Correction to: A high-quality cucumber genome assembly enhances computational comparative genomics.

The authors would like to correct the citation for the North European B10 line.

A high-quality cucumber genome assembly enhances computational comparative genomics.

Genetic variation is expressed by the presence of polymorphisms in compared genomes of individuals that can be transferred to next generations. The aim of this work was to reveal genome dynamics by predicting polymorphisms among the genomes of three individuals of the highly inbred B10 cucumber (Cucumis sativus L.) line. In this study, bioinformatic comparative genomics was used to uncover cucumber genome dynamics (also called real-time evolution). We obtained a new genome draft assembly from long single molecule real-time (SMRT) sequencing reads and used short paired-end read data from three individuals to analyse the polymorphisms. Using this approach, we uncovered differentiation aspects in the genomes of the inbred B10 line. The newly assembled genome sequence (B10v3) has the highest contiguity and quality characteristics among the currently available cucumber genome draft sequences. Standard and newly designed approaches were used to predict single nucleotide and structural variants that were unique among the three individual genomes. Some of the variant predictions spanned protein-coding genes and their promoters, and some were in the neighbourhood of annotated interspersed repetitive elements, indicating that the highly inbred homozygous plants remained genetically dynamic. This is the first bioinformatic comparative genomics study of a single highly inbred plant line. For this project, we developed a polymorphism prediction method with optimized precision parameters, which allowed the effective detection of small nucleotide variants (SNVs). This methodology could significantly improve bioinformatic pipelines for comparative genomics and thus has great practical potential in genomic metadata handling.

Molecular Cytogenetic Analysis of Cucumis Wild Species Distributed in Southern Africa: Physical Mapping of 5S and 45S rDNA with DAPI.

Wild Cucumis species have been divided into Australian/Asian and African groups using morphological and phylogenetic characteristics, and new species have been described recently. No molecular cytogenetic information is available for most of these species. The crossability between 5 southern African Cucumis species (C. africanus, C. anguria, C. myriocarpus, C. zeyheri, and C. heptadactylus) has been reported; however, the evolutionary relationship among them is still unclear. Here, a molecular cytogenetic analysis using FISH with 5S and 45 S ribosomal DNA (rDNA) was used to investigate these Cucumis species based on sets of rDNA-bearing chromosomes (rch) types I, II and III. The molecular cytogenetic and phylogenetic results suggested that at least 2 steps of chromosomal rearrangements may have occurred during the evolution of tetraploid C. heptadactylus. In step 1, an additional 45 S rDNA site was observed in the chromosome (type III). In particular, C. myriocarpus had a variety of rch sets. Our results suggest that chromosomal rearrangements may have occurred in the 45 S rDNA sites. We propose that polyploid evolution occurred in step 2. This study provides insights into the chromosomal characteristics of African Cucumis species and contributes to the understanding of chromosomal evolution in this genus.

Karyotype analysis and chromosomal distribution of repetitive DNA sequences of cucumis metuliferus using fluorescence in situ hybridization.

Cucumis metuliferus (2n = 24) is a cultivated species of the Cucumis genus which is a potential genetic resource for Cucumis crops. Although some cytogenetic research has been reported, there is no study of karyotyping in this species. Here, we used 4',6-diamidino-2-phenylindole and chromomycin A3 staining to identify 12 pairs of chromosomes in early-metaphase cells. Fluorescence in situ hybridization revealed the chromosomal distribution patterns of the 5S and 45S ribosomal DNA (rDNA) genes, telomeres, and 3 different satellite repeats. The 2 major signals of the 45S rDNA were located on the satellite of chromosome 11, and the 2 signals of the 5S rDNA and 2 minor signals of the 45S rDNA were located on chromosome 12. The telomere probes hybridized to the ends of all chromosomes. The 3 satellite DNAs were localized at the ends of chromosomes 1, 2, 4-10, and at the end of the short arm of chromosome 3. In summary, we reported the identification of all chromosomes of C. metuliferus. We also depicted the location of 5S and 45S rDNA, the telomere motif sequence, CmetSat1, CmetSatT2, and CmetmSat1 in an ideogram.

Comparative transcriptome analysis reveals new molecular pathways for cucumber genes related to sex determination.

