed. The Upland cotton is the most important fiber plant with highly salt tolerance. However, the molecular mechanism underlying salt tolerance in domesticated cotton was unclear. Here, seven full-length cDNA libraries were constructed for seedling roots of Upland cotton 'Zhong G 5' at 0, 3, 12 and 48 h after the treatment of control or 150 mM NaCl stress. About 3300 colonies in each library were selected robotically for 5'-end pyrosequencing, resulting in 20,358 expressed sequence tags (ESTs) totally. And 8516 uniESTs were then assembled, including 2914 contigs and 5602 singletons, and explored for Gene Ontology (GO) function. GO comparison between serial stress libraries and control reflected the growth regulation, stimulus response, signal transduction and biology regulation processes were conducted dynamically in response to salt stress. MYB, MYB-related, WRKY, bHLH, GRAS and ERF families of transcription factors were significantly enriched in the early response. 65 differentially expressed genes (DEGs), mainly associated with reactive oxygen species (ROS) scavenging, gibberellins (GAs) metabolism, signal transduction, transcription regulation, stress response and transmembrane transport, were identified and confirmed by quantitative real-time PCR. Overexpression of selected DEGs increased tolerance against salt stress in transgenic yeast. Results in this study supported that a ROS-GAs interacting signaling pathway of salt stress response was activated in Upland cotton. Our results provided valuable gene resources for further investigation of the molecular mechanism of salinity tolerance.

Distribution and evolution of cotton fiber development genes in the fibreless Gossypium raimondii genome

Cotton fiber represents the largest single cell in plants and they serve as models to study cell development. This study investigated the distribution and evolution of fiber Unigenes anchored to recombination hotspots between tetraploid cotton (Gossypium hirsutum) At and Dt subgenomes, and within a parental diploid cotton (Gossypium raimondii) D genome. Comparative analysis of At vs D and Dt vs D showed that 1) the D genome provides many fiber genes after its merger with another parental diploid cotton (Gossypium arboreum) A genome although the D genome itself does not produce any spinnable fiber; 2) similarity of fiber genes is higher between At vs D than between Dt vs D genomic hotspots. This is the first report that fiber genes have higher similarity between At and D than between Dt and D. The finding provides new insights into cotton genomic regions that would facilitate genetic improvement of natural fiber properties.

Toward Coalescing Gene Expression and Function with QTLs of Water-Deficit Stress in Cotton

Cotton exhibits moderately high vegetative tolerance to water-deficit stress but lint production is restricted by the available rainfed and irrigation capacity. We have described the impact of water-deficit stress on the genetic and metabolic control of fiber quality and production. Here we examine the association of tentative consensus sequences (TCs) derived from various cotton tissues under irrigated and water-limited conditions with stress-responsive QTLs. Three thousand sixteen mapped sequence-tagged-sites were used as anchored targets to examine sequence homology with 15,784 TCs to test the hypothesis that putative stress-responsive genes will map within QTLs associated with stress-related phenotypic variation more frequently than with other genomic regions not associated with these QTLs. Approximately 1,906 of 15,784 TCs were mapped to the consensus map. About 35% of the annotated TCs that mapped within QTL regions were genes involved in an abiotic stress response. By comparison, only 14.5% of the annotated TCs mapped outside these QTLs were classified as abiotic stress genes. A simple binomial probability calculation of this degree of bias being observed if QTL and non-QTL regions are equally likely to contain stress genes was P (x ≥ 85) = 7.99  × 10(-15). These results suggest that the QTL regions have a higher propensity to contain stress genes.

Molecular markers and cotton genetic improvement: current status and future prospects

Narrow genetic base and complex allotetraploid genome of cotton (Gossypium hirsutum L.) is stimulating efforts to avail required polymorphism for marker based breeding. The availability of draft genome sequence of G. raimondii and G. arboreum and next generation sequencing (NGS) technologies facilitated the development of high-throughput marker technologies in cotton. The concepts of genetic diversity, QTL mapping, and marker assisted selection (MAS) are evolving into more efficient concepts of linkage disequilibrium, association mapping, and genomic selection, respectively. The objective of the current review is to analyze the pace of evolution in the molecular marker technologies in cotton during the last ten years into the following four areas: (i) comparative analysis of low- and high-throughput marker technologies available in cotton, (ii) genetic diversity in the available wild and improved gene pools of cotton, (iii) identification of the genomic regions within cotton genome underlying economic traits, and (iv) marker based selection methodologies. Moreover, the applications of marker technologies to enhance the breeding efficiency in cotton are also summarized. Aforementioned genomic technologies and the integration of several other omics resources are expected to enhance the cotton productivity and meet the global fiber quantity and quality demands.

Homeologous genes involved in mannitol synthesis reveal unequal contributions in response to abiotic stress in Coffea arabica

Polyploid plants can exhibit transcriptional modulation in homeologous genes in response to abiotic stresses. Coffea arabica, an allotetraploid, accounts for 75% of the world's coffee production. Extreme temperatures, salinity and drought limit crop productivity, which includes coffee plants. Mannitol is known to be involved in abiotic stress tolerance in higher plants. This study aimed to investigate the transcriptional responses of genes involved in mannitol biosynthesis and catabolism in C. arabica leaves under water deficit, salt stress and high temperature. Mannitol concentration was significantly increased in leaves of plants under drought and salinity, but reduced by heat stress. Fructose content followed the level of mannitol only in heat-stressed plants, suggesting the partitioning of the former into other metabolites during drought and salt stress conditions. Transcripts of the key enzymes involved in mannitol biosynthesis, CaM6PR, CaPMI and CaMTD, were modulated in distinct ways depending on the abiotic stress. Our data suggest that changes in mannitol accumulation during drought and salt stress in leaves of C. arabica are due, at least in part, to the increased expression of the key genes involved in mannitol biosynthesis. In addition, the homeologs of the Coffea canephora subgenome did not present the same pattern of overall transcriptional response, indicating differential regulation of these genes by the same stimulus. In this way, this study adds new information on the differential expression of C. arabica homeologous genes under adverse environmental conditions showing that abiotic stresses can influence the homeologous gene regulation pattern, in this case, mainly on those involved in mannitol pathway.

Transcriptome dynamics during fibre development in contrasting genotypes of Gossypium hirsutum L

Understanding the contribution of genetic background in fibre quality traits is important for the development of future cotton varieties with superior fibre quality. We used Affymetrix microarray (Santa Clara, CA) and Roche 454 GSFLX (Branford, CT) for comparative transcriptome analysis between two superior and three inferior genotypes at six fibre developmental stages. Microarray-based analysis of variance (ANOVA) for 89 microarrays encompassing five contrasting genotypes and six developmental stages suggests that the stages of the fibre development have a more pronounced effect on the differentially expressed genes (DEGs) than the genetic background of genotypes. Superior genotypes showed enriched activity of cell wall enzymes, such as pectin methyl esterase, at early elongation stage, enriched metabolic activities such as lipid, amino acid and ribosomal protein subunits at peak elongation, and prolonged combinatorial regulation of brassinosteroid and auxin at later stages. Our efforts on transcriptome sequencing were focused on changes in gene expression at 25 DPA. Transcriptome sequencing resulted in the generation of 475 658 and 429 408 high-quality reads from superior and inferior genotypes, respectively. A total of 24 609 novel transcripts were identified manually for Gossypium hirsutum with no hits in NCBI 'nr' database. Gene ontology analyses showed that the genes for ribosome biogenesis, protein transport and fatty acid biosynthesis were over-represented in superior genotype, whereas salt stress, abscisic acid stimuli and water deprivation leading to the increased proteolytic activity were more pronounced in inferior genotype.

