Different fertility and meiotic regularity in allohexaploids derived from trigenomic hybrids between three cultivated Brassica allotetraploids and B. maurorum

The wild species Brassica maurorum Durieu (MM, 2n = 16) is useful for the improvement of Brassica crops. Herein, interspecific reciprocal crosses between B. maurorum and three cultivated Brassica allotetraploids were carried out with the aid of embryo rescue. Trigenomic hybrids with Brassica napus (AACC, 2n = 38) and Brassica carinata (BBCC, 2n = 34) were produced from reciprocal crosses, but the hybrids with Brassica juncea (AABB, 2n = 36) were obtained only when B. maurorum was used as female. All the hybrids were morphologically intermediate between their parents, and were male and female sterile. By in vitro chromosome doubling of the trigenomic hybrids, the allohexaploids (AACC.MM/MM.AACC, 2n = 54; BBCC.MM, 2n = 50; MM.AABB, 2n = 52) were established and characterized for their phenotype and cytology. The fertilities of three allohexaploids were different, for AACC.MM and MM.AACC failed to produce seeds by selfing, but BBCC.MM showed low seed-set and MM.AABB had good seed-set. They also expressed variable extents of male meiotic regularity as to chromosome pairing and segregation, with MM.AABB > BBCC.MM > AACC.MM/MM.AACC, the same order as their fertility. So their meiotic behavior contributed to the fertility. Finally, the potential of these allohexaploids as a bridge for genetic improvement of Brassica crops was discussed.