A bacterial artificial chromosome (BAC) library of sugar beet and a physical map of the region encompassing the bolting gene B

The genome of the recently domesticated crop plant sugar beet (Beta vulgaris)

Sugar beet (Beta vulgaris ssp. vulgaris) is an important crop of temperate climates which provides nearly 30% of the world's annual sugar production and is a source for bioethanol and animal feed. The species belongs to the order of Caryophylalles, is diploid with 2n = 18 chromosomes, has an estimated genome size of 714-758 megabases and shares an ancient genome triplication with other eudicot plants. Leafy beets have been cultivated since Roman times, but sugar beet is one of the most recently domesticated crops. It arose in the late eighteenth century when lines accumulating sugar in the storage root were selected from crosses made with chard and fodder beet. Here we present a reference genome sequence for sugar beet as the first non-rosid, non-asterid eudicot genome, advancing comparative genomics and phylogenetic reconstructions. The genome sequence comprises 567 megabases, of which 85% could be assigned to chromosomes. The assembly covers a large proportion of the repetitive sequence content that was estimated to be 63%. We predicted 27,421 protein-coding genes supported by transcript data and annotated them on the basis of sequence homology. Phylogenetic analyses provided evidence for the separation of Caryophyllales before the split of asterids and rosids, and revealed lineage-specific gene family expansions and losses. We sequenced spinach (Spinacia oleracea), another Caryophyllales species, and validated features that separate this clade from rosids and asterids. Intraspecific genomic variation was analysed based on the genome sequences of sea beet (Beta vulgaris ssp. maritima; progenitor of all beet crops) and four additional sugar beet accessions. We identified seven million variant positions in the reference genome, and also large regions of low variability, indicating artificial selection. The sugar beet genome sequence enables the identification of genes affecting agronomically relevant traits, supports molecular breeding and maximizes the plant's potential in energy biotechnology.

A sugar beet (Beta vulgaris L.) reference FISH karyotype for chromosome and chromosome-arm identification, integration of genetic linkage groups and analysis of major repeat family distribution

We developed a reference karyotype for B. vulgaris which is applicable to all beet cultivars and provides a consistent numbering of chromosomes and genetic linkage groups. Linkage groups of sugar beet were assigned to physical chromosome arms by FISH (fluorescent in situ hybridization) using a set of 18 genetically anchored BAC (bacterial artificial chromosome) markers. Genetic maps of sugar beet were correlated to chromosome arms, and North-South orientation of linkage groups was established. The FISH karyotype provides a technical platform for genome studies and can be applied for numbering and identification of chromosomes in related wild beet species. The discrimination of all nine chromosomes by BAC probes enabled the study of chromosome-specific distribution of the major repetitive components of sugar beet genome comprising pericentromeric, intercalary and subtelomeric satellites and 18S-5.8S-25S and 5S rRNA gene arrays. We developed a multicolor FISH procedure allowing the identification of all nine sugar beet chromosome pairs in a single hybridization using a pool of satellite DNA probes. Fiber-FISH was applied to analyse five chromosome arms in which the furthermost genetic marker of the linkage group was mapped adjacently to terminal repetitive sequences on pachytene chromosomes. Only on two arms telomere arrays and the markers are physically linked, hence these linkage groups can be considered as terminally closed making the further identification of distal informative markers difficult. The results support genetic mapping by marker localization, the anchoring of contigs and scaffolds for the annotation of the sugar beet genome sequence and the analysis of the chromosomal distribution patterns of major families of repetitive DNA.

Palaeohexaploid ancestry for Caryophyllales inferred from extensive gene-based physical and genetic mapping of the sugar beet genome (Beta vulgaris)

Sugar beet (Beta vulgaris) is an important crop plant that accounts for 30% of the world's sugar production annually. The genus Beta is a distant relative of currently sequenced taxa within the core eudicotyledons; the genomic characterization of sugar beet is essential to make its genome accessible to molecular dissection. Here, we present comprehensive genomic information in genetic and physical maps that cover all nine chromosomes. Based on this information we identified the proposed ancestral linkage groups of rosids and asterids within the sugar beet genome. We generated an extended genetic map that comprises 1127 single nucleotide polymorphism markers prepared from expressed sequence tags and bacterial artificial chromosome (BAC) end sequences. To construct a genome-wide physical map, we hybridized gene-derived oligomer probes against two BAC libraries with 9.5-fold cumulative coverage of the 758 Mbp genome. More than 2500 probes and clones were integrated both in genetic maps and the physical data. The final physical map encompasses 535 chromosomally anchored contigs that contains 8361 probes and 22 815 BAC clones. By using the gene order established with the physical map, we detected regions of synteny between sugar beet (order Caryophyllales) and rosid species that involves 1400-2700 genes in the sequenced genomes of Arabidopsis, poplar, grapevine, and cacao. The data suggest that Caryophyllales share the palaeohexaploid ancestor proposed for rosids and asterids. Taken together, we here provide extensive molecular resources for sugar beet and enable future high-resolution trait mapping, gene identification, and cross-referencing to regions sequenced in other plant species.

