Polyploidy promotes the restructuring of merged genomes within initial generations of resynthesized Brassica napus, possibly caused by homoeologous recombination at meiosis. However, little is known about the impact of the first confrontation of two genomes at the first meiosis which could lead to genome exchanges in progeny. Here, we assessed the role of the first meiosis in the genome instability of synthetic B. napus. We used three different newly resynthesized B. napus plants and established meiotic pairing frequencies for the A and C genomes. We genotyped the three corresponding progenies in a cross to a natural B. napus on the two homoeologous A1 and C1 chromosomes. Pairing at meiosis in a set of progenies with various rearrangements was scored. Here, we confirmed that the very first meiosis of resynthesized plants of B. napus acts as a genome blender, with many of the meiotic-driven genetic changes transmitted to the progenies, in proportions that depend significantly on the cytoplasm background inherited from the progenitors. We conclude that the first meiosis generates rearrangements on both genomes and promotes subsequent restructuring in further generations. Our study advances the knowledge on the timing of genetic changes and the mechanisms that may bias their transmission.

/pdf/the-first-meiosis-of-resynthesized-brassica-napus-a-genome-ejwsx493ue.pdf

The first meiosis of resynthesized Brassica napus, a genome blender.

POLYPLOIDY is a major evolutionary process of speciation in eukaryotes and particularly in plants where several economically important crop species including oilseed rape, wheat, and sugarcane are polyploid (Grant 1981; Wendel 2000). Moreover, an increasing number of diploid-considered species are shown to actually be paleopolyploids, such as maize (Gaut and Doebley 1997), Arabidopsis (Arabidopsis Genome Initiative 2000), and rice (Zhang et al. 2005). The high occurrence of polyploid plants has been the source of considerable interest, notably to determine why polyploidy represents such an indisputable evolutionary success. Several authors have noted that polyploidy must confer an immediate selective advantage (Osborn et al. 2003; Comai 2005) and have underlined the necessity to analyze the very first stages of polyploid formation. Accordingly, the study of newly synthesized polyploids has led to the description of structural changes such as chromosomal rearrangements (Pontes et al. 2004) and gains or losses of DNA sequences (Song et al. 1995; Liu et al. 1998; Madlung et al. 2005). Additionally, investigations have demonstrated the occurrence of modifications at the level of gene expression. Several studies on resynthesized Arabidopsis suecica (Comai et al. 2000; Wang et al. 2004, 2006), Gossypium allotetraploids (Adams et al. 2003, 2004), and allotetraploids and allohexaploids of wheat (Ni et al. 2000; Kashkush et al. 2002, 2003; He et al. 2003; Wu et al. 2003) have shown that some genes are silenced after polyploidization, while others are derepressed. Nearly all of these approaches were undertaken at the transcript level and most of them used qualitative or semiqualitative techniques like cDNA–AFLP display that allowed the detection of gene silencing or novel expression patterns, but lacked precision for more subtle variations (up- or downregulation). Other innovating techniques like cDNA–SSCP (Adams et al. 2003) give quantitative information on homeologous gene expression, but are restricted to a small number of genes. Genomewide analyses can now be undertaken using the promising cDNA microarray technology. This approach is planned for Brassica polyploids (Chen et al. 2004; Lee et al. 2004; Wang et al. 2005) and has been recently applied to the synthetic allopolyploids Senecio cambrensis (Hegarty et al. 2005) and A. suecica (Wang et al. 2006), revealing several modifications of gene expression.

Proteomic tools complement transcriptomic approaches: cellular biochemistry is essentially modulated by proteins rather than by transcripts, and the correlation between mRNA abundance and protein amounts is erratic due to post-transcriptional regulations (Gygi et al. 1999). Additionally, post-translational modifications play a key role in the cell as they modulate protein activity, turnover, subcellular location, or interactions with proteins or nucleic acids and can be investigated only at the protein level. Comparative proteomics can be achieved using the two-dimensional electrophoresis (2-DE) technique that allows the resolution of hundreds of polypeptides in a single gel. Accurate and reproducible quantification of the corresponding spots can be performed using dedicated software and the relevant proteins identified by mass spectrometry. Comparative proteomics provides pertinent and reliable quantitative results on genomewide expression and it is becoming more and more popular in plant research (Thiellement et al. 1999, 2002).

In this study, we used the Brassica napus model system to investigate the early consequences of allopolyploid formation on gene expression. Four independent neosynthesized B. napus amphidiploids, their diploid progenitors B. oleracea and B. rapa, as well as the amphihaploid hybrids were submitted to comparative proteomics. The additivity hypothesis (predicting a midparent proteome for the amphiploids) was tested in two distinct organs, the stem and root, and polypeptides displaying nonadditive patterns were identified using mass spectrometry.

https://www.genetics.org/content/genetics/173/2/1101.full.pdf

Numerous and Rapid Nonstochastic Modifications of Gene Products in Newly Synthesized Brassica Napus Allotetraploids

