Hybridization between different Brassica allohexaploid lineages produces viable, fertile progeny but does not improve genome stability or fertility in F1 hybrids

Background and Aims Hybridization events are crucial in the evolution of plants. Experimental hybridization between extant lineages can help us understand the evolutionary consequences of merging different species. Brassica allohexaploids (2n = AABBCC) offer a unique opportunity to test the effects of hybridization between allopolyploids from different species origins, and specifically whether these new hybrids are fertile and perform better than the parents. Methods We hand-pollinated between Brassica allohexaploid genotypes from different species origins to analyze the crossability between species: carirapa (2n = 54; B. carinata × B. rapa), junleracea (B. juncea × B. oleracea), naponigra (B. napus × B. nigra), and NCJ (B. napus × B. carinata × B. juncea). We also analyzed a subset of resulting F1 hybrids for fertility and genomic changes. Key results We obtained 9,052 new allohexaploid hybrid seeds, with a range of 0.0–4.6 seeds per flower bud crossed. Specific female and male parent genotypes affected the crossing success rate (number of seeds/bud crossed). The F­1 hybrids showed mid-parent heterosis for seed fertility ranging from -64 to 275%, while best-parent heterosis ranged from -79 to 241%. Most of the F­1 hybrids had similar quantities of copy number variation (CNV) events compared to the parents, with the majority of these events (76%) directly inherited from one of the parents. Conclusions We conclude that combining different allohexaploid types via hand pollination is feasible without any extra measures to ensure embryo survival, despite the different species origins, with strong maternal genotype effects on success rate. Novel hybrids between allohexaploid lineages showed similar fertility and stability to their parents, suggesting that there is no selection against CNVs in the hybridization event, nor is there an immediate gain in seed fertility associated with an increase in heterozygosity in the allohexaploids.