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The genus Brassica contains many agriculturally important crops, including rapeseed, cabbage, cauliflower, turnip and mustards. Three diploid Brassica species contain the A, B and C genomes (B. rapa, B. nigra and B. oleracea), and three allotetraploid species contain the AB, AC and BC genomes (B.
juncea, B. napus and B. carinata), making this an interesting species complex for investigation of questions related to polyploidy and hybridization. Although no naturally occurring allohexaploid Brassica (ABC) exists, production of synthetic allohexaploids and other trigenomic Brassica hybrids might prove of interest for greater exploitation of genetic diversity and heterosis for crop breeding in this genus. The Brassica species can all hybridize with each other, particularly when assisted by embryo
rescue, and are also amenable to colchicine-induced chromosome doubling, although both of these traits are influenced by genotype as well as by the species combination. We have to date produced hybrids and polyploids involving the six Brassica crop species in different combinations. However, most novel synthetic hybrids and polyploids in Brassica show a greater or lesser degree of homoeologous recombination during meiosis between
chromosomes belonging to different genomes. This hinders the establishment of stable lines for further propagation and the development of novel crop types. We have established that genotype-specific differences play a major role in this effect, and specifically implicate particular allelic variants of known meiosis genes as well as the degree of relationship between subgenomes. Generational selection for fertility in initially heterozygous material also seems a promising way forward to stabilise new polyploid crop types, relying on selective pressure for different alleles inherited from the parent species to stabilise meiosis (stable meiosis is associated with higher fertility than unstable meiosis). Our results suggest interesting future avenues for production of novel hybrid types in the Brassica genus.
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