BnaC.Tic40, a plastid inner membrane translocon originating from Brassica oleracea, is essential for tapetal function and microspore development in Brassica napus

Here, we describe the characteristics of a Brassica napus male sterile mutant 7365A with loss of the BnMs3 gene, which exhibits abnormal enlargement of the tapetal cells during meiosis. Later in development, the absence of the BnMs3 gene in the mutant results in a loss of the secretory function of the tapetum, as suggested by abortive callose dissolution and retarded tapetal degradation. The BnaC.Tic40 gene (equivalent to BnMs3) was isolated by a map-based cloning approach and was confirmed by genetic complementation. Sequence analyses suggested that BnaC.Tic40 originated from BolC.Tic40 on the Brassica oleracea linkage group C9, whereas its allele Bnms3 was derived from BraA.Tic40 on the Brassica rapa linkage group A10. The BnaC.Tic40 gene is highly expressed in the tapetum and encodes a putative plastid inner envelope membrane translocon, Tic40, which is localized into the chloroplast. Transmission electron microscopy (TEM) and lipid staining analyses suggested that BnaC.Tic40 is a key factor in controlling lipid accumulation in the tapetal plastids. These data indicate that BnaC.Tic40 participates in specific protein translocation across the inner envelope membrane in the tapetal plastid, which is required for tapetal development and function.     

Identification of FAD2 and FAD3 genes in Brassica napus genome and development of allele-specific markers for high oleic and low linolenic acid contents

Modification of oleic acid (C18:1) and linolenic acid (C18:3) contents in seeds is one of the major goals for quality breeding after removal of erucic acid in oilseed rape (Brassica napus). The fatty acid desaturase genes FAD2 and FAD3 have been shown as the major genes for the control of C18:1 and C18:3 contents. However, the genome structure and locus distributions of the two gene families in amphidiploid B. napus are still not completely understood to date. In the present study, all copies of FAD2 and FAD3 genes in the A- and C-genome of B. napus and its two diploid progenitor species, Brassica rapa and Brassica oleracea, were identified through bioinformatic analysis and extensive molecular cloning. Two FAD2 genes exist in B. rapa and B. oleracea, and four copies of FAD2 genes exist in B. napus. Three and six copies of FAD3 genes were identified in diploid species and amphidiploid species, respectively. The genetic control of high C18:1 and low C18:3 contents in a double haploid population was investigated through mapping of the quantitative trait loci (QTL) for the traits and the molecular cloning of the underlying genes. One major QTL of BnaA.FAD2.a located on A5 chromosome was responsible for the high C18:1 content. A deleted mutation in the BnaA.FAD2.a locus was uncovered, which represented a previously unidentified allele for the high oleic variation in B. napus species. Two major QTLs on A4 and C4 chromosomes were found to be responsible for the low C18:3 content in the DH population as well as in SW Hickory. Furthermore, several single base pair changes in BnaA.FAD3.b and BnaC.FAD3.b were identified to cause the phenotype of low C18:3 content. Based on the results of genetic mapping and identified sequences, allele-specific markers were developed for FAD2 and FAD3 genes. Particularly, single-nucleotide amplified polymorphisms markers for FAD3 alleles were demonstrated to be a reliable type of SNP markers for unambiguous identification of genotypes with different content of C18:3 in amphidiploid B. napus.     

Sequence,expression divergence,and complementation of homologous <Emphasis Type="Italic">ALCATRAZ</Emphasis> loci in <Emphasis Type="Italic">Brassica napus</Emphasis>

The genomic era provides new perspectives in understanding polyploidy evolution, mostly on the genome-wide scale. In this paper, we show the sequence and expression divergence between the homologous ALCATRAZ (ALC) loci in Brassica napus, responsible for silique dehiscence. We cloned two homologous ALC loci, namely BnaC.ALC.a and BnaA.ALC.a in B. napus. Driven by the 35S promoter, both the loci complemented to the alc mutation of Arabidopsis thaliana, yet only the expression of BnaC.ALC.a was detectable in the siliques of B. napus. Sequence alignment indicated that BnaC.ALC.a and BolC.ALC.a, or BnaA.ALC.a and BraA.ALC.a, possess a high level of similarity. The understanding of the sequence and expression divergence among homologous loci of a gene is of due importance for an effective gene manipulation and TILLING (or ECOTILLING) analysis for the allelic DNA variation at a given locus. S. Hua and I. H. Shamsi contributed equally to this work.     

