Chaobo Tong

Early allopolyploid evolution in the post-neolithic Brassica napus oilseed genome

The genomic origins of rape oilseed Many domesticated plants arose through the meeting of multiple genomes through hybridization and genome doubling, known as polyploidy. Chalhoub et al. sequenced the polyploid genome of Brassica napus, which originated from a recent combination of two distinct genomes approximately 7500 years ago and gave rise to the crops of rape oilseed (canola), kale, and rutabaga. B. napus has undergone multiple events affecting differently sized genetic regions where a gene from one progenitor species has been converted to the copy from a second progenitor species. Some of these gene conversion events appear to have been selected by humans as part of the process of domestication and crop improvement. Science, this issue p. 950 The polyploid genome of oilseed rape exhibits evolution through homologous gene conversion. Oilseed rape (Brassica napus L.) was formed ~7500 years ago by hybridization between B. rapa and B. oleracea, followed by chromosome doubling, a process known as allopolyploidy. Together with more ancient polyploidizations, this conferred an aggregate 72× genome multiplication since the origin of angiosperms and high gene content. We examined the B. napus genome and the consequences of its recent duplication. The constituent An and Cn subgenomes are engaged in subtle structural, functional, and epigenetic cross-talk, with abundant homeologous exchanges. Incipient gene loss and expression divergence have begun. Selection in B. napus oilseed types has accelerated the loss of glucosinolate genes, while preserving expansion of oil biosynthesis genes. These processes provide insights into allopolyploid evolution and its relationship with crop domestication and improvement.

The Brassica oleracea genome reveals the asymmetrical evolution of polyploid genomes

Polyploidization has provided much genetic variation for plant adaptive evolution, but the mechanisms by which the molecular evolution of polyploid genomes establishes genetic architecture underlying species differentiation are unclear. Brassica is an ideal model to increase knowledge of polyploid evolution. Here we describe a draft genome sequence of Brassica oleracea, comparing it with that of its sister species B. rapa to reveal numerous chromosome rearrangements and asymmetrical gene loss in duplicated genomic blocks, asymmetrical amplification of transposable elements, differential gene co-retention for specific pathways and variation in gene expression, including alternative splicing, among a large number of paralogous and orthologous genes. Genes related to the production of anticancer phytochemicals and morphological variations illustrate consequences of genome duplication and gene divergence, imparting biochemical and morphological variation to B. oleracea. This study provides insights into Brassica genome evolution and will underpin research into the many important crops in this genus.

Genome sequencing of the high oil crop sesame provides insight into oil biosynthesis

BackgroundSesame, Sesamum indicum L., is considered the queen of oilseeds for its high oil content and quality, and is grown widely in tropical and subtropical areas as an important source of oil and protein. However, the molecular biology of sesame is largely unexplored.ResultsHere, we report a high-quality genome sequence of sesame assembled de novo with a contig N50 of 52.2 kb and a scaffold N50 of 2.1 Mb, containing an estimated 27,148 genes. The results reveal novel, independent whole genome duplication and the absence of the Toll/interleukin-1 receptor domain in resistance genes. Candidate genes and oil biosynthetic pathways contributing to high oil content were discovered by comparative genomic and transcriptomic analyses. These revealed the expansion of type 1 lipid transfer genes by tandem duplication, the contraction of lipid degradation genes, and the differential expression of essential genes in the triacylglycerol biosynthesis pathway, particularly in the early stage of seed development. Resequencing data in 29 sesame accessions from 12 countries suggested that the high genetic diversity of lipid-related genes might be associated with the wide variation in oil content. Additionally, the results shed light on the pivotal stage of seed development, oil accumulation and potential key genes for sesamin production, an important pharmacological constituent of sesame.ConclusionsAs an important species from the order Lamiales and a high oil crop, the sesame genome will facilitate future research on the evolution of eudicots, as well as the study of lipid biosynthesis and potential genetic improvement of sesame.

Bolbase: a comprehensive genomics database for Brassica oleracea

BackgroundBrassica oleracea is a morphologically diverse species in the family Brassicaceae and contains a group of nutrition-rich vegetable crops, including common heading cabbage, cauliflower, broccoli, kohlrabi, kale, Brussels sprouts. This diversity along with its phylogenetic membership in a group of three diploid and three tetraploid species, and the recent availability of genome sequences within Brassica provide an unprecedented opportunity to study intra- and inter-species divergence and evolution in this species and its close relatives.DescriptionWe have developed a comprehensive database, Bolbase, which provides access to the B. oleracea genome data and comparative genomics information. The whole genome of B. oleracea is available, including nine fully assembled chromosomes and 1,848 scaffolds, with 45,758 predicted genes, 13,382 transposable elements, and 3,581 non-coding RNAs. Comparative genomics information is available, including syntenic regions among B. oleracea, Brassica rapa and Arabidopsis thaliana, synonymous (Ks) and non-synonymous (Ka) substitution rates between orthologous gene pairs, gene families or clusters, and differences in quantity, category, and distribution of transposable elements on chromosomes. Bolbase provides useful search and data mining tools, including a keyword search, a local BLAST server, and a customized GBrowse tool, which can be used to extract annotations of genome components, identify similar sequences and visualize syntenic regions among species. Users can download all genomic data and explore comparative genomics in a highly visual setting.ConclusionsBolbase is the first resource platform for the B. oleracea genome and for genomic comparisons with its relatives, and thus it will help the research community to better study the function and evolution of Brassica genomes as well as enhance molecular breeding research. This database will be updated regularly with new features, improvements to genome annotation, and new genomic sequences as they become available. Bolbase is freely available at http://ocri-genomics.org/bolbase.