KEY MESSAGE: Transcriptome data and qPCR analysis revealed new insight into genes regulatory mechanism related to cucumber sex determination. Cucumber (Cucumis sativus L.) is an economically important crop cultivated worldwide. Enhancing the genomic resources for cucumber may enable the regulation of traits relevant to crop productivity and quality. Sequencing technologies and bioinformatics tools provide opportunities for the development of such resources. The aims of this study were to identify and characterize the genes involved in sex determination and flower morphogenesis in cucumber isogenic lines that differed regarding flower sex type. We obtained transcripts for 933 genes related to shoot apex development, among which 310 were differentially expressed genes (DEGs) among the male, female, and hermaphroditic lines. We performed gene ontology and molecular network analyses and explored the DEGs related to already known processes like: hormone synthesis and signaling, lipid and sugar metabolism; and also newly discovered processes related to cell wall, membrane, and cytoskeleton modifications; ion homeostasis which appears to be important for ethylene perception and signaling, and genes expression mediated by transcription factors related to floral organ identities. We proposed a new model of regulatory mechanism network of sex development in cucumber. Our results may be useful for clarifying the molecular genetics and the functional mechanisms underlying the sex determination processes.

The construction and characteristics of a BAC library for Cucumis sativus L. 'B10'.

Cloning using bacterial artificial chromosomes (BACs) can yield high quality genomic libraries, which are used for the physical mapping, identification and isolation of genes, and for gene sequencing. A BAC genomic library was constructed from high molecular weight DNA (HMW DNA) obtained from nuclei of the cucumber (Cucumis sativus L. cv. Borszczagowski; B10 line). The DNA was digested with the HindIII restriction enzyme and ligated into the pCC1BAC vector. The library consists of 34,560 BAC clones with an average insert size of 135 kb, and 12.7x genome coverage. Screening the library for chloroplast and mitochondrial DNA content indicated an exceptionally low 0.26% contamination with chloroplast DNA and 0.3% with mitochondrial DNA.

In situ reverse transcription PCR on plant tissues.

In situ detection techniques allow specific nucleic acid sequences to be exposed in morphologically preserved tissue sections. In combination with immunocytochemistry, in situ detection can relate microscopic topological information to gene activity at the transcript or protein levels in specific tissues. The advantage of in situ methods over the conventional techniques (e.g., Northern blot, reverse transcription polymerase chain reaction [RT-PCR], or real-time PCR) is that they allow the investigation of the putative spatial distribution of nucleic acid products activity in a heterogeneous cell population. In this chapter, we describe a protocol for in situ RT-PCR detection of specific messenger RNA in cucumber (Cucumis sativus), although this protocol can be used for any plant species, floral buds, and somatic embryo tissue sections on glass microscope slides. A successful in situ RT-PCR procedure requires the optimization of many conditions related to the tissue types used, for example, a cell's age, size, and composition, which may influence the detection of RT-PCR products, as well as specific transcript availability. Moreover, parameters, such as the fixation time, thermal cycling set-up, and the time of detection of RT-PCR products, also should be optimized. The importance of the other factors also is estimated in the protocol. In addition several types of controls that are necessary for a trustworthy in situ RT-PCR method are being discussed.

Next generation sequencing and omics in cucumber (Cucumis sativus L.) breeding directed research.

In the post-genomic era the availability of genomic tools and resources is leading us to novel generation methods in plant breeding, as they facilitate the study of the genotype and its relationship with the phenotype, in particular for complex traits. In this study we have mainly concentrated on the Cucumis sativus and (but much less) Cucurbitaceae family several important vegetable crops. There are many reports on research conducted in Cucurbitaceae plant breeding programs on the ripening process, phloem transport, disease resistance, cold tolerance and fruit quality traits. This paper presents the role played by new omic technologies in the creation of knowledge on the mechanisms of the formation of the breeding features. The analysis of NGS (NGS-next generation sequencing) data allows the discovery of new genes and regulatory sequences, their positions, and makes available large collections of molecular markers. Genome-wide expression studies provide breeders with an understanding of the molecular basis of complex traits. Firstly a high density map should be created for the reference genome, then each re-sequencing data could be mapped and new markers brought out into breeding populations. The paper also presents methods that could be used in the future for the creation of variability and genomic modification of the species in question. It has been shown also the state and usefulness in breeding the chloroplastomic and mitochondriomic study.

Identification of genes up-regulated during somatic embryogenesis of cucumber.

