Genome structure affects the rate of autosyndesis and allosyndesis in AABC, BBAC and CCAB Brassica interspecific hybrids

Abstract

Gene introgression into allopolyploid crop species from diploid or polyploid ancestors can be accomplished through homologous or homoeologous chromosome pairing during meiosis. We produced trigenomic Brassica interspecific hybrids (genome complements AABC, BBAC and CCAB) from the amphidiploid species Brassica napus (AACC), Brassica juncea (AABB) and Brassica carinata (BBCC) in order to test whether the structure of each genome affects frequencies of homologous and homoeologous (both allosyndetic and autosyndetic) pairing during meiosis. AABC hybrids produced from three genotypes of B. napus were included to assess the genetic control of homoeologous pairing. Multi-colour fluorescent in situ hybridisation was used to quantify homologous pairing (e.g. A-genome bivalents in AABC), allosyndetic associations (e.g. B-C in AABC) and autosyndetic associations (e.g. B-B in AABC) at meiosis. A high percentage of homologous chromosomes formed pairs (97.5-99.3%), although many pairs were also involved in autosyndetic and allosyndetic associations. Allosyndesis was observed most frequently as A-C genome associations (mean 4.0 per cell) and less frequently as A-B genome associations (0.8 per cell) and B-C genome associations (0.3 per cell). Autosyndesis occurred most frequently in the haploid A genome (0.75 A-A per cell) and least frequently in the haploid B genome (0.13 B-B per cell). The frequency of C-C autosyndesis was greater in BBAC hybrids (0.75 per cell) than in any other hybrid. The frequency of A-B, A-C and B-C allosyndesis was affected by the genomic structure of the trigenomic hybrids. Frequency of allosyndesis was also influenced by the genotype of the B. napus paternal parent for the three AABC (B. juncea × B. napus) hybrid types. Homoeologous pairing between the Brassica A, B and C genomes in interspecific hybrids may be influenced by complex interactions between genome structure and allelic composition.

Analysis of B-Genome Chromosome Introgression in Interspecific Hybrids of Brassica napus × B. carinata

“…High levels of segregation distortion for the B-genome chromosomes were observed in this study at the BC 3 level. This is due to the aneuploid nature of the BC 3 plants, which can be explained by the interspecific origin of the material and the fact that homeologous chromosomes from the B and A genomes do not pair very frequently (Parkin and Lydiate 1997;Ky et al 2000;Lorieux et al 2000;Mason et al 2010a). This was previously reported in interspecific Brassica hybrids that contained distantly related genomes (Chèvre et al 1998(Chèvre et al , 2007.…”

“…As studied before, this can be highly influenced by the genotype of either parent of the hybrid (Mason et al 2010a,b). Homeologous pairing between the three genomes of Brassica in interspecific crosses is complexly influenced by genome structure and allelic composition (Mason et al 2010a). …”

“…Cytological observations in digenomic triploids (BBC and CCB) generated from interspecific hybridization between B. carinata and B. nigra and between B. carinata and B. oleracea indicated that the Brassica B genome chromosomes may not form homeologous pairs with chromosomes of the A and C genomes in interspecific crosses (Meng et al 1998). Likewise on the basis of molecular cytogenetic evidence, the structure of the genome affects the frequency of homologous and homeologous pairing during meiosis, where there were more A-C genome associations observed than A-B or B-C in an interspecific cross of B. napus and B. carinata (Mason et al 2010a). On the basis of these data and early mapping studies (Lagercrantz and Lydiate 1996), it was recognized that the B genome has significantly diverged from the A and C genomes and was not considered homeologous to any A/C genome chromosomes (Warwick et al 1992;Axelsson et al 2000).…”

The Fate of Chromosomes and Alleles in an AllohexaploidBrassicaPopulation

“…FISH was carried out according to protocols detailed in Mason et al (2010), at INRA, Le Rheu, France. The B genome was detected using Texas Red-labeled B. nigra DNA as a probe (GISH, Genomic In Situ Hybridisation), and the C genome was detected using Alexa 488-labeled BAC BoB014O06 to highlight a C-genome-specific repetitive sequence distributed over the nine C-genome chromosomes (Howell et al 2002;Leflon et al 2006).…”

Cytoplasmic and Genomic Effects on Meiotic Pairing in Brassica Hybrids and Allotetraploids from Pair Crosses of Three Cultivated Diploids

“…Darmor-bzh (Nicolas et al 2007). In trigenomic interspecific hybrids (AABC, BBAC, and CCAB) from the crosses of B. napus (AACC), B. juncea (AABB), and B. carinata (BBCC), a maximum of three A-A pairs, two B-B pairs, or two C-C pairs were observed across all hybrid types (Mason et al 2010), although these genomes from natural allotetraploids have experienced the evolutionary process. A maximum of two B-B pairs appeared in trigenomic hybrids from crosses between natural and synthetic B. napus and B. nigra (AC.B, A.C.B) and between B. carinata and B. rapa (BC.A) (Ge and Li 2007), while the B genome was from allotetraploid B. carinata or diploid B. nigra.…”