Conservation and divergence of autonomous pathway genes in the flowering regulatory network of Beta vulgaris

The transition from vegetative growth to reproductive development is a complex process that requires an integrated response to multiple environmental cues and endogenous signals. In Arabidopsis thaliana, which has a facultative requirement for vernalization and long days, the genes of the autonomous pathway function as floral promoters by repressing the central repressor and vernalization-regulatory gene FLC. Environmental regulation by seasonal changes in daylength is under control of the photoperiod pathway and its key gene CO. The root and leaf crop species Beta vulgaris in the caryophyllid clade of core eudicots, which is only very distantly related to Arabidopsis, is an obligate long-day plant and includes forms with or without vernalization requirement. FLC and CO homologues with related functions in beet have been identified, but the presence of autonomous pathway genes which function in parallel to the vernalization and photoperiod pathways has not yet been reported. Here, this begins to be addressed by the identification and genetic mapping of full-length homologues of the RNA-regulatory gene FLK and the chromatin-regulatory genes FVE, LD, and LDL1. When overexpressed in A. thaliana, BvFLK accelerates bolting in the Col-0 background and fully complements the late-bolting phenotype of an flk mutant through repression of FLC. In contrast, complementation analysis of BvFVE1 and the presence of a putative paralogue in beet suggest evolutionary divergence of FVE homologues. It is further shown that BvFVE1, unlike FVE in Arabidopsis, is under circadian clock control. Together, the data provide first evidence for evolutionary conservation of components of the autonomous pathway in B. vulgaris, while also suggesting divergence or subfunctionalization of one gene. The results are likely to be of broader relevance because B. vulgaris expands the spectrum of evolutionarily diverse species which are subject to differential developmental and/or environmental regulation of floral transition.

Epigenetic profiling of heterochromatic satellite DNA

Sugar beet (Beta vulgaris) chromosomes consist of large heterochromatic blocks in pericentromeric, centromeric, and intercalary regions comprised of two different highly abundant DNA satellite families. To investigate DNA methylation at single base resolution at heterochromatic regions, we applied a method for strand-specific bisulfite sequencing of more than 1,000 satellite monomers followed by statistical analyses. As a result, we uncovered diversity in the distribution of different methylation patterns in both satellite families. Heavily methylated CG and CHG (H=A, T, or C) sites occur more frequently in intercalary heterochromatin, while CHH sites, with the exception of CAA, are only sparsely methylated, in both intercalary and pericentromeric/centromeric heterochromatin. We show that the difference in DNA methylation intensity is correlated to unequal distribution of heterochromatic histone H3 methylation marks. While clusters of H3K9me2 were absent from pericentromeric heterochromatin and restricted only to intercalary heterochromatic regions, H3K9me1 and H3K27me1 were observed in all types of heterochromatin. By sequencing of a small RNA library consisting of 6.76 million small RNAs, we identified small interfering RNAs (siRNAs) of 24 nucleotides in size which originated from both strands of the satellite DNAs. We hypothesize an involvement of these siRNAs in the regulation of DNA and histone methylation for maintaining heterochromatin.

High-throughput identification of genetic markers using representational oligonucleotide microarray analysis

We describe a novel approach for high-throughput development of genetic markers using representational oligonucleotide microarray analysis. We test the performance of the method in sugar beet (Beta vulgaris L.) as a model for crop plants with little sequence information available. Genomic representations of both parents of a mapping population were hybridized on microarrays containing in total 146,554 custom made oligonucleotides based on sugar beet bacterial artificial chromosome (BAC) end sequences and expressed sequence tags (ESTs). Oligonucleotides showing a signal with one parental line only, were selected as potential marker candidates and placed onto an array, designed for genotyping of 184 F(2) individuals from the mapping population. Utilizing known co-dominant anchor markers we obtained 511 new dominant markers (392 derived from BAC end sequences, and 119 from ESTs) distributed over all nine sugar beet linkage groups and calculated genetic maps. Further improvements for large-scale application of the approach are discussed and its feasibility for the cost-effective and flexible generation of genetic markers is presented.