Chromosomal localization and characterization of rDNA loci in the Brassica A and C genomes.

Using fluorescence in situ hybridization, we located ribosomal DNA loci on prometaphase chromosomes of the diploid species Brassica rapa and Brassica oleracea and their amphidiploid Brassica napus. Based on comparisons of chromosome morphology and hybridization patterns, we characterized the individual B. napus rDNA loci according to their presumed origins in the Brassica A and C genomes. As reported in other studies, the sum of rDNA loci observed on B. rapa (AA genome) and B. oleracea (CC genome) chromosomes was one greater than the total number of loci seen in their amphidiploid B. napus (AACC). Evidence is presented that this reduction in B. napus rDNA locus number results from the loss of the smallest A genome rDNA site in the amphidiploid.     

The CACTA transposon Bot1 played a major role in Brassica genome divergence and gene proliferation

We isolated and characterized a Brassica C genome-specific CACTA element, which was designated Bot1 (Brassica oleracea transposon 1). After analysing phylogenetic relationships, copy numbers and sequence similarity of Bot1 and Bot1 analogues in B. oleracea (C genome) versus Brassica rapa (A genome), we concluded that Bot1 has encountered several rounds of amplification in the oleracea genome only, and has played a major role in the recent rapa and oleracea genome divergence. We performed in silico analyses of the genomic organization and internal structure of Bot1, and established which segment of Bot1 is C-genome specific. Our work reports a fully characterized Brassica repetitive sequence that can distinguish the Brassica A and C chromosomes in the allotetraploid Brassica napus, by fluorescent in situ hybridization. We demonstrated that Bot1 carries a host S locus-associated SLL3 gene copy. We speculate that Bot1 was involved in the proliferation of SLL3 around the Brassica genome. The present study reinforces the assumption that transposons are a major driver of genome and gene evolution in higher plants.     

Conservation of the microstructure of genome segments in Brassica napus and its diploid relatives

The cultivated Brassica species are the group of crops most closely related to Arabidopsis thaliana (Arabidopsis). They represent models for the application in crops of genomic information gained in Arabidopsis and provide an opportunity for the investigation of polyploid genome formation and evolution. The scientific literature contains contradictory evidence for the dynamics of the evolution of polyploid genomes. We aimed at overcoming the inherent complexity of Brassica genomes and clarify the effects of polyploidy on the evolution of genome microstructure in specific segments of the genome. To do this, we have constructed bacterial artificial chromosome (BAC) libraries from genomic DNA of B. rapa subspecies trilocularis (JBr) and B. napus var Tapidor (JBnB) to supplement an existing BAC library from B. oleracea. These allowed us to analyse both recent polyploidization (under 10,000 years in B. napus) and more ancient polyploidization events (ca. 20 Myr for B. rapa and B. oleracea relative to Arabidopsis), with an analysis of the events occurring on an intermediate time scale (over the ca. 4 Myr since the divergence of the B. rapa and B. oleracea lineages). Using the Arabidopsis genome sequence and clones from the JBr library, we have analysed aspects of gene conservation and microsynteny between six regions of the genome of B. rapa with the homoeologous regions of the genomes of B. oleracea and Arabidopsis. Extensive divergence of gene content was observed between the B. rapa paralogous segments and their homoeologous segments within the genome of Arabidopsis. A pattern of interspersed gene loss was identified that is similar, but not identical, to that observed in B. oleracea. The conserved genes show highly conserved collinearity with their orthologues across genomes, but a small number of species-specific rearrangements were identified. Thus the evolution of genome microstructure is an ongoing process. Brassica napus is a recently formed polyploid resulting from the hybridization of B. rapa (containing the Brassica A genome) and B. oleracea (containing the Brassica C genome). Using clones from the JBnB library, we have analysed the microstructure of the corresponding segments of the B. napus genome. The results show that there has been little or no change to the microstructure of the analysed segments of the Brassica A and C genomes as a consequence of the hybridization event forming natural B. napus. The observations indicate that, upon polyploid formation, these segments of the genome did not undergo a burst of evolution discernible at the scale of microstructure.     

