The novel allo-cytoplasmic male sterility (CMS) system, Nsa CMS line, and the corresponding restorer system, NR1 line, have been successfully developed from somatic hybrids between Brassica napus (oilseed rape) and its wild relative Sinapis arvensis (Yeyou 18, Xinjiang wild mustard from northwestern China) by fusing mesophyll protoplasts [1, 2]. Yeyou 18, a Chinese wild population cataloged into S. arvensis based on genetic analyses , possesses valuable agricultural traits such as enhanced resistance to Sclerotinia sclerotiorum, Leptosphaeria maculans and insects, greater tolerance to low temperatures and drought, low contents of erucic acid and glucosinolates , as well as a low incidence of pod shattering [5, 6]. The Nsa CMS line contains the S. arvensis cytoplasm, which is essentially different from other rapeseed CMS systems such as ogu, nap, pol, tour and hau, based on their origins and molecular characterization . And the cytoplasmic sterile gene in the Nsa CMS line was likely derived from the S. arvensis parent . Furthermore, the Nsa CMS line is more stable to temperature changes compared to pol and nap. NR1, as a B. napus
S. arvensis disomic alien addition line, carries one pair of homologous chromosomes from S. arvensis and 19 chromosome pairs from B. napus, and displays important agricultural characters which arise from the alien chromosomes, such as fertility restoration ability to Nsa CMS line, low erucic acid and low glucosinolate contents, S. sclerotiorum resistance and pod shattering resistance .
Fertile and sterile plants were derived from the self-pollinated offspring of the F1 hybrid between the novel restorer line NR1 and the Nsa CMS line. Because NR1 contains one S. arvensis homologous chromosome pair, on which the restorer genes reside, F1 hybrids from NR1 crossed to Nsa CMS line are monosomic . The fertility segregation was observed in self-pollinated plants of F1 hybrid because of the loss of added chromosome, producing fertile and sterile plants, which possess the identical cytoplasmic genetic background arising from Nsa CMS line and similar nuclear genetic background arising from B. napus, except one or two members of the added S. arvensis alien chromosome pair in fertile plants. Floral morphology of fertile plants are normal, whereas sterile plants have stamens reduced in size, abnormal anthers and no pollen produced.
To elucidate gene expression and regulation caused by the A and C subgenomes, alien chromosome and cytoplasm from S. arvensis during the development of young floral bud, especially stamens, we performed a genome-wide high-throughput transcriptomic sequencing for young floral buds of sterile and fertile plants. The transcriptome is the complete set and quantity of transcripts in a cell at a specific developmental stage or under a physiological condition, providing information on gene expression, gene regulation, and amino acid content of proteins . Therefore, transcriptome analysis is essential to interpret the functional elements of the genome and reveals the molecular constituents of cells and tissues. Because of the deep coverage and single base-pair resolution provided by the next-generation sequencing instrument, digital gene-expression (DGE), driven by Solexa/Illumina technology, is an efficient method to analyze transcriptome data. Base on genome-wide expression profiles by sequencing, DGE is able to identify, quantify, and annotate expressed genes on the whole genome level without prior sequence knowledge, opening doors to higher confidence target discovery and pathway studies. This technique has also been widely used in plant research. DGE analysis using Solexa sequencing was performed to identify candidate genes encoding enzymes responsible for the Siraitia grosvenorii triterpene biosynthesis . High-throughput tag-sequencing analysis based on the Solexa Genome Analyzer platform was applied to analyze the gene expression profiling of cucumber plant and revealed the comprehensive mechanisms of waterlogging-responsive transcription . Using the Solexa sequencing system, the transcriptomes were compared between seedlings of two soybean varieties to find genes associated with nitrogen use efficiency . Early developing cotton fiber was analyzed by deep-sequencing, and differential expression patterns of genes in a fuzzless/lintless mutant were revealed . DGE signatures were also used to study maize development, and the results from that study provided a basis for the analysis of short-read expression data and resolved specific expression signatures that will help define mechanisms of action of the maize RA3 gene . In addition, Solexa / Illumina technology was used to analyze gene expression during female flower development  and gene expression of Sinapis alba leaves under drought stress and rewatering growth conditions . Overall, the DGE approach has provided more valuable tools for qualitative and quantitative gene expression analysis than the previous micro array-based assays.
Currently, no progress has been made in identifying the genes that specify CMS and fertility restoration in Nsa CMS and NR1 system, and little is understood about how expression of these genes influences the physiological, developmental or biochemical processes such that pollen formation is disrupted. In this study, the transcriptomes of young floral buds were compared between fertile and sterile plants using the Solexa sequencing system. By investigating the expression of genes related to young floral bud development, a number of candidate genes that are important in this process were identified. Here, to our knowledge, this study is the first to characterize genome-wide comparative analysis of gene expression in young floral buds between fertile and sterile plants. And these sequencing datasets will serve as a valuable resource for novel gene discovery from Yeyou 18, the Chinese wild population of S. arvensis, laying the foundation for elucidating the mechanisms of cytoplasmic male sterility, fertility restoration and improved resistance.