Somatic embryogenesis is a method of plant regeneration, but it can also be used as a model to study plant development. A normalized library of cDNA fragments representing genes up-regulated after the induction of somatic embryogenesis in cucumber suspension cultures was constructed using the suppression subtractive hybridization technique. Candidate cDNA fragments (119) were classified according to their similarity to genes encoding known proteins and the presence of potential functional domains. Of the translation products with homology to known proteins, about 23% were possibly involved in metabolism, 13% represented proteins with a probable role in cellular communication and signal transduction, about 12% were likely to participate in protein synthesis, while around 10% were potential transcription factors. The genes corresponding to four of the cDNAs were subsequently analyzed in more detail: CsSEF2, CsSEM1 and CsSESTK1 encoding putative transcription factors or co-activators, and CsSECAD1 encoding cinnamyl alcohol dehydrogenase. Full-length cDNAs were isolated and analyzed. RT-PCR confirmed the up-regulation of these genes after the induction of somatic embryogenesis and showed the presence of their transcripts in other tissues. The in situ localization of transcripts of the CsSEF2 and CsSEM1 genes demonstrated that signalling in somatic embryo tissues involving these factors is concentrated in the cotyledon primordia and roots.

The genome sequence of the North-European cucumber (Cucumis sativus L.) unravels evolutionary adaptation mechanisms in plants.

Cucumber (Cucumis sativus L.), a widely cultivated crop, has originated from Eastern Himalayas and secondary domestication regions includes highly divergent climate conditions e.g. temperate and subtropical. We wanted to uncover adaptive genome differences between the cucumber cultivars and what sort of evolutionary molecular mechanisms regulate genetic adaptation of plants to different ecosystems and organism biodiversity. Here we present the draft genome sequence of the Cucumis sativus genome of the North-European Borszczagowski cultivar (line B10) and comparative genomics studies with the known genomes of: C. sativus (Chinese cultivar--Chinese Long (line 9930)), Arabidopsis thaliana, Populus trichocarpa and Oryza sativa. Cucumber genomes show extensive chromosomal rearrangements, distinct differences in quantity of the particular genes (e.g. involved in photosynthesis, respiration, sugar metabolism, chlorophyll degradation, regulation of gene expression, photooxidative stress tolerance, higher non-optimal temperatures tolerance and ammonium ion assimilation) as well as in distributions of abscisic acid-, dehydration- and ethylene-responsive cis-regulatory elements (CREs) in promoters of orthologous group of genes, which lead to the specific adaptation features. Abscisic acid treatment of non-acclimated Arabidopsis and C. sativus seedlings induced moderate freezing tolerance in Arabidopsis but not in C. sativus. This experiment together with analysis of abscisic acid-specific CRE distributions give a clue why C. sativus is much more susceptible to moderate freezing stresses than A. thaliana. Comparative analysis of all the five genomes showed that, each species and/or cultivars has a specific profile of CRE content in promoters of orthologous genes. Our results constitute the substantial and original resource for the basic and applied research on environmental adaptations of plants, which could facilitate creation of new crops with improved growth and yield in divergent conditions.

AFLP marker polymorphism in cucumber (Cucumis sativus L.) near isogenic lines differing in sex expression.

The AFLP technique was used to evaluate the level of polymorphism between two pairs of isogenic cucumber (Cucumis sativus L.) lines (NIL) differing in flower sex expression. The BSA techniques were also applied to find molecular markers linked to sex determination genes (dominant alleles) in those cucumber lines. Sex determination in cucumber is controlled by three main loci F, M and Gy. The interaction of these loci is responsible for the formation of the various phenotypes of flowers in respect to sex in the analyzed lines [corrected]. A female line 2gg with a ff/MM/gygy genotype, isogenic to a monoecious line B10 (genotype ff/MM/GyGy), and a female line Gy3 with a FF/MM/GyGy genotype, isogenic to a hermaphroditic line HGy3 (genotype FF/mm/GyGy). Using 56 combinations of AFLP primers, used for the analysis of lines 2gg and B10, gave 3794 bands, of which 155 (4.1%) were polymorphic. Ten bands distinguished gynoecious and monoecious bulks appearing at the same time in the appropriate parent; they are believed to be linked to the Gy locus. The isogenic lines Gy3 and HGy3 showed a higher level of polymorphism (14.2%). In this case, 55 combinations of primers gave 2996 reaction products, of which 430 showed variation. Twenty bands occurred in one bulk and in one parent, so they are probably associated with the M locus. Using the AFLP technique, the isogenicity of the lines was evaluated. The level of polymorphism (per pair of primer) between lines 2gg and B10 is 0.072% and is four times lower than that between the Gy3 and HGy3 lines (0.27%). The differences in the isogenicity of the lines can result from the degree of their relatedness, which may reflect the way they were derived.