Polyploidy and the petal transcriptome of Gossypium

Background

Genes duplicated by polyploidy (homoeologs) may be differentially expressed in plant tissues. Recent research using DNA microarrays and RNAseq data have described a cacophony of complex expression patterns during development of cotton fibers, petals, and leaves. Because of its highly canalized development, petal tissue has been used as a model tissue for gene expression in cotton. Recent advances in cotton genome annotation and assembly now permit an enhanced analysis of duplicate gene deployment in petals from allopolyploid cotton.

Results

Homoeologous gene expression levels were quantified in diploid and tetraploid flower petals of Gossypium using the Gossypium raimondii genome sequence as a reference. In the polyploid, most homoeologous genes were expressed at equal levels, though a subset had an expression bias of A_T and D_T copies. The direction of gene expression bias was conserved in natural and recent polyploids of cotton. Conservation of direction of bias and additional comparisons between the diploids and tetraploids suggested different regulation mechanisms of gene expression. We described three phases in the evolution of cotton genomes that contribute to gene expression in the polyploid nucleus.

Conclusions

Compared to previous studies, a surprising level of expression homeostasis was observed in the expression patterns of polyploid genomes. Conserved expression bias in polyploid petals may have resulted from cis-acting modifications that occurred prior to polyploidization. Some duplicated genes were intriguing exceptions to general trends. Mechanisms of gene regulation for these and other genes in the cotton genome warrants further investigation.

Comparative evolutionary and developmental dynamics of the cotton (Gossypium hirsutum) fiber transcriptome

The single-celled cotton (Gossypium hirsutum) fiber provides an excellent model to investigate how human selection affects phenotypic evolution. To gain insight into the evolutionary genomics of cotton domestication, we conducted comparative transcriptome profiling of developing cotton fibers using RNA-Seq. Analysis of single-celled fiber transcriptomes from four wild and five domesticated accessions from two developmental time points revealed that at least one-third and likely one-half of the genes in the genome are expressed at any one stage during cotton fiber development. Among these, ∼5,000 genes are differentially expressed during primary and secondary cell wall synthesis between wild and domesticated cottons, with a biased distribution among chromosomes. Transcriptome data implicate a number of biological processes affected by human selection, and suggest that the domestication process has prolonged the duration of fiber elongation in modern cultivated forms. Functional analysis suggested that wild cottons allocate greater resources to stress response pathways, while domestication led to reprogrammed resource allocation toward increased fiber growth, possibly through modulating stress-response networks. This first global transcriptomic analysis using multiple accessions of wild and domesticated cottons is an important step toward a more comprehensive systems perspective on cotton fiber evolution. The understanding that human selection over the past 5,000+ years has dramatically re-wired the cotton fiber transcriptome sets the stage for a deeper understanding of the genetic architecture underlying cotton fiber synthesis and phenotypic evolution.

Ever since Darwin biologists have recognized that comparative study of crop plants and their wild relatives offers a powerful framework for generating insights into the mechanisms that underlie evolutionary change. Here, we study the domestication process in cotton, Gossypium hirsutum, an allopolyploid species (containing two different genomes) which initially was domesticated approximately 5000 years ago, and which primarily is grown for its single-celled seed fibers. Strong directional selection over the millennia was accompanied by transformation of the short, coarse, and brown fibers of wild plants into the long, strong, and fine white fibers of the modern cotton crop plant. To explore the evolutionary genetics of cotton domestication, we conducted transcriptome profiling of developing cotton fibers from multiple accessions of wild and domesticated cottons. Comparative analysis revealed that the domestication process dramatically rewired the transcriptome, affecting more than 5,000 genes, and with a more evenly balanced usage of the duplicated copies arising from genome doubling. We identify many different biological processes that were involved in this transformation, including those leading to a prolongation of fiber elongation and a reallocation of resources toward increased fiber growth in modern forms. The data provide a rich resource for future functional analyses targeting crop improvement and evolutionary objectives.

RNA-Seq transcriptome profiling of upland cotton (Gossypium hirsutum L.) root tissue under water-deficit stress

An RNA-Seq experiment was performed using field grown well-watered and naturally rain fed cotton plants to identify differentially expressed transcripts under water-deficit stress. Our work constitutes the first application of the newly published diploid D5 Gossypium raimondii sequence in the study of tetraploid AD1 upland cotton RNA-seq transcriptome analysis. A total of 1,530 transcripts were differentially expressed between well-watered and water-deficit stressed root tissues, in patterns that confirm the accuracy of this technique for future studies in cotton genomics. Additionally, putative sequence based genome localization of differentially expressed transcripts detected A2 genome specific gene expression under water-deficit stress. These data will facilitate efforts to understand the complex responses governing transcriptomic regulatory mechanisms and to identify candidate genes that may benefit applied plant breeding programs.

CottonGen: a genomics, genetics and breeding database for cotton research

CottonGen (http://www.cottongen.org) is a curated and integrated web-based relational database providing access to publicly available genomic, genetic and breeding data for cotton. CottonGen supercedes CottonDB and the Cotton Marker Database, with enhanced tools for easier data sharing, mining, visualization and data retrieval of cotton research data. CottonGen contains annotated whole genome sequences, unigenes from expressed sequence tags (ESTs), markers, trait loci, genetic maps, genes, taxonomy, germplasm, publications and communication resources for the cotton community. Annotated whole genome sequences of Gossypium raimondii are available with aligned genetic markers and transcripts. These whole genome data can be accessed through genome pages, search tools and GBrowse, a popular genome browser. Most of the published cotton genetic maps can be viewed and compared using CMap, a comparative map viewer, and are searchable via map search tools. Search tools also exist for markers, quantitative trait loci (QTLs), germplasm, publications and trait evaluation data. CottonGen also provides online analysis tools such as NCBI BLAST and Batch BLAST.

Generation and analysis of a large-scale expressed sequence Tag database from a full-length enriched cDNA library of developing leaves of Gossypium hirsutum L

Background

Cotton (Gossypium hirsutum L.) is one of the world’s most economically-important crops. However, its entire genome has not been sequenced, and limited resources are available in GenBank for understanding the molecular mechanisms underlying leaf development and senescence.