Conserved patterns of chromosome pairing and recombination in Brassica napus crosses.

The patterns of chromosome pairing and recombination in two contrasting Brassica napus F1 hybrids were deduced. One hybrid was from a winter oilseed rape (WOSR) x spring oilseed rape cross, the other from a resynthesized B. napus x WOSR cross. Segregation at 211 equivalent loci assayed in the population derived from each hybrid produced two collinear genetic maps. Alignment of the maps indicated that B. napus chromosomes behaved reproducibly as 19 homologous pairs and that the 19 distinct chromosomes of B. napus each recombined with unique chromosomes from the interspecific hybrid between Brassica rapa and Brassica oleracea. This result indicated that the genomes of the diploid progenitors of amphidiploid B. napus have remained essentially unaltered since the formation of the species and that the progenitor genomes were similar to those of modern-day B. rapa and B. oleracea. The frequency and distribution of crossovers were almost indistinguishable in the two populations, suggesting that the recombination machinery of B. napus could cope easily with different degrees of genetic divergence between homologous chromosomes. Efficient recombination in wide crosses will facilitate the introgression of novel alleles into oilseed rape from B. rapa and B. oleracea (via resynthesized B. napus) and reduce linkage drag.     

Expression, mapping, and genetic variability of Brassica napus disease resistance gene analogues.

Numerous sequences analogous to resistance (R) genes exist in plant genomes and could be involved in resistance traits. The aim of this study was to identify a large number of Brassica napus sequences related to R genes and also to test the adequacy of specific PCR-based tools for studying them. Different consensus primers were compared for their efficiency in amplifying resistance-gene analogues (RGAs) related to the nucleotide-binding-site subgroup of R genes. Specific primers were subsequently designed to fine-study the different RGAs and we tested their efficiency in three species related to B. napus: Brassica oleracea, Brassica rapa, and Arabidopsis thaliana. Forty-four B. napus RGAs were identified. Among 29 examined, at least one-third were expressed. Eighteen RGAs were mapped on 10 of the 19 B. napus linkage groups. The high variability within these sequences permitted discrimination of each genotype within a B. napus collection. The RGA-specific primers amplified RGAs in the B. oleracea and B. rapa genomes, but the sequences appear to be poorly conserved in A. thaliana. Specific RGA primers are a precise tool for studying known-sequence RGAs. These sequences represent interesting markers that could be correlated with resistance traits in B. napus or related Brassica genomes.     

Targeting of the Arabidopsis homomeric acetyl-coenzyme A carboxylase to plastids of rapeseeds.

Acetyl-coenzyme A carboxylase (ACCase) occurs in at least two forms in rapeseed (Brassica napus): a homomeric (HO) and presumably cytosolic isozyme and a heteromeric, plastidial isozyme. We investigated whether the HO-ACCase of Arabidopsis can be targeted to plastids of B. napus seeds. A chloroplast transit peptide and the napin promoter were fused to the Arabidopsis ACC1 gene and transformed into B. napus, with the following results. (a) The small subunit transit peptide was sufficient to provide import of this very large protein into developing seed plastids. (b) HO-ACCase in isolated plastids was found to be biotinylated at a level comparable to extraplastidial HO-ACCase. (c) In vitro assays of HO-ACCase in isolated plastids from developing seeds indicate that it occurs as an enzymatically active form in the plastidial compartment. (d) ACCase activity in mature B. napus seeds is normally very low; however, plants expressing the SSU/ACC1 gene had 10- to 20-fold higher ACCase activity in mature seeds, suggesting that plastid localization prevents the turnover of HO-ACCase. (e) ACCase over-expression altered seed fatty acid composition, with the largest effect being an increase approximately 5% by the expression of HO-ACCase in plastids.     

