Allele segregation analysis of F1 hybrids between independent Brassica allohexaploid lineages

In the Brassica genus we find both diploid species (one genome) and allotetraploid species (two different genomes) but no naturally occurring hexaploid species (three different genomes, AABBCC). Although hexaploids can be produced via human intervention, these neo- polyploids have quite unstable genomes and usually suffer from severe genome reshuffling. Whether these genome rearrangements continue in later generations and whether genomic arrangements follow similar, reproducible patterns between different lines is still unknown. We crossed Brassica hexaploids resulting from different species combinations to produce five F1 hybrids, and analyzed the karyotypes of the parents and the F1 hybrids, as well as allele segregation in a resulting test-cross population via molecular karyotyping using SNP array genotyping. Although some genomic regions were found to be more likely to be duplicated, deleted or rearranged, a consensus pattern was not shared between genotypes. Brassica hexaploids had a high tolerance for fixed structural rearrangements, but which rearrangements occur and become fixed over many generations does not seem to show either strong reproducibility or to indicate selection for stability. On average, we observed 10 de novo chromosome rearrangements contributed almost equally from both parents to the F1 hybrids. At the same time, the F1 hybrid meiosis produced on average 8.6 new rearrangements. Hence, the increased heterozygosity in the F1 hybrid did not significantly improve genome stability in our hexaploid hybrids, and might have had the opposite effect. However, hybridization between lineages was readily achieved and may be exploited for future genetics and breeding purposes.