Genome-wide comparative analysis of NBS-encoding genes between Brassica species and Arabidopsis thaliana

BackgroundPlant disease resistance (R) genes with the nucleotide binding site (NBS) play an important role in offering resistance to pathogens. The availability of complete genome sequences of Brassica oleracea and Brassica rapa provides an important opportunity for researchers to identify and characterize NBS-encoding R genes in Brassica species and to compare with analogues in Arabidopsis thaliana based on a comparative genomics approach. However, little is known about the evolutionary fate of NBS-encoding genes in the Brassica lineage after split from A. thaliana.ResultsHere we present genome-wide analysis of NBS-encoding genes in B. oleracea, B. rapa and A. thaliana. Through the employment of HMM search and manual curation, we identified 157, 206 and 167 NBS-encoding genes in B. oleracea, B. rapa and A. thaliana genomes, respectively. Phylogenetic analysis among 3 species classified NBS-encoding genes into 6 subgroups. Tandem duplication and whole genome triplication (WGT) analyses revealed that after WGT of the Brassica ancestor, NBS-encoding homologous gene pairs on triplicated regions in Brassica ancestor were deleted or lost quickly, but NBS-encoding genes in Brassica species experienced species-specific gene amplification by tandem duplication after divergence of B. rapa and B. oleracea. Expression profiling of NBS-encoding orthologous gene pairs indicated the differential expression pattern of retained orthologous gene copies in B. oleracea and B. rapa. Furthermore, evolutionary analysis of CNL type NBS-encoding orthologous gene pairs among 3 species suggested that orthologous genes in B. rapa species have undergone stronger negative selection than those in B .oleracea species. But for TNL type, there are no significant differences in the orthologous gene pairs between the two species.ConclusionThis study is first identification and characterization of NBS-encoding genes in B. rapa and B. oleracea based on whole genome sequences. Through tandem duplication and whole genome triplication analysis in B. oleracea, B. rapa and A. thaliana genomes, our study provides insight into the evolutionary history of NBS-encoding genes after divergence of A. thaliana and the Brassica lineage. These results together with expression pattern analysis of NBS-encoding orthologous genes provide useful resource for functional characterization of these genes and genetic improvement of relevant crops.

Shifts in the evolutionary rate and intensity of purifying selection between two Brassica genomes revealed by analyses of orthologous transposons and relics of a whole genome triplication.

Recent sequencing of the Brassica rapa and Brassica oleracea genomes revealed extremely contrasting genomic features such as the abundance and distribution of transposable elements between the two genomes. However, whether and how these structural differentiations may have influenced the evolutionary rates of the two genomes since their split from a common ancestor are unknown. Here, we investigated and compared the rates of nucleotide substitution between two long terminal repeats (LTRs) of individual orthologous LTR-retrotransposons, the rates of synonymous and non-synonymous substitution among triplicated genes retained in both genomes from a shared whole genome triplication event, and the rates of genetic recombination estimated/deduced by the comparison of physical and genetic distances along chromosomes and ratios of solo LTRs to intact elements. Overall, LTR sequences and genic sequences showed more rapid nucleotide substitution in B. rapa than in B. oleracea. Synonymous substitution of triplicated genes retained from a shared whole genome triplication was detected at higher rates in B. rapa than in B. oleracea. Interestingly, non-synonymous substitution was observed at lower rates in the former than in the latter, indicating shifted densities of purifying selection between the two genomes. In addition to evolutionary asymmetry, orthologous genes differentially regulated and/or disrupted by transposable elements between the two genomes were also characterized. Our analyses suggest that local genomic and epigenomic features, such as recombination rates and chromatin dynamics reshaped by independent proliferation of transposable elements and elimination between the two genomes, are perhaps partially the causes and partially the outcomes of the observed inter-specific asymmetric evolution.

