Meiotic stability and fertility in interspecific allotetraploid hybrids in the genus Brassica

Abstract

The Brassica genus is the most important in the Brassicaceae family. The six most important cultivated species of this genus consist of three diploid species B. rapa, AA (2n = 2x = 20), B. nigra, BB (2n = 2x = 18) and B. oleracea, CC (2n = 2x = 18). Pairwise hybridization between these diploids gave rise to three allotetraploids B. juncea AABB (2n = 2x = 36), B. carinata, BBCC (2n = 2x = 34) and B. napus, AACC (2n = 2x = 38). These Brassica allotetraploids can easily hybridize to produce trigenomic hybrids with initial 2n = AABC, 2n = BBAC and 2n = CCAB containing a diploid genome and two haploid genomes. Interspecific crosses or newly formed hybrids have been shown to encounter a variety of challenges, the most important of which are chromosomes pairing abnormalities and infertility. The fate of these newly formed hybrid lineages is also relatively understudied. Understanding the mechanisms behind these challenges and how to manipulate them will be important in helping us utilize the best strategies when trying to introgress new traits and when trying to synthesize new, fertile and stable hybrids. This thesis describes the chromosome pairing behavior, inheritance, meiotic stability and fertility of Brassica trigenomic hybrids AABC, BBAC and CCAB formed by pairwise hybridization of Brassica allotetraploids in the early F1 and S1 generations for AABC, BBAC, and CCAB, and in the S1 - S6 generations for BBAC hybrids following self-pollination and selection for fertility. Despite the presence of complete diploid genomes in these hybrids, hybrid fertility was found to be extremely low. All confirmed CCAB hybrids in the second generation (S1) were found to be sterile producing no seeds. A strong bias towards retention rather than loss of haploid genomes was observed suggesting that subgenomes in the Brassica allotetraploids are already highly interdependent such that loss of one subgenome is detrimental to fertility. With the CCAB hybrids becoming sterile in the S1 generation and the AABC hybrids having extremely low fertility, the BBAC hybrids were self-pollinated for six generations with selection for fertility. The results show there was a significant improvement in meiotic pairing behavior from the F1 generation to the S5/6 generation possibly due to the close relationship of the A and C genomes. There was also an increase in the hybrid seed fertility to parent levels. Additionally, while the B genome chromosomes remained intact, new stable A/C genome chromosomes were also formed by recombination of the A and C genome chromosomes. The results of this thesis suggest that relationships between subgenomes determine hybridization outcomes in these species. Additionally, the results provide experimental evidence that two genomes can come together to form new, restructured genomes in hybridization events between two allotetraploid species that share a common genome. This mechanism should be considered in interpreting phylogenies in taxa with multiple allotetraploids.