甘蓝型油菜扩展蛋白家族的全基因组鉴定及其对缺硼胁迫响应的差异分析

以甘蓝型油菜(Brassica napus L.)硼高效品种‘青油10号’和硼低效品种‘Westar 10’为研究对象,采用生物信息学分析、转录组测序和实时荧光定量PCR技术,鉴定其基因组中扩展蛋白的家族成员,并对该基因家族响应缺硼胁迫的表达差异进行分析。结果显示,甘蓝型油菜基因组中包含109个扩展蛋白,可分为4个亚家族,包括:79个扩展蛋白A(BnaEXPAs)、21个扩展蛋白B(BnaEXPBs)、5个类扩展蛋白A(BnaEXLAs)和4个类扩展蛋白B(BnaEXLBs)。同一亚家族中的扩展蛋白具有相对保守的基因结构和蛋白质基序组成。这些扩展蛋白基因分布在19条染色体上,其中10个位于硼高效QTL区间内。转录组测序分析结果表明,缺硼胁迫时‘青油10号’的根、幼叶和老叶中分别有40、18和30个扩展蛋白基因显著上调或下调表达;而‘Westar10’中分别有27、24和41个扩展蛋白基因显著上调或下调表达。其中‘青油10号’根中的BnaC04.EXPA6a,幼叶中的BnaA09.EXPA5以及老叶中的BnaA09.EXPA16、BnaC04.EXPA3、BnaCnn.EXPA5b和BnaA03.EXPA8基因的表达水平均显著高于‘Westar10’。研究结果说明甘蓝型油菜基因组中扩展蛋白基因家族数量庞大,其中高、低效品种间和不同硼水平中差异表达的扩展蛋白可能在甘蓝型油菜低硼适应性中发挥重要作用。     

Phylogenetic analysis of the acetyl-CoA carboxylase and 3-phosphoglycerate kinase loci in wheat and other grasses

We have applied a two-gene system based on the sequences of nuclear genes encoding multi-domain plastid acetyl-CoA carboxylase (ACCase) and plastid 3-phosphoglycerate kinase (PGK) to study grass evolution. Our analysis revealed that these genes are single-copy in most of the grass species studied, allowing the establishment of orthologous relationships between them. These relationships are consistent with the known facts of their evolution: the eukaryotic origin of the plastid ACCase, created by duplication of a gene encoding the cytosolic multi-domain ACCase gene early in grass evolution, and the prokaryotic (endosymbiont) origin of the plastid PGK. The major phylogenetic relationships among grasses deduced from the nucleotide sequence comparisons of ACCase and PGK genes are consistent with each other and with the milestones of grass evolution revealed by other methods. Nucleotide substitution rates were calculated based on multiple pairwise sequence comparisons. On a relative basis, with the divergence of the Pooideae and Panicoideae subfamilies set at 60 million years ago (MYA), events leading to the Triticum/Aegilops complex occurred at the following intervals: divergence of Lolium (Lolium rigidum) at 35 MYA, divergence of Hordeum (Hordeum vulgare) at 11 MYA and divergence of Secale (Secale cereale) at 7 MYA. On the same scale, gene duplication leading to the multi-domain plastid ACCase in grasses occurred at 129 MYA, divergence of grass and dicot plastid PGK genes at 137 MYA, and divergence of grass and dicot cytosolic PGK genes at 155 MYA. The ACCase and PGK genes provide a well-understood two-locus system to study grass phylogeny, evolution and systematics.     

Sequence-level analysis of the diploidization process in the triplicated FLOWERING LOCUS C region of Brassica rapa

Strong evidence exists for polyploidy having occurred during the evolution of the tribe Brassiceae. We show evidence for the dynamic and ongoing diploidization process by comparative analysis of the sequences of four paralogous Brassica rapa BAC clones and the homologous 124-kb segment of Arabidopsis thaliana chromosome 5. We estimated the times since divergence of the paralogous and homologous lineages. The three paralogous subgenomes of B. rapa triplicated 13 to 17 million years ago (MYA), very soon after the Arabidopsis and Brassica divergence occurred at 17 to 18 MYA. In addition, a pair of BACs represents a more recent segmental duplication, which occurred approximately 0.8 MYA, and provides an exception to the general expectation of three paralogous segments within the B. rapa genome. The Brassica genome segments show extensive interspersed gene loss relative to the inferred structure of the ancestral genome, whereas the Arabidopsis genome segment appears little changed. Representatives of all 32 genes in the Arabidopsis genome segment are represented in Brassica, but the hexaploid complement of 96 has been reduced to 54 in the three subgenomes, with compression of the genomic region lengths they occupy to between 52 and 110 kb. The gene content of the recently duplicated B. rapa genome segments is identical, but intergenic sequences differ.     

