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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. [ABSTRACT FROM AUTHOR]

Polyploidy has played a major role in the evolution of higher plants. Precise control of chromosome pairing is vital for conferring meiotic regularity, and hence reproductive stability in allopolyploids. In this review, we examine whether strong evidence has accumulated for the presence and activity of pairing control genes in different allopolyploid species that are entirely bivalent forming and that display a strict disomic inheritance. We show that very good evidence has been adduced in Triticum species, Arena sativa, Festuca arundinacea, Brassica napus, Gossypium hirsutum, and G. barbadense, and in amphidiploids related to the diploid species Lolium perenne, L. multiflorum, and L. rigidum. More circumstantial evidence has been obtained for polyploids in the genera Aegilops, Hordeum, Nicotiana, and Coffea, which have received far less attention than the other species. Although these pairing regulators seem to control different processes operating throughout the premeiotic interphase and the meiotic prophase, little is known about their precise mode of action. We present three hypotheses that have been proposed to explain the origin and evolution of pairing control genes; none of them has been supported by direct evidence, and the origin of most pairing suppressors is still unknown. Accordingly, the study of pairing control genes is still an important task for understanding the stabilization and establishment of allopolyploid species. [ABSTRACT FROM AUTHOR]

The assessment of the likelihood of transgene spread from cultivated species to their wild relatives is relevant for oilseed rape ( Brassica napus , AACC, 2 n =38) as it is partially allogamous and presents numerous wild relatives growing nearby in cultivated areas and having an overlapping of the flowering period. We have assessed the probability of F1 interspecific hybrid formation between oilseed rape and three main weeds encountered in France, wild mustard ( Sinapis arvensis , SarSar, 2 n =18), hoary mustard ( Hirschfeldia incana , AdAd, 2 n =14) and wild radish ( Raphanus raphanistrum , RrRr, 2 n =18), under optimal conditions. Because of the higher frequency of interspecific hybrids observed with this latter weed, complementary field experiments were carried out under normal agronomic conditions. The F1 hybrids obtained showed different genomic structures, either 2 n =28, or 2 n =37, or 2 n =56. These different hybrids were cultivated in the presence of wild radish and the succeeding generations were studied. [ABSTRACT FROM AUTHOR]