Methodology/Principal Findings

In this study, 9,874 high-quality ESTs were generated from a normalized, full-length cDNA library derived from pooled RNA isolated from throughout leaf development during the plant blooming stage. After clustering and assembly of these ESTs, 5,191 unique sequences, representative 1,652 contigs and 3,539 singletons, were obtained. The average unique sequence length was 682 bp. Annotation of these unique sequences revealed that 84.4% showed significant homology to sequences in the NCBI non-redundant protein database, and 57.3% had significant hits to known proteins in the Swiss-Prot database. Comparative analysis indicated that our library added 2,400 ESTs and 991 unique sequences to those known for cotton. The unigenes were functionally characterized by gene ontology annotation. We identified 1,339 and 200 unigenes as potential leaf senescence-related genes and transcription factors, respectively. Moreover, nine genes related to leaf senescence and eleven MYB transcription factors were randomly selected for quantitative real-time PCR (qRT-PCR), which revealed that these genes were regulated differentially during senescence. The qRT-PCR for three GhYLSs revealed that these genes express express preferentially in senescent leaves.

Conclusions/Significance

These EST resources will provide valuable sequence information for gene expression profiling analyses and functional genomics studies to elucidate their roles, as well as for studying the mechanisms of leaf development and senescence in cotton and discovering candidate genes related to important agronomic traits of cotton. These data will also facilitate future whole-genome sequence assembly and annotation in G. hirsutum and comparative genomics among Gossypium species.

BAC-pool sequencing and analysis of large segments of A12 and D12 homoeologous chromosomes in upland cotton

Although new and emerging next-generation sequencing (NGS) technologies have reduced sequencing costs significantly, much work remains to implement them for de novo sequencing of complex and highly repetitive genomes such as the tetraploid genome of Upland cotton (Gossypium hirsutum L.). Herein we report the results from implementing a novel, hybrid Sanger/454-based BAC-pool sequencing strategy using minimum tiling path (MTP) BACs from Ctg-3301 and Ctg-465, two large genomic segments in A12 and D12 homoeologous chromosomes (Ctg). To enable generation of longer contig sequences in assembly, we implemented a hybrid assembly method to process ~35x data from 454 technology and 2.8-3x data from Sanger method. Hybrid assemblies offered higher sequence coverage and better sequence assemblies. Homology studies revealed the presence of retrotransposon regions like Copia and Gypsy elements in these contigs and also helped in identifying new genomic SSRs. Unigenes were anchored to the sequences in Ctg-3301 and Ctg-465 to support the physical map. Gene density, gene structure and protein sequence information derived from protein prediction programs were used to obtain the functional annotation of these genes. Comparative analysis of both contigs with Arabidopsis genome exhibited synteny and microcollinearity with a conserved gene order in both genomes. This study provides insight about use of MTP-based BAC-pool sequencing approach for sequencing complex polyploid genomes with limited constraints in generating better sequence assemblies to build reference scaffold sequences. Combining the utilities of MTP-based BAC-pool sequencing with current longer and short read NGS technologies in multiplexed format would provide a new direction to cost-effectively and precisely sequence complex plant genomes.

Insights into the evolution of cotton diploids and polyploids from whole-genome re-sequencing

Understanding the composition, evolution, and function of the Gossypium hirsutum (cotton) genome is complicated by the joint presence of two genomes in its nucleus (A_T and D_T genomes). These two genomes were derived from progenitor A-genome and D-genome diploids involved in ancestral allopolyploidization. To better understand the allopolyploid genome, we re-sequenced the genomes of extant diploid relatives that contain the A₁ (Gossypium herbaceum), A₂ (Gossypium arboreum), or D₅ (Gossypium raimondii) genomes. We conducted a comparative analysis using deep re-sequencing of multiple accessions of each diploid species and identified 24 million SNPs between the A-diploid and D-diploid genomes. These analyses facilitated the construction of a robust index of conserved SNPs between the A-genomes and D-genomes at all detected polymorphic loci. This index is widely applicable for read mapping efforts of other diploid and allopolyploid Gossypium accessions. Further analysis also revealed locations of putative duplications and deletions in the A-genome relative to the D-genome reference sequence. The approximately 25,400 deleted regions included more than 50% deletion of 978 genes, including many involved with starch synthesis. In the polyploid genome, we also detected 1,472 conversion events between homoeologous chromosomes, including events that overlapped 113 genes. Continued characterization of the Gossypium genomes will further enhance our ability to manipulate fiber and agronomic production of cotton.

Analyses of the NAC transcription factor gene family in Gossypium raimondii Ulbr.: chromosomal location, structure, phylogeny, and expression patterns

NAC domain proteins are plant-specific transcription factors known to play diverse roles in various plant developmental processes. In the present study, we performed the first comprehensive study of the NAC gene family in Gossypium raimondii Ulbr., incorporating phylogenetic, chromosomal location, gene structure, conserved motif, and expression profiling analyses. We identified 145 NAC transcription factor (NAC-TF) genes that were phylogenetically clustered into 18 distinct subfamilies. Of these, 127 NAC-TF genes were distributed across the 13 chromosomes, 80 (55%) were preferentially retained duplicates located in both duplicated regions and six were located in triplicated chromosomal regions. The majority of NAC-TF genes showed temporal-, spatial-, and tissue-specific expression patterns based on transcriptomic and qRT-PCR analyses. However, the expression patterns of several duplicate genes were partially redundant, suggesting the occurrence of sub-functionalization during their evolution. Based on their genomic organization, we concluded that genomic duplications contributed significantly to the expansion of the NAC-TF gene family in G. raimondii. Comprehensive analysis of their expression profiles could provide novel insights into the functional divergence among members of the NAC gene family in G. raimondii.

Using genome-referenced expressed sequence tag assembly to analyze the origin and expression patterns of Gossypium hirsutum transcripts

We assembled a total of 297,239 Gossypium hirsutum (Gh, a tetraploid cotton, AADD) expressed sequence tag (EST) sequences that were available in the National Center for Biotechnology Information database, with reference to the recently published G. raimondii (Gr, a diploid cotton, DD) genome, and obtained 49,125 UniGenes. The average lengths of the UniGenes were increased from 804 and 791 bp in two previous EST assemblies to 1,019 bp in the current analysis. The number of putative cotton UniGenes with lengths of 3 kb or more increased from 25 or 34 to 1,223. As a result, thousands of originally independent G. hirsutum ESTs were aligned to produce large contigs encoding transcripts with very long open reading frames, indicating that the G. raimondii genome sequence provided remarkable advantages to assemble the tetraploid cotton transcriptome. Significant different distribution patterns within several GO terms, including transcription factor activity, were observed between D- and A-derived assemblies. Transcriptome analysis showed that, in a tetraploid cotton cell, 29,547 UniGenes were possibly derived from the D subgenome while another 19,578 may come from the A subgenome. Finally, some of the in silico data were confirmed by reverse transcription polymerase chain reaction experiments to show the changes in transcript levels for several gene families known to play key role in cotton fiber development. We believe that our work provides a useful platform for functional and evolutionary genomic studies in cotton.

