A new method for producing allohexaploid Brassica through unreduced gametes

We trialled a two-step method of producing allohexaploid Brassica with three genomes (A, B, and C) derived from pair-wise crossing among three allotetraploid Brassica species. In the first step, the three allotetraploid Brassica species (Brassica juncea, A^jA^jB^jB^j; Brassica napus, AⁿAⁿCⁿCⁿ; and Brassica carinata, B^cB^cC^cC^c) were intercrossed in pairs to produce unbalanced trigenomic hybrids: A^jAⁿB^jCⁿ, B^jB^cA^jC^c and CⁿC^cAⁿB^c. In the second step, these hybrids were crossed with the complementary allotetraploid parent, that is, A^jAⁿB^jCⁿ × B^cB^cC^cC^c (B. carinata), B^jB^cA^jC^c × AⁿAⁿCⁿCⁿ (B. napus) and CⁿC^cAⁿB^c × A^jA^jB^jB^j (B. juncea). We hypothesised that the unbalanced trigenomic hybrids would produce high levels of unreduced gametes with the same genome composition as the hybrid. These unreduced gametes would unite with reduced gametes from the complementary allotetraploids to form allohexaploid Brassica progeny (A^jAⁿB^cB^jC^cCⁿ). From 112 s generation interspecific progeny, two progeny were shown by locus-specific simple sequence repeat markers to be near-allohexaploids derived from an unreduced gamete from CⁿC^cAⁿB^c and a reduced gamete from B. juncea (A^jB^j). One of these plants was highly self-fertile, had 50 chromosomes, and inherited patterns of marker alleles (A^jAⁿB^cB^jC^cCⁿ) that were predicted from first division restitution at meiosis in the CⁿC^cAⁿB^c parent. The second near-allohexaploid had 60 chromosomes, was sterile, and inherited patterns of marker alleles that indicated second division restitution in the CⁿC^cAⁿB^c parent. Trigenomic hybrid Brassica produced by these methods will be valuable bridges to move alleles between allotetraploid species, and may contribute useful meiotic stability alleles to a future allohexaploid species.