A useful protocol for in situ RT-PCR on plant tissues.

This study describes an effective method of in situ RT-PCR (RT-ISPCR) that was developed to localize gene expression in plant tissues. This RT-PCR technique was performed on sectioned tissues of female buds of the cucumber GY3 inbred line. The CUS1 gene, encoding the MADS-box type (agamus-like) protein, the expression pattern of which was described earlier, was used as a marker gene for optimisation of steps in the in situ RT-PCR inside the cells. For the identification of RT-PCR products inside the cells of the female buds, they were fixed in FAA solution, embedded in Paraplast Plus and cut into 7 microm thick sections which were dewaxed by immersion in HistoClear and dehydrated with ethanol. They were washed in water, then in 0.02M HCl, 2xSSC and PBS buffer. In the next step of tissue pretreatment, the sections were digested with 1% pectinase. As shown, the pectinase treatment proved to be a crucial step in the tissue preparation procedure to get successful RT-PCR products. After washing in PBS buffer, the sections were digested with protease K followed by incubation with RNase-free DNase I, and subsequently washed in 2xSSC, 1xSSC and 0.5xSSC and finally in DEPC-treated water. Then the sections were covered with 50 microl of the RT-PCR reaction mixture supplemented with 0.5 microM digoxigenin dUTP and sealed with a coverslip. After amplification in situ the PCR products were identified with anti-digoxigenin antibody (Roche Molecular Biochemicals), conjugated with alkaline phosphatase. The data obtained showed that specific signals reflecting CUS1 gene expression were detected in the female flower buds of cucumber. The specificity of the in situ RT-PCR protocol was confirmed by dot blot hybridization of RT-PCR products with CUS1 cDNA probe.

Polymorphom of sexually different cucumber (Cucumis sativus L.) NIL lines.

Isolations of polymorphic sequences of two pairs of the NIL lines of cucumber (Cucumis sativus L.), which differ with respect to sex, were carried out using the subtraction hybridization methods of DSC (Differential Subtraction Chain) and GDDSC (Genetically Directed DSC). 266 DSC tags were isolated from the entire genome region, and 38 GDDSC tags were isolated from the region containing the sex genes. Based on the obtained results, the methods used may be considered highly effective. The attained sequences, like 11 AFLP clones obtained earlier [Witkowicz, J. et al. Cell. Mol. Biol. Lett. 8 (2003) 375-381], were characterized by analyzing their hybridization with differential (dhaom) and subtractive cDNA libraries (cDNAsubtractom) from 1- to 2- mm floral buds of the same lines, and by the sequencing of 28 tags. A high average degree of homology was found to exist in the genpolom to dhom and cDNAsubtractom, particularly in the case of "dominant" (when the tester used was a line in which the sex of the plants was dependent upon the dominant allele). This indicates a significant share of coding sequences in the polymorphic genomic tags as well as their share in flower formation. Many of these sequences originate from the sex gene region. Analysis of the sequenced tags showed their interesting composition, including many organelle sequences which transferred into the nucleus, and coding.

The isolation of cDNA clones from cucumber (Cucumis sativus L.) floral buds coming from plants differing in sex.

In this study, we found flower cDNA clones which may be connected with the development of flower sex in cucumber. Two pairs of nearly-isogenic lines: gynoecious GY3 (FFMMGG) versus hermaphrodite HGY3 (FFmmGG) and monoecious B10 (ffMMGG) versus gynoecious 2gg (ffMMgg) were used for clone isolation. To obtain differentially-expressed clones, we applied the differential screening method. 454 clones from GY3 and 478 from B10 cDNA libraries were isolated. The results of RFLP analysis with 56 cDNA clones showed no clones which cosegregated with sex in cucumber. The 28 cDNA B10 and 33 cDNA GY3 clones isolated using the differential screening method were sequenced. Some of them seem to may play a role in cell differentiation or flower development. Among the 61 identified clones, 14 show high homology to plant proteins, although of unknown function. 11 show high homology to known proteins, and the possible function of some of them is discussed. For 3 clones, no significant similarity was found. The 31 clones displayed high homology to plant cDNA in EST database. The patterns of expression of five differential cDNA clones, 35GY3, 216GY3, 47GY3, 100B10 and 157B10, were analyzed in cucumber flower buds using in situ RT-PCR. The most interesting clone is 35GY3, because of its possible role in the inhibition of the development of male specific elements in the female cucumber flower.