A genetic and metabolic analysis revealed that cotton fiber cell development was retarded by flavonoid naringenin

The cotton (Gossypium spp.) fiber is a unique elongated cell that is useful for investigating cell differentiation. Previous studies have demonstrated the importance of factors such as sugar metabolism, the cytoskeleton, and hormones, which are commonly known to be involved in plant cell development, while the secondary metabolites have been less regarded. By mining public data and comparing analyses of fiber from two cotton species (Gossypium hirsutum and Gossypium barbadense), we found that the flavonoid metabolism is active in early fiber cell development. Different flavonoids exhibited distinct effects on fiber development during ovule culture; among them, naringenin (NAR) could significantly retard fiber development. NAR is a substrate of flavanone 3-hydroxylase (F3H), and silencing the F3H gene significantly increased the NAR content of fiber cells. Fiber development was suppressed following F3H silencing, but the overexpression of F3H caused no obvious effects. Significant retardation of fiber growth was observed after the introduction of the F3H-RNA interference segment into the high-flavonoid brown fiber G. hirsutum T586 line by cross. A greater accumulation of NAR as well as much shorter fibers were also observed in the BC1 generation plants. These results suggest that NAR is negatively associated with fiber development and that the metabolism mediated by F3H is important in fiber development, thus highlighting that flavonoid metabolism represents a novel pathway with the potential for cotton fiber improvement.

Characterization of expressed sequence tags from developing fibers of Gossypium barbadense and evaluation of insertion-deletion variation in tetraploid cultivated cotton species

Background

Cotton is the leading fiber crop worldwide. Gossypium barbadense is an important species of cotton because of its extra-long staple fibers with superior luster and silkiness. However, a systematic analysis and utilization of cDNA sequences from G. barbadense fiber development remains understudied.

Results

A total of 21,079 high quality sequences were generated from two non-normalized cDNA libraries prepared by using a mixture of G. barbadense Hai7124 fibers and ovules. After assembly processing, a set of 8,653 unigenes were obtained. Of those, 7,786 were matched to known proteins and 7,316 were assigned to functional categories. The molecular functions of these unigenes were mostly related to binding and catalytic activity, and carbohydrate, amino acid, and energy metabolisms were major contributors among the subsets of metabolism. Sequences comparison between G. barbadense and G. hirsutum revealed that 8,245 unigenes from G. barbadense were detected the similarity with those released publicly in G. hirsutum, however, the remaining 408 sequences had no hits against G. hirsutum unigenes database. Furthermore, 13,275 putative ESTs InDels loci involved in the orthologous and/or homoeologous differences between/within G. barbadense and G. hirsutum were discovered by in silico analyses, and 2,160 InDel markers were developed by ESTs with more than five insertions or deletions. By gel electrophoresis combined with sequencing verification, 71.11% candidate InDel loci were reconfirmed orthologous and/or homoeologous loci polymorphisms using G. hirsutum acc TM-1 and G. barbadense cv Hai7124. Blastx result showed among 2,160 InDel loci, 81 with significant function similarity with known genes associated with secondary wall synthesis process, indicating the important roles in fiber quality in tetraploid cultivated cotton species.

Conclusion

Sequence comparisons and InDel markers development will lay the groundwork for promoting the identification of genes related to superior agronomic traits, genetic differentiation and comparative genomic studies between G. hirsutum and G. barbadense.

Whole-genome thermodynamic analysis reduces siRNA off-target effects

Small interfering RNAs (siRNAs) are important tools for knocking down targeted genes, and have been widely applied to biological and biomedical research. To design siRNAs, two important aspects must be considered: the potency in knocking down target genes and the off-target effect on any nontarget genes. Although many studies have produced useful tools to design potent siRNAs, off-target prevention has mostly been delegated to sequence-level alignment tools such as BLAST. We hypothesize that whole-genome thermodynamic analysis can identify potential off-targets with higher precision and help us avoid siRNAs that may have strong off-target effects. To validate this hypothesis, two siRNA sets were designed to target three human genes IDH1, ITPR2 and TRIM28. They were selected from the output of two popular siRNA design tools, siDirect and siDesign. Both siRNA design tools have incorporated sequence-level screening to avoid off-targets, thus their output is believed to be optimal. However, one of the sets we tested has off-target genes predicted by Picky, a whole-genome thermodynamic analysis tool. Picky can identify off-target genes that may hybridize to a siRNA within a user-specified melting temperature range. Our experiments validated that some off-target genes predicted by Picky can indeed be inhibited by siRNAs. Similar experiments were performed using commercially available siRNAs and a few off-target genes were also found to be inhibited as predicted by Picky. In summary, we demonstrate that whole-genome thermodynamic analysis can identify off-target genes that are missed in sequence-level screening. Because Picky prediction is deterministic according to thermodynamics, if a siRNA candidate has no Picky predicted off-targets, it is unlikely to cause off-target effects. Therefore, we recommend including Picky as an additional screening step in siRNA design.

PolyCat: a resource for genome categorization of sequencing reads from allopolyploid organisms

Read mapping is a fundamental part of next-generation genomic research but is complicated by genome duplication in many plants. Categorizing DNA sequence reads into their respective genomes enables current methods to analyze polyploid genomes as if they were diploid. We present PolyCat-a pipeline for mapping and categorizing all types of next-generation sequence data produced from allopolyploid organisms. PolyCat uses GSNAP's single-nucleotide polymorphism (SNP)-tolerant mapping to minimize the mapping efficiency bias caused by SNPs between genomes. PolyCat then uses SNPs between genomes to categorize reads according to their respective genomes. Bisulfite-treated reads have a significant reduction in nucleotide complexity because nucleotide conversion events are confounded with transition substitutions. PolyCat includes special provisions to properly handle bisulfite-treated data. We demonstrate the functionality of PolyCat on allotetraploid cotton, Gossypium hirsutum, and create a functional SNP index for efficiently mapping sequence reads to the D-genome sequence of G. raimondii. PolyCat is appropriate for all allopolyploids and all types of next-generation genome analysis, including differential expression (RNA sequencing), differential methylation (bisulfite sequencing), differential DNA-protein binding (chromatin immunoprecipitation sequencing), and population diversity.

Isolation, characterization and mapping of genes differentially expressed during fibre development between Gossypium hirsutum and G. barbadense by cDNA-SRAP

Gossypium hirsutum and G. barbadense are two cultivated tetraploid cotton species with differences in fibre quality. The fibre of G. barbadense is longer, stronger and finer than that of G. hirsutum. To isolate genes expressed differently between the two species during fibre development, cDNA-SRAP (sequence-related amplified polymorphism) was applied. This technique was used to analyse genes at different stages of fibre development in G. hirsutum cv. Emian22 and G. barbadense acc. 3-79, the parents of our interspecific mapping population. A total of 4096 SRAP primer combinations were used to screen polymorphism between the DNA of the parents, and 275 highly polymorphic primers were picked out to analyse DNA and RNA from leaves and fibres at different developmental stages of the parents. A total of 168 DNA fragments were isolated from gels and sequenced: 54, 30, 38 and 41 from fibres of 5, 10, 15 and 20 days post-anthesis, respectively, and five from multi stages. To genetically map these sequences, 104 sequence-specific primers were developed and were used to screened polymorphism between the mapping parents. Finally, six markers were mapped on six chromosomes of our backbone interspecific genetic map. This work can give us a primary knowledge of differences in mechanism of fibre development between G. hirsutum and G. barbadense.

