Three Genome Meiosis in Brassica Hybrids

The agriculturally important Brassica genus contains six species with an interesting genomic relationship: three diploids have genome complements 2n = AA, BB and CC and three allotetraploids have genome complements 2n = AABB, AACC and BBCC. Although no natural allohexaploid Brassica with 2n = AABBCC exists, this genome composition is of particular interest for development of a new crop type benefiting from hybrid vigour, as well as for investigation of hybrid speciation processes. However, newly synthesised Brassica hybrids containing the A and C genomes usually suffer from poor control of non-homologous chromosome pairing during meiosis, resulting in loss of chromosomes and chromosome fragments and hence failure to establish true-breeding, genomically-stable lines. We investigated meiotic behavior in three-genome interspecific hybrids from crosses between B. juncea, B. napus and B. carinata, including allohexaploid types, using a combination of classical and molecular cytogenetics and high-throughput genotyping approaches. Homologous chromosome pairing was mostly normal despite the different species origins, but non-homologous chromosome interactions (A-B, B-C and A-C) did occur at low frequencies between most chromosomes. Surprisingly, strong selective pressure for 2n = AABBCC euploid karyotypes was not observed, and most chromosomes present in only one copy (univalents) were equally likely to be lost as to be retained in next-generation progeny. Meiotic behaviour was observed to be influenced by both genotype and genomic structure in different allohexaploid genotypes and combinations. Ongoing analysis may detect identification of factors associated with meiotic stability in Brassica, and facilitate selection and production of new, stable allohexaploid germplasm.