Cytogenetics Of Synthetic Brassica Allohexaploid

Three cultivated brassica diploid species(B. rapa, B. oleracea, B. nigra) are hybridized to produce three allotetraploid species(B. napus, B. juncea, B. carinata). There is no natural higher ploid species than allotetraploid. In the present investigation, four allohexaploid(2n = 54, AABBCC) were obtained by three different crosses.The serial selfed progenies were analyzed by morphological, pollen fertility, and molecular cytogenetics. Main findings were as follows:1.Through three order of hybridization and chromosome doubling between diploid([B.rapa × B. oleracea var. alboglabra] × B. nigra, [B. oleracea var. alboglabra × B. rapa] ×B. nigra) two different source hexaploid(Ar Ar.CoCo.BiBi/CoCo.ArAr.BiBi were produced,genome superscript symbols represented the source of the parent species, such as from B.rapa chromosomes as ArAr) which respectively have B. rapa and B. oleracea cytoplasm.B. juncea(2n=36, AABB)as maternal parent crossed with B. oleracea to obtain allohexaploid AjAjBjBj.CoCo, and the progenies were selfed three generations. Previously,allohexaploid obtained between B. carinata and B. rapa was also selfed cross for three genenations(S5-S7).2.Four types of trigenomic Brassica allohexaploids(2n=54, AABBCC) were produced by different crossing approaches and their self-pollinated progenies of successive generations(S0-S7) were used for this study. Constituent Brassica subgenomes were distinguished by their species of origin. The first two allohexaploids(CoCo.ArAr.BiBi and ArAr.CoCo.BiBi) were obtained by treating the cloned plantlets of two trigenomic hybrids(Co.Ar.Bi and Ar.Co.Bi).They were derived from the sequential crosses of the same three Brassica diploids, and contained the same chromosome complement but had the cytoplasm from B. rapa and B. oleracea var. alboglabra, respectively. The third allohexaploid(AjAjBjBj.CoCo) was synthesized by chromosome doubling of the trigenomic hybrid(AjBj.Co) derived from the B. juncea(L.) Czern.(2n=36, AABB,genotype XZ-1) × B. oleracea var. alboglabra L.(2n=18, CC, the same genotype Chi Jie Lan). The fourth allohexaploid(BcBc CcCc.ArAr) was produced previously from the B.carinata(2n=34, BBCC, accession GO-7) × B. rapa ssp. chinensis cv. Shanghaiqing(AA).3.Those four trigenomic Brasscia allohexaploids approaches at successive generations were investigated for phenotype, fertility, and chromosome complement andgenome stability by multi-color fluorescence in situ hybridization(FISH). By multi-color FISH analysis with C-genome and B-genome specific probes, it was feasible to determine the chromosome complements of three genomes in these allohexploids and their progenies with variable chromosome numbers, and to identify the meiotic pairing configurations involving intra- and inter-genomic chromosomes. Both euploid and aneuploid progenies were identified. The most aneuploids maintained all B- and A-genome chromosomes and variable number of C-genome chromosomes, suggesting that genome stability was B>A>C. In the extreme case, loss of whole set of C-genome chromosomes led to the production of B. juncea-type progeny. In particular, the so-called hidden aneuploid progenies had the same number of chromosomes(2n=54) as the euploid,but the simultaneous loss and gain of A- and C-genome chromosomes. The diploidized and non-diploidized meiotic behaviors co-occurred in all allohexaploid individuals of consecutive generations. The aberrant chromosome pairing and segregation mainly involved the chromosomes of A and C genomes, which resulted in aneuploidy in self-pollinated progenies.To summarize, the three genomes of newly formed Brassica allohexaploids from several crosses showed different chromosome stability in consecutive self-pollinating generations. The B-genome chromosomes were completely retained in the majority of progeny, while C- genome chromosomes were most vulnerable to be lost and the change of A-genome chromosomes varied in less range than the C-genome chromosomes.Variation in chromosome number occurred only between plants, not within a plant,suggesting that the chromosome elimination did not occur during mitotic division. In general, these allohexaploids mainly showed normal chromosome pairing and segregation,contributing to the transgenerational breeding of the allohexaploids with euploid complement. Aberrant pairing and segregation biased to A and C genomes would cause successive loss of the A- and C-genome chromosomes in the progenies of these allohexaploids. Specially, all C-genome chromosomes were completely eliminated in AjAjBjBj.CoCoafter three generations, leading to reappearance of parental B. juncea type.Similarly, the B. juncea-type progeny(ArAr BcBc) was recovered in the S7 generation of the BcBcCcCc.ArAr, which harbored A-genome from natural B. rapa and B genome from natural B. carinata. So the chromosome number and complement of the self-pollinated progenies varied from those of the euploid allohexaploids and B. juncea(2n=36, AABB).The diploidized and non-diploidized meiotic behavior of these allohexaploids atsuccessive generations contributed to the production of the euploid and aneuploid progenies. The much wide range of chromosome variation in these allohexaploids than that in the synthesized allotetraploid B. napus and allohexaploid wheat suggested genomic and ploidy effects on the differential stability of the constituent genomes in these newly synthesized allopolyploids.The meiosis-driven distinct genome stability in these synthesized allohexaploids was likely associated with the difference in inherent nature of three genomes and also their relationships. The results provide some new insights into the cytological behavior of different genomes in the early stages of allopolyploidization.