Identification and annotation of abiotic stress responsive candidate genes in peanut ESTs

Peanut (Arachis hypogaea L.) ranks fifth among the world oil crops and is widely grown in India and neighbouring countries. Due to its large and unknown genome size, studies on genomics and genetic modification of peanut are still scanty as compared to other model crops like Arabidopsis, rice, cotton and soybean. Because of its favourable cultivation in semi-arid regions, study on abiotic stress responsive genes and its regulation in peanut is very much important. Therefore, we aim to identify and annotate the abiotic stress responsive candidate genes in peanut ESTs. Expression data of drought stress responsive corresponding genes and EST sequences were screened from dot blot experiments shown as heat maps and supplementary tables, respectively as reported by Govind et al. (2009). Some of the screened genes having no information about their ESTs in above mentioned supplementary tables were retrieved from NCBI. A phylogenetic analysis was performed to find a group of utmost similar ESTs for each selected gene. Individual EST of the said group were further searched in peanut ESTs (1,78,490 whole EST sequences) using stand alone BLAST. For the prediction as well as annotation of abiotic stress responsive selected genes, various tools (like Vec-Screen, Repeat Masker, EST-Trimmer, DNA Baser, WISE2 and I-TASSER) were used. Here we report the predicted result of Contigs, domain as well as 3D structure for HSP 17.3KDa protein, DnaJ protein and Type 2 Metallothionein protein.

Deep sequencing reveals differences in the transcriptional landscapes of fibers from two cultivated species of cotton

Cotton (Gossypium) fiber is the most prevalent natural product used in the textile industry. The two major cultivated species, G. hirsutum (Gh) and G. barbadense (Gb), are allotetraploids with contrasting fiber quality properties. To better understand the molecular basis for their fiber differences, EST pyrosequencing was used to document the fiber transcriptomes at two key development stages, 10 days post anthesis (dpa), representing the peak of fiber elongation, and 22 dpa, representing the transition to secondary cell wall synthesis. The 617,000 high quality reads (89% of the total 692,000 reads) from 4 libraries were assembled into 46,072 unigenes, comprising 38,297 contigs and 7,775 singletons. Functional annotation of the unigenes together with comparative digital gene expression (DGE) revealed a diverse set of functions and processes that were partly linked to specific fiber stages. Globally, 2,770 contigs (7%) showed differential expression (>2-fold) between 10 and 22 dpa (irrespective of genotype), with 70% more highly expressed at 10 dpa, while 2,248 (6%) were differentially expressed between the genotypes (irrespective of stage). The most significant genes with differential DGE at 10 dpa included expansins and lipid transfer proteins (higher in Gb), while at 22 dpa tubulins, cellulose, and sucrose synthases showed higher expression in Gb. DGE was compared with expression data of 10 dpa-old fibers from Affymetrix microarrays. Among 543 contigs showing differential expression on both platforms, 74% were consistent in being either over-expressed in Gh (242 genes) or in Gb (161 genes). Furthermore, the unigene set served to identify 339 new SSRs and close to 21,000 inter-genotypic SNPs. Subsets of 88 SSRs and 48 SNPs were validated through mapping and added 65 new loci to a RIL genetic map. The new set of fiber ESTs and the gene-based markers complement existing available resources useful in basic and applied research for crop improvement in cotton.

Targeted capture of homoeologous coding and noncoding sequence in polyploid cotton

Targeted sequence capture is a promising technology in many areas in biology. These methods enable efficient and relatively inexpensive sequencing of hundreds to thousands of genes or genomic regions from many more individuals than is practical using whole-genome sequencing approaches. Here, we demonstrate the feasibility of target enrichment using sequence capture in polyploid cotton. To capture and sequence both members of each gene pair (homeologs) of wild and domesticated Gossypium hirsutum, we created custom hybridization probes to target 1000 genes (500 pairs of homeologs) using information from the cotton transcriptome. Two widely divergent samples of G. hirsutum were hybridized to four custom NimbleGen capture arrays containing probes for targeted genes. We show that the two coresident homeologs in the allopolyploid nucleus were efficiently captured with high coverage. The capture efficiency was similar between the two accessions and independent of whether the samples were multiplexed. A significant amount of flanking, nontargeted sequence (untranslated regions and introns) was also captured and sequenced along with the targeted exons. Intraindividual heterozygosity is low in both wild and cultivated Upland cotton, as expected from the high level of inbreeding in natural G. hirsutum and bottlenecks accompanying domestication. In addition, levels of heterozygosity appeared asymmetrical with respect to genome (A_T or D_T) in cultivated cotton. The approach used here is general, scalable, and may be adapted for many different research inquiries involving polyploid plant genomes.

Duplicate gene evolution, homoeologous recombination, and transcriptome characterization in allopolyploid cotton

BACKGROUND

Modern allotetraploid cotton contains an "A" and "D" genome from an ancestral polyploidy event that occurred approximately 1-2 million years ago. Diploid A- and D-genome species can be compared to the A- and D-genomes found within these allotetraploids to make evolutionary inferences about polyploidy. In this paper we present a comprehensive EST assembly derived from diploid and model allotetraploid cottons and demonstrate several evolutionary inferences regarding genic evolution that can be drawn from these data.

RESULTS

We generated a set of cotton expressed sequence tags (ESTs), comprising approximately 4.4 million Sanger and next-generation (454) transcripts supplemented by approximately 152 million Illumina reads from diploid and allotetraploid cottons. From the EST alignments we inferred 259,192 genome-specific single nucleotide polymorphisms (SNPs). Molecular evolutionary analyses of protein-coding regions demonstrate that the rate of nucleotide substitution has increased among both allotetraploid genomes relative to the diploids, and that the ratio of nonsynonymous to synonymous substitutions has increased in one of the two polyploid lineages we sampled. We also use these SNPs to show that a surprisingly high percentage of duplicate genes (~7 %) show a signature of non-independent evolution in the allotetraploid nucleus, having experienced one or more episodes of nonreciprocal homoeologous recombination (NRHR).

CONCLUSIONS

In this study we characterize the functional and mutational properties of the cotton transcriptome, produce a large genome-specific SNP database, and detect illegitimate genetic exchanges between duplicate genomes sharing a common allotetraploid nucleus. Our findings have important implications for our understanding of the consequences of polyploidy and duplicate gene evolution. We demonstrate that cotton genes have experienced an increased rate of molecular evolution following duplication by polyploidy, and that polyploidy has enabled considerable levels of nonreciprocal exchange between homoeologous genes.

