Interspecific Hybridizations Between Crop Brassicas And A Wild Species Brassica Maurorum And Their Genomic Relationships

Brassica is the core genus in the family Cruciferae, containing many important edible oil, vegetable and feed crops. There are many wild species related to brassica crops, which can be used as germplasm for the genetic improvement of the cultivated species. The meiotic analysis of the wide hybrids and progenies provides important information for the genetic relationships between the genomes of these species. Doubling chromosomes of the hybrids is a main way to the artificial production of allopolyploids. The Brassica wild species Brassica maurorum Durieu (2n=16, MM) carries the resistance to white rust and alternaria blight. In the present study, several hybrid combinations were made between B. maurorum and brassica crops. The meiosis in the F1 plants was investigated by applying the method of genomic in situ hybridization (GISH) to gain an insight into the genomic relationships among these genomes from B. maurorum and brassica crops. The F1 plants were treated with colchicine and allohexaploids were obtained with their cytologg analyzed. The main results are as follows:1. GISH analysis of B. maurorum×B. rapa (M.A,2n=18) and B. maurorum×B. napus (M.AC,2n=27) hybrids. With the labeled DNA of B. maurorum as probe, the parental chromosomes in these two hybrids are distinguished. In M.A, the chromosomes from A and M genomes are present as unvalents in 28% poll mother cells (PMCs). Up to 2 allosyndetic bivalents are formed between A and M genomes, averged 0.48 bivalent. Up to two autosyndetic bivalents are found within A genome, averged 0.27. The maximum mumber of bivalents formed within M genome is one and the averge is 0.12. In M.AC, two bivalents are formed maximally in the M genome with the averge 0.11. Up to two allosyndetic bivalents are formed between M and A/C gnomes, with an averge of 0.78. Two to seven bivalents are found among A/C chromosomes.2. Dual-color GISH analysis of hybrids from the cross B. maurorum×B. juncea M.AB,2n=26) and the reciprocal crosses B. maurorum×B. carinata (M.BC, BC.M, 2n=25). By using dual-color GISH with the simultaneous application of the labeled genomic DNA of B. maurorum and B. nigra as probes, the chromosomes of three genomes in these hybrids could be distinguished unequivocally, and then autosyndesis within each genome and allosyndesis between any two genomes were identified. In M.AB, two autosyndetic bivalents are formed in A, B and M genomes and the average number is 0.39,0.23,0.26, respectively. A maximum of three allosyndetic bivalents appeared between A-B, A-M, and B-M genomes, respectively, while the averge mumbers are 0.90, 0.43,0.72, respectively. The average of the trivalents involving two or three different genomes is 0.06 per PMC. In M.BC, a maximum of one autosyndetic bivalent is found for B and M genomes, but two for C genome. The average mumber of autosyndetic bivalents formed in B, C, M genome is 0.15,0.35,0.14, respectively. Allosyndetic bivalents from 0 to 2 are observed between B-C, B-M, and C-M genomes. The average mumber of allosyndetic bivalents formed between B-C, B-M and C-M genome is 0.64,0.39,0.31, respectively. The average of the trivalents is 0.05 per cell. The similar pairings in M.BC and BC.M hybrids suggest that the cytoplasm of B. maurorum or B. carinata have no obvious effect on chromosome pairing. The B-M allosyndesis frequency is higher than that of A-M or C-M in these hybrids, revealing their closer relationship. The higher allosyndesis frequency among A, B and C genomes than their autosyndesis in these combinations suggests that the intergenomic homoeology is higher than intragenomic homology.3. Artificial synthesis of allohexaploids. Allohexaploids (AACCMM,2n=54; MMAABB,2n=52; BBCCMM,2n=50) are obtained by doubling chromosomes of the hybrids AC.M, M.AB, BC.M, repectively. All the allohexaploid plants exhibit polyploid characteristics, e.g. larger stems, thicker leaves, larger flowers, etc. The AACCMM is totally male sterile but can produce a few seeds after being pollinated abundantly by parental B. napus. Partial flowers in BBCCMM have viable pollen grains and can produce some seeds after selfcrossing. The MMAABB has high fertility and shows good seed-sets. The AACCMM and BBCCMM show abnormal chromosome pairings with some multivalents and unequal chromosome separations at anaphaseⅠandⅡ, frequently together with laggards. The meiosis in the PMCs of the MMAABB is normal.4. Backcrossing progenies of allohexaploids. After pollinated by B. napus, the male sterile allohexaploids (AACCMM) from B. napus×B. maurorum produced the BC1 progenies with the expected complement (2n=46, AACCM), which are still male sterile and give rise to BC2 plants. All the BC2 plants are also sterile and have variable phenotypes. They show different chromosome mumbers and the additional M-genome chromosomes and allosyndesis between M and A/C chromosomes are detected.