An abundant and heavily truncated non-LTR retrotransposon (LINE) family in Beta vulgaris

We describe a non-LTR retrotransposon family,BvL, of the long interspersed nuclear elements L1 clade isolated from sugar beet (Beta vulgaris). Characteristic molecular domains of three full-length BvL elements were determined in detail, showing that coding sequences are interrupted and most likely non-functionally. In addition,eight highly conserved endonuclease regions were defined by comparison with other plant LINEs. The abundant BvL family is widespread within the genus Beta, however, the vast majority of BvL copies are extremely 50 truncated indicating an error-prone reverse transcriptase activity. The dispersed distribution of BvL copies on all sugar beet chromosomes with exclusion of most heterochromatic regions was shown by fluorescent in situ hybridization. The analysis of BvL 30 end sequences and corresponding flanking regions, respectively, revealed the preferred integration of BvL into A/T-rich regions of the sugar beet genome, but no specific target sequences.

Construction and characterization of a sugar beet (Beta vulgaris) fosmid library

A sugar beet (Beta vulgaris) fosmid library from the doubled haploid accession KWS2320 encompassing 115 200 independent clones was constructed and characterized. The average insert size of the fosmid library was determined by pulsed field gel electrophoresis to be 39 kbp on average, thus representing 5.9-fold coverage of the sugar beet genome (758 Mbp). PCR screening of plate pools with primer pairs against nine sugar beet genes supported the insert size estimation. BLAST searches with 2951 fosmid end-sequences originating from 1510 clones (1536 clones attempted) revealed little contamination with organellar DNA (2.1% chloroplast DNA, 0.3% mitochondrial DNA). The sugar beet fosmid library will be integrated in the presently ongoing efforts to determine the sequence of the sugar beet genome. Fosmids will be publicly available in the format of plate pools and individual clones.

Haplotype divergence in Beta vulgaris and microsynteny with sequenced plant genomes

We characterized two overlapping sugar beet (Beta vulgaris) bacterial artificial chromosome (BAC) clones representing different haplotypes. A total of 254 kbp of the genomic sequence was determined, of which the two BACs share 92 kbp. Eleven of 15 genes discovered in the sequenced interval locate to the overlap region. The haplotypes differ in exons by 1% (nucleotide level) and in non-coding regions by 9% (6% mismatches, 3% gaps; alignable regions only). Large indels or high sequence divergence comprised 11% of either sequence. Of such indels, 68 and 45%, respectively, could be attributed to haplotype-specific integration of transposable elements. We identified novel repeat candidates by comparing the two BAC sequences to a set of genomic sugar beet sequences. Synteny was found with Arabidopsis chromosome 1 (At1), At2 and At4, Medicago chromosome 7, Vitis chromosome 15 and paralogous regions on poplar chromosomes II and XIV.

Diversity of a complex centromeric satellite and molecular characterization of dispersed sequence families in sugar beet (Beta vulgaris)

BACKGROUND AND AIMS

The aim of this work was the identification and molecular characterization of novel sugar beet (Beta vulgaris) repetitive sequences to unravel the impact of repetitive DNA on size and evolution of Beta genomes via amplification and diversification.

METHODS

Genomic DNA and a pool of B. vulgaris repetitive sequences were separately used as probes for a screening of high-density filters from a B. vulgaris plasmid library. Novel repetitive motifs were identified by sequencing and further used as probes for Southern analyses in the genus Beta. Chromosomal localization of the repeats was analysed by fluorescent in situ hybridization on chromosomes of B. vulgaris and two other species of the section Beta.

KEY RESULTS

Two dispersed repetitive families pDvul1 and pDvul2 and the tandemly arranged repeat family pRv1 were isolated from a sugar beet plasmid library. The dispersed repetitive families pDvul1 and pDvul2 were identified in all four sections of the genus Beta. The members of the pDvul1 and pDvul2 family are scattered over all B. vulgaris chromosomes, although amplified to a different extent. The pRv1 satellite repeat is exclusively present in species of the section Beta. The centromeric satellite pBV1 by structural variations of the monomer and interspersion of pRv1 units forms complex satellite structures, which are amplified in different degrees on the centromeres of 12 chromosomes of the three species of the Beta section.