Duplicate gene evolution, homoeologous recombination, and transcriptome characterization in allopolyploid cotton

BACKGROUND

Modern allotetraploid cotton contains an "A" and "D" genome from an ancestral polyploidy event that occurred approximately 1-2 million years ago. Diploid A- and D-genome species can be compared to the A- and D-genomes found within these allotetraploids to make evolutionary inferences about polyploidy. In this paper we present a comprehensive EST assembly derived from diploid and model allotetraploid cottons and demonstrate several evolutionary inferences regarding genic evolution that can be drawn from these data.

RESULTS

We generated a set of cotton expressed sequence tags (ESTs), comprising approximately 4.4 million Sanger and next-generation (454) transcripts supplemented by approximately 152 million Illumina reads from diploid and allotetraploid cottons. From the EST alignments we inferred 259,192 genome-specific single nucleotide polymorphisms (SNPs). Molecular evolutionary analyses of protein-coding regions demonstrate that the rate of nucleotide substitution has increased among both allotetraploid genomes relative to the diploids, and that the ratio of nonsynonymous to synonymous substitutions has increased in one of the two polyploid lineages we sampled. We also use these SNPs to show that a surprisingly high percentage of duplicate genes (~7 %) show a signature of non-independent evolution in the allotetraploid nucleus, having experienced one or more episodes of nonreciprocal homoeologous recombination (NRHR).

CONCLUSIONS

In this study we characterize the functional and mutational properties of the cotton transcriptome, produce a large genome-specific SNP database, and detect illegitimate genetic exchanges between duplicate genomes sharing a common allotetraploid nucleus. Our findings have important implications for our understanding of the consequences of polyploidy and duplicate gene evolution. We demonstrate that cotton genes have experienced an increased rate of molecular evolution following duplication by polyploidy, and that polyploidy has enabled considerable levels of nonreciprocal exchange between homoeologous genes.

Genome-wide identification of differentially expressed genes under water deficit stress in upland cotton (Gossypium hirsutum L.)

BACKGROUND

Cotton is the world's primary fiber crop and is a major agricultural commodity in over 30 countries. Like many other global commodities, sustainable cotton production is challenged by restricted natural resources. In response to the anticipated increase of agricultural water demand, a major research direction involves developing crops that use less water or that use water more efficiently. In this study, our objective was to identify differentially expressed genes in response to water deficit stress in cotton. A global expression analysis using cDNA-Amplified Fragment Length Polymorphism was conducted to compare root and leaf gene expression profiles from a putative drought resistant cotton cultivar grown under water deficit stressed and well watered field conditions.

RESULTS

We identified a total of 519 differentially expressed transcript derived fragments. Of these, 147 transcript derived fragment sequences were functionally annotated according to their gene ontology. Nearly 70 percent of transcript derived fragments belonged to four major categories: 1) unclassified, 2) stress/defense, 3) metabolism, and 4) gene regulation. We found heat shock protein-related and reactive oxygen species-related transcript derived fragments to be among the major parts of functional pathways induced by water deficit stress. Also, twelve novel transcripts were identified as both water deficit responsive and cotton specific. A subset of differentially expressed transcript derived fragments was verified using reverse transcription-polymerase chain reaction. Differential expression analysis also identified five pairs of duplicated transcript derived fragments in which four pairs responded differentially between each of their two homologues under water deficit stress.

CONCLUSIONS

In this study, we detected differentially expressed transcript derived fragments from water deficit stressed root and leaf tissues in tetraploid cotton and provided their gene ontology, functional/biological distribution, and possible roles of gene duplication. This discovery demonstrates complex mechanisms involved with polyploid cotton's transcriptome response to naturally occurring field water deficit stress. The genes identified in this study will provide candidate targets to manipulate the water use characteristics of cotton at the molecular level.

Genome-wide SNP identification in multiple morphotypes of allohexaploid tall fescue (Festuca arundinacea Schreb)

BACKGROUND

Single nucleotide polymorphisms (SNPs) provide essential tools for the advancement of research in plant genomics, and the development of SNP resources for many species has been accelerated by the capabilities of second-generation sequencing technologies. The current study aimed to develop and use a novel bioinformatic pipeline to generate a comprehensive collection of SNP markers within the agriculturally important pasture grass tall fescue; an outbreeding allopolyploid species displaying three distinct morphotypes: Continental, Mediterranean and rhizomatous.

RESULTS

A bioinformatic pipeline was developed that successfully identified SNPs within genotypes from distinct tall fescue morphotypes, following the sequencing of 414 polymerase chain reaction (PCR) - generated amplicons using 454 GS FLX technology. Equivalent amplicon sets were derived from representative genotypes of each morphotype, including six Continental, five Mediterranean and one rhizomatous. A total of 8,584 and 2,292 SNPs were identified with high confidence within the Continental and Mediterranean morphotypes respectively. The success of the bioinformatic approach was demonstrated through validation (at a rate of 70%) of a subset of 141 SNPs using both SNaPshot™ and GoldenGate™ assay chemistries. Furthermore, the quantitative genotyping capability of the GoldenGate™ assay revealed that approximately 30% of the putative SNPs were accessible to co-dominant scoring, despite the hexaploid genome structure. The sub-genome-specific origin of each SNP validated from Continental tall fescue was predicted using a phylogenetic approach based on comparison with orthologous sequences from predicted progenitor species.

CONCLUSIONS

Using the appropriate bioinformatic approach, amplicon resequencing based on 454 GS FLX technology is an effective method for the identification of polymorphic SNPs within the genomes of Continental and Mediterranean tall fescue. The GoldenGate™ assay is capable of high-throughput co-dominant SNP allele detection, and minimises the problems associated with SNP genotyping in a polyploid by effectively reducing the complexity to a diploid system. This SNP collection may now be refined and used in applications such as cultivar identification, genetic linkage map construction, genome-wide association studies and genomic selection in tall fescue. The bioinformatic pipeline described here represents an effective general method for SNP discovery within outbreeding allopolyploid species.

Development and mapping of SNP assays in allotetraploid cotton

A narrow germplasm base and a complex allotetraploid genome have made the discovery of single nucleotide polymorphism (SNP) markers difficult in cotton (Gossypium hirsutum). To generate sequence for SNP discovery, we conducted a genome reduction experiment (EcoRI, BafI double digest, followed by adapter ligation, biotin-streptavidin purification, and agarose gel separation) on two accessions of G. hirsutum and two accessions of G. barbadense. From the genome reduction experiment, a total of 2.04 million genomic sequence reads were assembled into contigs with an N(50) of 508 bp and analyzed for SNPs. A previously generated assembly of expressed sequence tags (ESTs) provided an additional source for SNP discovery. Using highly conservative parameters (minimum coverage of 8× at each SNP and 20% minor allele frequency), a total of 11,834 and 1,679 non-genic SNPs were identified between accessions of G. hirsutum and G. barbadense in genome reduction assemblies, respectively. An additional 4,327 genic SNPs were also identified between accessions of G. hirsutum in the EST assembly. KBioscience KASPar assays were designed for a portion of the intra-specific G. hirsutum SNPs. From 704 non-genic and 348 genic markers developed, a total of 367 (267 non-genic, 100 genic) mapped in a segregating F(2) population (Acala Maxxa × TX2094) using the Fluidigm EP1 system. A G. hirsutum genetic linkage map of 1,688 cM was constructed based entirely on these new SNP markers. Of the genic-based SNPs, we were able to identify within which genome ('A' or 'D') each SNP resided using diploid species sequence data. Genetic maps generated by these newly identified markers are being used to locate quantitative, economically important regions within the cotton genome.