Identification of the A and C genomes of amphidiploid Brassica napus (oilseed rape).

A genetic linkage map consisting of 399 RFLP-defined loci was generated from a cross between resynthesized Brassica napus (an interspecific B. rapa x B. oleracea hybrid) and "natural" oilseed rape. The majority of loci exhibited disomic inheritance of parental alleles demonstrating that B. rapa chromosomes were each pairing exclusively with recognisable A-genome homologues in B. napus and that B. oleracea chromosomes were pairing similarly with C-genome homologues. This behaviour identified the 10 A genome and 9 C genome linkage groups of B. napus and demonstrated that the nuclear genomes of B. napus, B. rapa, and B. oleracea have remained essentially unaltered since the formation of the amphidiploid species, B. napus. A range of unusual marker patterns, which could be explained by aneuploidy and nonreciprocal translocations, were observed in the mapping population. These chromosome abnormalities were probably caused by associations between homoeologous chromosomes at meiosis in the resynthesized parent and the F1 plant leading to nondisjunction and homoeologous recombination.     

Comparison of Flowering Time Genes in Brassica Rapa, B. Napus and Arabidopsis Thaliana

The major difference between annual and biennial cultivars of oilseed Brassica napus and B. rapa is conferred by genes controlling vernalization-responsive flowering time. These genes were compared between the species by aligning the map positions of flowering time quantitative trait loci (QTLs) detected in a segregating population of each species. The results suggest that two major QTLs identified in B. rapa correspond to two major QTLs identified in B. napus. Since B. rapa is one of the hypothesized diploid parents of the amphidiploid B. napus, the vernalization requirement of B. napus probably originated from B. rapa. Brassica genes also were compared to flowering time genes in Arabidopsis thaliana by mapping RFLP loci with the same probes in both B. napus and Arabidopsis. The region containing one pair of Brassica QTLs was collinear with the top of chromosome 5 in A. thaliana where flowering time genes FLC, FY and CO are located. The region containing the second pair of QTLs showed fractured collinearity with several regions of the Arabidopsis genome, including the top of chromosome 4 where FRI is located. Thus, these Brassica genes may correspond to two genes (FLC and FRI) that regulate flowering time in the latest flowering ecotypes of Arabidopsis.     

A and C genome distinction and chromosome identification in brassica napus by sequential fluorescence in situ hybridization and genomic in situ hybridization

The two genomes (A and C) of the allopolyploid Brassica napus have been clearly distinguished using genomic in situ hybridization (GISH) despite the fact that the two extant diploids, B. rapa (A, n = 10) and B. oleracea (C, n = 9), representing the progenitor genomes, are closely related. Using DNA from B. oleracea as the probe, with B. rapa DNA and the intergenic spacer of the B. oleracea 45S rDNA as the block, hybridization occurred on 9 of the 19 chromosome pairs along the majority of their length. The pattern of hybridization confirms that the two genomes have remained distinct in B. napus line DH12075, with no significant genome homogenization and no large-scale translocations between the genomes. Fluorescence in situ hybridization (FISH)-with 45S rDNA and a BAC that hybridizes to the pericentromeric heterochromatin of several chromosomes-followed by GISH allowed identification of six chromosomes and also three chromosome groups. Our procedure was used on the B. napus cultivar Westar, which has an interstitial reciprocal translocation. Two translocated segments were detected in pollen mother cells at the pachytene stage of meiosis. Using B. oleracea chromosome-specific BACs as FISH probes followed by GISH, the chromosomes involved were confirmed to be A7 and C6.     