• Polyploids can be produced by the union of unreduced gametes or through somatic doubling of F(1) interspecific hybrids. The first route is suspected to produce allopolyploid species under natural conditions, whereas experimental data have only been thoroughly gathered for the latter. • We analyzed the meiotic behavior of an F(1) interspecific hybrid (by crossing Brassica oleracea and B.rapa, progenitors of B.napus) and the extent to which recombined homoeologous chromosomes were transmitted to its progeny. These results were then compared with results obtained for a plant generated by somatic doubling of this F₁ hybrid (CD.S₀) and an amphidiploid (UG.S₀) formed via a pathway involving unreduced gametes; we studied the impact of this method of polyploid formation on subsequent generations. • This study revealed that meiosis of the F₁ interspecific hybrid generated more gametes with recombined chromosomes than did meiosis of the plant produced by somatic doubling, although the size of these translocations was smaller. In the progeny of the UG.S₀ plant, there was an unexpected increase in the frequency at which the C1 chromosome was replaced by the A1 chromosome. • We conclude that polyploid formation pathways differ in their genetic outcome. Our study opens up perspectives for the understanding of polyploid origins.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.2011.03729.x

Polyploid formation pathways have an impact on genetic rearrangements in resynthesized Brassica napus

Polyploidization is a major evolutionary process in eukaryotes. In plants, genetic and epigenetic changes occur rapidly after formation of allopolyploids. Hybridization, rather than genome doubling itself, is considered as the main cause for the resulting differential gene expression. We studied the consequences of genome doubling alone in an autopolyploid model, by comparing two-dimensional gel electrophoresis (2-DE) gels of haploid, diploid, and tetraploid Brassica oleracea cabbages. Two fully homozygous lines, HDEM and RC, as well as two organs, leaf and stem, were studied. For the 558 common spots found present in all the 29 2-DE gels of the experiment, inter-organ and -genotype differences were the major sources of the variation in protein amounts: 41 and 10-13%, respectively. HDEM leaf and stem proteomes were not significantly affected by the ploidy level, since no qualitative variation was detected and since the number of quantitative variations could be due to chance. For RC, no qualitative variations were observed, but a few spots were significantly variable in protein amount. However, the number of inter-ploidy variations was of the same range as the number of intra-ploidy variations. In conclusion, whatever the ploidy level, leaf and stem proteomes remained globally unchanged in both cabbage lines.

Autopolyploidy in cabbage (Brassica oleracea L.) does not alter significantly the proteomes of green tissues

We introduce and review Brassica crop improvement via trigenomic bridges. Six economically important Brassica species share three major genomes (A, B, and C), which are arranged in diploid (AA, BB, and CC) and allotetraploid (AABB, AACC, and BBCC) species in the classical triangle of U. Trigenomic bridges are Brassica interspecific hybrid plants that contain the three genomes in various combinations, either triploid (ABC), unbalanced tetraploid (e.g., AABC), pentaploid (e.g., AABCC) or hexaploid (AABBCC). Through trigenomic bridges, Brassica breeders can access all the genetic resources in the triangle of U for genetic improvement of existing species and development of new agricultural species. Each of the three Brassica genomes occurs in several species, where they are distinguished as subgenomes with a tag to identify the species of origin. For example, the A subgenome in B. juncea (2n = AABB) is denoted as Aj and the A subgenome in B. napus (2n = AACC) as An. Trigenomic bridges have been used to increase genetic diversity in allopolyploid Brassica crop species, such as a new-type B. napus with subgenomes from B. rapa (Ar) and B. carinata (Cc). Recently, trigenomic bridges from several sources have been crossed together as the ‘founders’ of a potentially new allohexaploid Brassica species (AABBCC). During meiosis in a trigenomic bridge, crossovers are expected to form between homologous chromosomes of related subgenomes (for example Ar and An), but cross-overs may also occur between non-homologous chromosomes (for example between A and C genome chromosomes). Irregular meiosis is a common feature of new polyploids, and any new allotetraploid or allohexaploid Brassica genotypes derived from a trigenomic bridge must achieve meiotic stability through a process of diploidisation. New sequencing technologies, at the genomic and epigenomic level, may reveal the genetic and molecular basis of diploidization, and accelerate selection of stable allotetraploids or allohexaploids. Armed with new genetic resources from trigenomic bridges, Brassica breeders will be able to improve yield and broaden adaptation of Brassica crops to meet human demands for food and biofuel, particularly in the face of abiotic constraints caused by climate change.

Trigenomic bridges for Brassica improvement

Des graines en germination de quatre lignees diploides de chou fourrager ont ete traitees a la colchicine. Seules les plantules presentant un nombre moyen de chloroplastes par cellules stomatiques superieur a celui des temoins ont ete retenues. Un nouveau denombrement au stade adulte a permis d'etablir une bonne correlation entre les resultats obtenus par cette methode et le niveau de ploidie determine par denombrement chromosomique. La proportion de plantes autotetraploides, au sein du materiel selectionne, depend de la lignee d'origine. Leur comportement meiotique est de type diploide et leur fertilite satisfaisante

/pdf/etudes-cytologiques-de-choux-tetraploides-brassica-oleracea-54vdza4dl6.pdf

G. Thomas

Papers

Etudes cytologiques de choux tétraploïdes (Brassica oleracea L. ssp. acephala) obtenus à partir de lignées diploïdes après traitement à la colchicine