The cytonuclear dimension of allopolyploid evolution: an example from cotton using rubisco

During allopolyploid speciation, two divergent nuclear genomes merge, yet only one (usually the maternal) of the two sets of progenitor organellar genomes is maintained. Rubisco (1,5-bisphosphate carboxylase/oxygenase) is composed of nuclear-encoded small subunits (SSUs) and plastome-encoded large subunits (LSUs), providing an ideal system to explore the evolutionary process of cytonuclear accommodation. Here, we take initial steps in this direction, using Gossypium allopolyploids as our model. SSU copies from divergent (5-10 My) progenitor diploids ("A" and "D" genomes) were combined at the time of polyploid formation 1-2 Ma, with the LSU encoded by the maternal A-genome parent. LSU genes from A- and D-genome diploids and AD-genome allopolyploids were sequenced, revealing several nonsynonymous substitutions and suggesting the possibility of differential selection on the nuclear-encoded rbcS partner following allopolyploid formation. Sequence data for the rbcS gene family revealed nonreciprocal homoeologous recombination between A- and D-rbcS homoeologs in all polyploid species but not in a synthetic intergenomic F1 hybrid, demonstrating "gene conversion" during allopolyploid evolution. All progenitor rbcS genes are retained and expressed in the five extant allopolyploid species, but analysis of the leaf transcriptome showed that A-homoeologs are preferentially expressed in both the allopolyploid and hybrid, consistent with the maternal origin of rbcL. Although rbcS genes from both progenitor genomes are expressed, some appear to have experienced mutations that may represent cytonuclear coevolution.

Comprehensive functional analyses of expressed sequence tags in common wheat (Triticum aestivum)

About 1 million expressed sequence tag (EST) sequences comprising 125.3 Mb nucleotides were accreted from 51 cDNA libraries constructed from a variety of tissues and organs under a range of conditions, including abiotic stresses and pathogen challenges in common wheat (Triticum aestivum). Expressed sequence tags were assembled with stringent parameters after processing with inbuild scripts, resulting in 37,138 contigs and 215,199 singlets. In the assembled sequences, 10.6% presented no matches with existing sequences in public databases. Functional characterization of wheat unigenes by gene ontology annotation, mining transcription factors, full-length cDNA, and miRNA targeting sites were carried out. A bioinformatics strategy was developed to discover single-nucleotide polymorphisms (SNPs) within our large EST resource and reported the SNPs between and within (homoeologous) cultivars. Digital gene expression was performed to find the tissue-specific gene expression, and correspondence analysis was executed to identify common and specific gene expression by selecting four biotic stress-related libraries. The assembly and associated information cater a framework for future investigation in functional genomics.

Development and mapping of SNP assays in allotetraploid cotton

A narrow germplasm base and a complex allotetraploid genome have made the discovery of single nucleotide polymorphism (SNP) markers difficult in cotton (Gossypium hirsutum). To generate sequence for SNP discovery, we conducted a genome reduction experiment (EcoRI, BafI double digest, followed by adapter ligation, biotin-streptavidin purification, and agarose gel separation) on two accessions of G. hirsutum and two accessions of G. barbadense. From the genome reduction experiment, a total of 2.04 million genomic sequence reads were assembled into contigs with an N(50) of 508 bp and analyzed for SNPs. A previously generated assembly of expressed sequence tags (ESTs) provided an additional source for SNP discovery. Using highly conservative parameters (minimum coverage of 8× at each SNP and 20% minor allele frequency), a total of 11,834 and 1,679 non-genic SNPs were identified between accessions of G. hirsutum and G. barbadense in genome reduction assemblies, respectively. An additional 4,327 genic SNPs were also identified between accessions of G. hirsutum in the EST assembly. KBioscience KASPar assays were designed for a portion of the intra-specific G. hirsutum SNPs. From 704 non-genic and 348 genic markers developed, a total of 367 (267 non-genic, 100 genic) mapped in a segregating F(2) population (Acala Maxxa × TX2094) using the Fluidigm EP1 system. A G. hirsutum genetic linkage map of 1,688 cM was constructed based entirely on these new SNP markers. Of the genic-based SNPs, we were able to identify within which genome ('A' or 'D') each SNP resided using diploid species sequence data. Genetic maps generated by these newly identified markers are being used to locate quantitative, economically important regions within the cotton genome.

Development of New Candidate Gene and EST-Based Molecular Markers for Gossypium Species

New source of molecular markers accelerate the efforts in improving cotton fiber traits and aid in developing high-density integrated genetic maps. We developed new markers based on candidate genes and G. arboreum EST sequences that were used for polymorphism detection followed by genetic and physical mapping. Nineteen gene-based markers were surveyed for polymorphism detection in 26 Gossypium species. Cluster analysis generated a phylogenetic tree with four major sub-clusters for 23 species while three species branched out individually. CAP method enhanced the rate of polymorphism of candidate gene-based markers between G. hirsutum and G. barbadense. Two hundred A-genome based SSR markers were designed after datamining of G. arboreum EST sequences (Mississippi Gossypium arboreum  EST-SSR: MGAES). Over 70% of MGAES markers successfully produced amplicons while 65 of them demonstrated polymorphism between the parents of G. hirsutum and G. barbadense RIL population and formed 14 linkage groups. Chromosomal localization of both candidate gene-based and MGAES markers was assisted by euploid and hypoaneuploid CS-B analysis. Gene-based and MGAES markers were highly informative as they were designed from candidate genes and fiber transcriptome with a potential to be integrated into the existing cotton genetic and physical maps.

Generation of ESTs for flowering gene discovery and SSR marker development in upland cotton

Background

Upland cotton, Gossypium hirsutum L., is one of the world's most important economic crops. In the absence of the entire genomic sequence, a large number of expressed sequence tag (EST) resources of upland cotton have been generated and used in several studies. However, information about the flower development of this species is rare.