CONCLUSIONS

The complexity of the pBV1 satellite family observed in the section Beta of the genus Beta and, in particular, the strong amplification of the pBV1/pRv1 satellite in the domesticated B. vulgaris indicates the dynamics of centromeric satellite evolution during species radiation within the genus. The dispersed repeat families pDvul1 and pDvul2 might represent derivatives of transposable elements.

A complete physical map of a wild beet (Beta procumbens) translocation in sugar beet

Two sugar beet lines carry homologous translocations of the wild beet Beta procumbens. Long-range restriction mapping with rare cutting enzymes revealed that both translocations are different in size, however, an overlapping region of about 350 kb could be identified. Both lines are resistant to the beet cyst nematode but only TR520 carries the previously cloned resistance gene Hs1pro-1. Hence, a second gene for nematode resistance (Hs1-1) must be located within this region. A bacterial artificial chromosome (BAC) library was constructed from line TR520. The library was screened with a number of B. procumbens specific probes and 61 BAC clones were identified. Five BAC clones formed a minimal tiling path of 580 kb to cover the overlapping region between both translocations including the translocation breakpoint. The five BACs from the overlapping region and one additional BAC distal from that contig were sequenced. The total sequence length from the five BACs of the overlapping region amounted to 524 kb which is 74.35% of the total insert size of these BACs. The frequency of retrotransposon sequences ranged between 14.7 and 43.3%. A total of 133 ORFs were identified, none of these showed similarity to known disease resistance genes. Of these, 12 ORFs showed homology to genes involved in biotic stress resistance reactions or to transcription factors. This paper demonstrates how genome specific probes can be employed for cloning an alien gene introgression into a cultivated species.

Sugar beet BAC library construction and assembly of a contig spanning Rf1, a restorer-of-fertility gene for Owen cytoplasmic male sterility

Rf1 is a nuclear gene that controls fertility restoration in cases of cytoplasmic male sterility caused by the Owen cytoplasm in sugar beet. In order to isolate the gene by positional cloning, a BAC library was constructed from a restorer line, NK198, with the genotype Rf1Rf1. The library contained 32,180 clones with an average insert size of 97.8 kb, providing 3.4 genome equivalents. Five AFLP markers closely linked to Rf1 were used to screen the library. As a result, we identified eight different BAC clones that were clustered into two contigs. The gap between the two contigs was filled by chromosome walking. To map the Rf1 region in more detail, we developed five cleaved amplified polymorphic sequence (CAPS) markers from the BAC DNAs identified, and carried out genotyping of 509 plants in the mapping population with the Rf1-flanking AFLP and CAPS markers. Thirteen plants in which recombination events had occurred in the vicinity of the Rf1 locus were identified and used to map the molecular markers relative to each other and to Rf1. In this way, we were able to restrict the possible location of the Rf1 gene to a minimum of six BAC clones spanning an interval of approximately 250 kb.

Bacterial artificial chromosome-derived molecular markers for early bolting in sugar beet

Early bolting in sugar beet (Beta vulgaris L.) is controlled by the dominant gene B. From an incomplete physical map around the B gene, 18 bacterial artificial chromosomes (BACs) were selected for marker development. Three BACs were shotgun-sequenced, and 61 open reading frames (ORFs) were identified. Together with 104 BAC ends from 54 BACs, a total number of 55,464 nucleotides were sequenced. Of these, 37 BAC ends and 12 ORFs were selected for marker development. Thirty-one percent of the sequences were found to be single copy and 24%, low copy. From these sequences, 15 markers from ten different BACs were developed. Ten polymorphisms were determined by simple agarose gel electrophoresis of either restricted or non-restricted PCR products. Another five markers were determined by tetra-primer amplification refractory mutation system-PCR. In order to select candidate BACs for cloning the gene, genetic linkage between seven markers and the bolting gene was calculated using 1,617 plants from an F2 population segregating for early bolting. The recombination values ranged between 0.0033 and 0.0201. In addition, a set of 41 wild and cultivated Beta accessions differing in their early bolting character was genotyped with seven markers. A common haplotype encompassing two marker loci and the b allele was found in all sugar beet varieties, indicating complete linkage disequilibrium between these loci. This suggests that the bolting gene is located in close vicinity to these markers, and the corresponding BACs can be used for cloning the gene.