Performance of aneuploid backcross hybrids between the crop Brassica napus and its wild relative B. rapa

A wide range of chromosome numbers result from unreduced gamete production in Brassica juncea × B. napus (AABC) interspecific hybrids

The establishment of successful interspecies hybrids requires restoration of a stable "2n" chromosome complement which can produce viable "n" gametes. This may occur (rarely) via recombination between non-homologous chromosomes, or more commonly is associated with a doubling of parental chromosome number to produce new homologous pairing partners in the hybrid. The production of unreduced "2n" gametes (gametes with the somatic chromosome number) may therefore be evolutionarily useful by serving as a key pathway for the formation of new polyploid hybrids, as might specific mechanisms permitting recombination between non-homologous chromosomes. Here, we investigated chromosome complements and fertility in third generation interspecific hybrids (AABC) resulting from a cross between allopolyploids Brassica juncea (AABB) × B. napus (AACC) followed by self-pollination for two generations. Chromosome numbers ranged from 2n = 48-74 in the experimental population (35 plants), with 9-16 B genome chromosomes and up to 4 copies of A genome chromosomes. Unreduced gamete production leading to a putative genome structure of approximately AAAABBCC was hence predicted to explain the high chromosome numbers observed. Additionally, the estimation of nuclei number in post-meiotic sporads revealed a higher frequency of unreduced gametes (0.04-5.21%) in the third generation AABC interspecific hybrids compared to the parental Brassica juncea (0.07%) and B. napus (0.13%). Our results suggest that unreduced gamete production in the subsequent generations following interspecific hybridization events may play a critical role in restoration of more stable, fertile chromosome complements.