Fertile allohexaploid Brassica hybrids obtained from crosses between B. oleracea and B. juncea via ovule rescue and colchicine treatment of cuttings

An allohexaploid Brassica crop (2n = AABBCC) does not exist naturally, but is of interest for its potential to combine useful traits found in the six cultivated Brassica species which share combinations of the A, B and C genomes with additional allelic heterosis. In this study, we aimed to produce 2n = AABBCC hybrids by crosses between B. juncea and a number of Brassica C genome species. We used ovule rescue to overcome hybridization barriers and different colchicine treatment methods to induce chromosome doubling of ABC hybrids to AABBCC allohexaploids, thus restoring fertility. Only the cross B. oleracea × B. juncea was successful, with six triploid hybrids produced from one genotype combination. Colchicine-containing regeneration media was unsuccessful in doubling chromosome number in these hybrids, but treatment of cuttings with 0.05 to 0.25% colchicine successfully produced ~ 200 S₁ allohexaploid seeds. The S₁ plants produced 7–84% viable pollen and set 0–390 seeds per plant, with 23–27 bivalents and 0–3 univalents during metaphase I of meiosis. Our results highlight the difficulties in working with the wild C genome species, but showed that our methods have utility for producing euploid, chromosome-doubled progeny in this cross combination. Further, Brassica oleracea × B. juncea allohexaploid hybrids may contain useful genetic factors for improved meiotic stability and fertility in allohexaploid germplasm pools.