Analysis of expression sequence tags from a full-length-enriched cDNA library of developing sesame seeds (Sesamum indicum)

BackgroundSesame (Sesamum indicum) is one of the most important oilseed crops with high oil contents and rich nutrient value. However, genetic improvement efforts in sesame could not get benefit from molecular biology technology due to poor DNA and RNA sequence resources. In this study, we carried out a large scale of expressed sequence tags (ESTs) sequencing from developing sesame seeds and further conducted analysis on seed storage products-related genes.ResultsA normalized and full-length enriched cDNA library from 5 ~ 30 days old immature seeds was constructed and randomly sequenced, leading to generation of 41,248 expressed sequence tags (ESTs) which then formed 4,713 contigs and 27,708 singletons with 44.9% uniESTs being putative full-length open reading frames. Approximately 26,091 of all these uniESTs have significant matches to the counterparts in Nr database of GenBank, and 21,628 of them were assigned to one or more Gene ontology (GO) terms. Homologous genes involved in oil biosynthesis were identified including some conservative transcription factors regulating oil biosynthesis such as LEAFY COTYLEDON1 (LEC1), PICKLE (PKL), WRINKLED1 (WRI1) and majority of them were found for the first time in sesame seeds. One hundred and 17 ESTs were identified possibly involved in biosynthesis of sesame lignans, sesamin and sesamolin. In total, 9,347 putative functional genes from developing seeds were identified, which accounts for one third of total genes in the sesame genome. Further analysis of the uniESTs identified 1,949 non-redundant simple sequence repeats (SSRs).ConclusionsThis study has provided an overview of genes expressed during sesame seed development. This collection of sesame full-length cDNAs covered a wide variety of genes in seeds, in particular, candidate genes involved in biosynthesis of sesame oils and lignans. These EST sequences enriched with full length will contribute to comparative genomic studies on sesame and other oilseed plants and serve as an abundant information platform for functional marker development and functional gene study.

Genomic identification, characterization and differential expression analysis of SBP-box gene family in Brassica napus

BackgroundSBP-box genes belong to one of the largest families of transcription factors. Though members of this family have been characterized to be important regulators of diverse biological processes, information of SBP-box genes in the third most important oilseed crop Brassica napus is largely undefined.ResultsIn the present study, by whole genome bioinformatics analysis and transcriptional profiling, 58 putative members of SBP-box gene family in oilseed rape (Brassica napus L.) were identified and their expression pattern in different tissues as well as possible interaction with miRNAs were analyzed. In addition, B. napus lines with contrasting branch angle were used for investigating the involvement of SBP-box genes in plant architecture regulation. Detailed gene information, including genomic organization, structural feature, conserved domain and phylogenetic relationship of the genes were systematically characterized. By phylogenetic analysis, BnaSBP proteins were classified into eight distinct groups representing the clear orthologous relationships to their family members in Arabidopsis and rice. Expression analysis in twelve tissues including vegetative and reproductive organs showed different expression patterns among the SBP-box genes and a number of the genes exhibit tissue specific expression, indicating their diverse functions involved in the developmental process. Forty-four SBP-box genes were ascertained to contain the putative miR156 binding site, with 30 and 14 of the genes targeted by miR156 at the coding and 3′UTR region, respectively. Relative expression level of miR156 is varied across tissues. Different expression pattern of some BnaSBP genes and the negative correlation of transcription levels between miR156 and its target BnaSBP gene were observed in lines with different branch angle.ConclusionsTaken together, this study represents the first systematic analysis of the SBP-box gene family in Brassica napus. The data presented here provides base foundation for understanding the crucial roles of BnaSBP genes in plant development and other biological processes.

RNA sequencing of Brassica napus reveals cellular redox control of Sclerotinia infection

Protection against Sclerotinia sclerotiorum in Brassica napus is mediated via dynamic transcription factor networks and cellular redox homeostasis directly at the site of infection.

Cysteine Protease 51 (CP51), an anther-specific cysteine protease gene, is essential for pollen exine formation in Arabidopsis

Cysteine proteases play important roles in intracellular protein degradation, programmed cell death and responses to environmental stimuli in plant. Although subclassification and biochemical analysis of major plant papain-like cysteine proteases (PLCPs) have been studied, the biological function of many PLCPs remained unknown. In this study, we identified a PLCP gene Cysteine Protease 51 (CP51) which participates in exine formation and anther development in Arabidopsis thaliana. Promoter-GUS fusion detection showed its specific expression in anthers at stages 9–12. RNA interference (RNAi) transgenic plants with reduced CP51 transcriptional levels exhibited a male sterile phenotype with aborted microspores, shortened siliques and fewer or no seeds. Cytological analysis indicated that pollen abortion occurred due to defective pollen exine and the tapetum degraded earlier during the transition from the uninucleated stage to the binucleated stage. Scanning electron microscopy demonstrated that aborted microspores lacked complete or normal reticulate exine, and the intine membrane was extruded in the pollens of CP51-RNAi plants. Transmission electron microscopy further revealed that the tapetum degeneration was initiated early and that normal tectum connections to the bacula were missing in anthers of CP51-RNAi plants. Taken together, these results suggested that CP51 critically mediates tapetum stability and pollen exine formation.