芸苔属（Brassica）植物中microRNAs的生物信息学预测与分析

MicroRNA (miRNA) 是一类调控基因转录后表达的非编码的小分子RNA．它在生物的发育、细胞增殖、凋亡以及胁迫响应等生物过程中发挥着重要的调控作用．目前,分离和鉴定miRNA的方法主要包括实验方法（遗传筛选、直接克隆）和生物信息学方法．MiRNA存在表达丰度低,表达组织特异性,以及受特殊诱导等问题,用传统的实验法常难发现和鉴定miRNA．通过生物信息学方法在已有的各种基因库中寻找未知miRNA,大大提高了人们发现miRNA及其靶基因的效率．芸苔属（Brassica）的成员包括油菜、芜青、甘蓝等,是世界各国主要油料和食用作物．目前,油菜的miRNA的分离和鉴定工作已有文献报道,而其它的尚属空白．本文将拟南芥、水稻等植物已知的miRNA分别与芜青、甘蓝、野芥菜、黑芥菜、埃塞俄比亚芥的GSS和EST数据库进行比对搜索,采用一系列标准进行筛选,最后分别在芜青和甘蓝中预测到67个和95个miRNA．再把这些预测得到的miRNA分别与芜青和甘蓝的mRNA数据库进行比对搜索,分别找到120个和111个靶基因,除去未知功能及功能不详的,各有62个和48个靶基因．分析结果表明,上述大多数靶基因编码的产物为转录因子及重要代谢酶类,涉及植物的生长发育调控,信号转导及胁迫响应等方面．     

Molecular characterisation and chromosomal localisation of a telomere-like repetitive DNA sequence highly enriched in the C genome of Brassica

The aim of this work was to find C genome specific repetitive DNA sequences able to differentiate the homeologous A (B. rapa) and C (B. oleracea) genomes of Brassica, in order to assist in the physical identification of B. napus chromosomes. A repetitive sequence (pBo1.6) highly enriched in the C genome of Brassica was cloned from B. oleracea and its chromosomal organisation was investigated through fluorescent in situ hybridisation (FISH) in B. oleracea (2n = 18, CC), B. rapa (2n = 20, AA) and B. napus (2n = 38, AACC) genomes. The sequence was 203 bp long with a GC content of 48.3%. It showed up to 89% sequence identity with telomere-like DNA from many plant species. This repeat was clearly underrepresented in the A genome and the in situ hybridisation showed its B. oleracea specificity at the chromosomal level. Sequence pBo1.6 was localised at interstitial and/or telomeric/subtelomeric regions of all chromosomes from B. oleracea, whereas in B. rapa no signal was detected in most of the cells. In B. napus 18 to 24 chromosomes hybridised with pBo1.6. The discovery of a sequence highly enriched in the C genome of Brassica opens the opportunity for detailed studies regarding the subsequent evolution of DNA sequences in polyploid genomes. Moreover, pBo1.6 may be useful for the determination of the chromosomal location of transgenic DNA in genetically modified oilseed rape.     

Structural and functional comparative mapping between the Brassica A genomes in allotetraploid Brassica napus and diploid Brassica rapa

Brassica napus (AACC genome) is an important oilseed crop that was formed by the fusion of the diploids B. rapa (AA) and B. oleracea (CC). The complete genomic sequence of the Brassica A genome will be available soon from the B. rapa genome sequencing project, but it is not clear how informative the A genome sequence in B. rapa (A(r)) will be for predicting the structure and function of the A subgenome in the allotetraploid Brassica species B. napus (A(n)). In this paper, we report the results of structural and functional comparative mapping between the A subgenomes of B. napus and B. rapa based on genetic maps that were anchored with bacterial artificial chromosomes (BACs)-sequence of B. rapa. We identified segmental conservation that represented by syntenic blocks in over one third of the A genome; meanwhile, comparative mapping of quantitative trait loci for seed quality traits identified a dozen homologous regions with conserved function in the A genome of the two species. However, several genomic rearrangement events, such as inversions, intra- and inter-chromosomal translocations, were also observed, covering totally at least 5% of the A genome, between allotetraploid B. napus and diploid B. rapa. Based on these results, the A genomes of B. rapa and B. napus are mostly functionally conserved, but caution will be necessary in applying the full sequence data from B. rapa to the B. napus as a result of genomic rearrangements in the A genome between the two species.