Methodology/Principal Findings

To clarify the molecular mechanism of flower development in upland cotton, 22,915 high-quality ESTs were generated and assembled into 14,373 unique sequences consisting of 4,563 contigs and 9,810 singletons from a normalized and full-length cDNA library constructed from pooled RNA isolated from shoot apexes, squares, and flowers. Comparative analysis indicated that 5,352 unique sequences had no high-degree matches to the cotton public database. Functional annotation showed that several upland cotton homologs with flowering-related genes were identified in our library. The majority of these genes were specifically expressed in flowering-related tissues. Three GhSEP (G. hirsutum L. SEPALLATA) genes determining floral organ development were cloned, and quantitative real-time PCR (qRT-PCR) revealed that these genes were expressed preferentially in squares or flowers. Furthermore, 670 new putative microsatellites with flanking sequences sufficient for primer design were identified from the 645 unigenes. Twenty-five EST–simple sequence repeats were randomly selected for validation and transferability testing in 17 Gossypium species. Of these, 23 were identified as true-to-type simple sequence repeat loci and were highly transferable among Gossypium species.

Conclusions/Significance

A high-quality, normalized, full-length cDNA library with a total of 14,373 unique ESTs was generated to provide sequence information for gene discovery and marker development related to upland cotton flower development. These EST resources form a valuable foundation for gene expression profiling analysis, functional analysis of newly discovered genes, genetic linkage, and quantitative trait loci analysis.

Genome-wide functional analysis of the cotton transcriptome by creating an integrated EST database

A total of 28,432 unique contigs (25,371 in consensus contigs and 3,061 as singletons) were assembled from all 268,786 cotton ESTs currently available. Several in silico approaches [comparative genomics, Blast, Gene Ontology (GO) analysis, and pathway enrichment by Kyoto Encyclopedia of Genes and Genomes (KEGG)] were employed to investigate global functions of the cotton transcriptome. Cotton EST contigs were clustered into 5,461 groups with a maximum cluster size of 196 members. A total of 27,956 indel mutants and 149,616 single nucleotide polymorphisms (SNPs) were identified from consensus contigs. Interestingly, many contigs with significantly high frequencies of indels or SNPs encode transcription factors and protein kinases. In a comparison with six model plant species, cotton ESTs show the highest overall similarity to grape. A total of 87 cotton miRNAs were identified; 59 of these have not been reported previously from experimental or bioinformatics investigations. We also predicted 3,260 genes as miRNAs targets, which are associated with multiple biological functions, including stress response, metabolism, hormone signal transduction and fiber development. We identified 151 and 4,214 EST-simple sequence repeats (SSRs) from contigs and raw ESTs respectively. To make these data widely available, and to facilitate access to EST-related genetic information, we integrated our results into a comprehensive, fully downloadable web-based cotton EST database (www.leonxie.com).

Genomically biased accumulation of seed storage proteins in allopolyploid cotton

Allopolyploidy is an important process during plant evolution that results in the reunion of two divergent genomes into a common nucleus. Many of the immediate as well as longer-term genomic and epigenetic responses to polyploidy have become appreciated. To investigate the modifications of gene expression at the proteome level caused by allopolyploid formation, we conducted a comparative analysis of cotton seed proteomes from the allopolyploid Gossypium hirsutum (AD genome) and its model A-genome and D-genome diploid progenitors. An unexpectedly high level of divergence among the three proteomes was found, with about one-third of all protein forms being genome specific. Comparative analysis showed that there is a higher degree of proteomic similarity between the allopolyploid and its D-genome donor than its A-genome donor, reflecting a biased accumulation of seed proteins in the allopolyploid. Protein identification and genetic characterization of high-abundance proteins revealed that two classes of seed storage proteins, vicilins and legumins, compose the major component of cotton seed proteomes. Analyses further indicate differential regulation or modification of homoeologous gene products, as well as novel patterns in the polyploid proteome that may result from the interaction between homoeologous gene products. Our findings demonstrate that genomic merger and doubling have consequences that extend beyond the transcriptome into the realm of the proteome and that unequal expression of proteins from diploid parental genomes may occur in allopolyploids.

Generation, annotation and analysis of first large-scale expressed sequence tags from developing fiber of Gossypium barbadense L

Background

Cotton fiber is the world's leading natural fiber used in the manufacture of textiles. Gossypium is also the model plant in the study of polyploidization, evolution, cell elongation, cell wall development, and cellulose biosynthesis. G. barbadense L. is an ideal candidate for providing new genetic variations useful to improve fiber quality for its superior properties. However, little is known about fiber development mechanisms of G. barbadense and only a few molecular resources are available in GenBank.

Methodology and Principal Findings

In total, 10,979 high-quality expressed sequence tags (ESTs) were generated from a normalized fiber cDNA library of G. barbadense. The ESTs were clustered and assembled into 5852 unigenes, consisting of 1492 contigs and 4360 singletons. The blastx result showed 2165 unigenes with significant similarity to known genes and 2687 unigenes with significant similarity to genes of predicted proteins. Functional classification revealed that unigenes were abundant in the functions of binding, catalytic activity, and metabolic pathways of carbohydrate, amino acid, energy, and lipids. The function motif/domain-related cytoskeleton and redox homeostasis were enriched. Among the 5852 unigenes, 282 and 736 unigenes were identified as potential cell wall biosynthesis and transcription factors, respectively. Furthermore, the relationships among cotton species or between cotton and other model plant systems were analyzed. Some putative species-specific unigenes of G. barbadense were highlighted.

Conclusions/Significance

The ESTs generated in this study are from the first large-scale EST project for G. barbadense and significantly enhance the number of G. barbadense ESTs in public databases. This knowledge will contribute to cotton improvements by studying fiber development mechanisms of G. barbadense, establishing a breeding program using marker-assisted selection, and discovering candidate genes related to important agronomic traits of cotton through oligonucleotide array. Our work will also provide important resources for comparative genomics, polyploidization, and genome evolution among Gossypium species.

Metabolite and mineral analyses of cotton near-isogenic lines introgressed with QTLs for productivity and drought-related traits

Quantitative trait loci (QTLs) for yield and drought-related traits were exchanged via marker-assisted selection between elite cultivars of two cotton species, Gossypium barbadense (GB) cv. F-177 and Gossypium hirsutum (GH) cv. Siv'on. Three of the resultant near-isogenic lines (NILs), each introgressed with a different QTL region, expressed an advantage in osmotic adjustment (OA) and other drought-related traits relative to their recipient parents. These NILs and the parental genotypes were field-grown under well-watered and water-limited conditions, and characterized for their metabolic and mineral compositions. Comparisons were then made between (1) GB and GH genotypes, (2) the contrasting water regimes and (3) each NIL and its recipient parent. Hierarchical clustering analysis clearly distinguished between GB and GH genotypes based on either metabolite or mineral composition. Comparisons between well-watered and water-limited conditions in each of the genotypes showed differing trends in the various solutes. The greater concentrations of potassium, magnesium and calcium under water stress, when compared with well-watered conditions, may have enhanced OA or osmoprotection. All NILs exhibited significantly modified solute composition relative to their recipient parents. In particular, increased levels of alanine, aspartic acid, citric acid, malic acid, glycerol, myoinositol, threonic acid, potassium, magnesium and calcium were found under drought conditions in one or more of the NILs relative to their recipient parents. The increased values of these solutes could contribute to the superior capacity of these NILs to cope with drought.