| The phenomenon of complete and partial parent genome separation was found inhybrids between six Brassica species and Orychophragmus violaceus (L.) O.E.Schulz(2n=24). Results from the hybridization also indicated that the different chromosomebehavior in different combinations might be arised by differetn maternal parent genome.The different chromosome behavior of hybrids with three Brassica diploids (B. rape, B.nigra and B. oleracea) might contribute to the different cytogenetics of hybrids with threetetraploids (B. napus, B. juncea and B. carinata). Those findings are new chromosomebehaviors in plant wide hybridization beside pseudogamy, semigamy and chromosomeelimination and enriched the knowledge of plant genetics. To further study the mechanismbehind the phenomenon, we synthesized Brassica allohexaploid from differentcombinations and cross with O.violaceus as maternal parent. Cytology, genomic in situhybridization (GISH), amplified fragment length polymorphisms (AFLP) andsingle-strand DNA conformation polymorphisms (SSCP) analyses were applied tohybrids. To obtain different allohexaploid, three different tri-genomic hybrids (ABC,2n=27) from crosses BC.A: B.carinata (G0-11, BBCC, 2n=34)×B.camprists (shanghaiqin,AA, 2n=20), AC. B: B. napus (zhong you 821, AACC, 2n=38)×B. nigra (Koch, BB, 2n=16),A. C. B: artificial B. napus×B. nigra were used for chromosome double. Three typeallohexaploids were named hexaploidâ… ,â…¡,â…¢respectively. Main results were describedas follows:1. A relatively high proportion of bivalents were formed in all tri-genomic hybridsfrom different combinations. Hybrids from combination of BC. A, AC. B, A. C. B had88. 2%, 96. 8% and 91. 2% PMCs (Pollen Mother Cells) with 6-10 bivalents, 14.7%, 24.5%and 9.8% PMCs with univalents less than 8 respectively. GISH analyses also revealed thateach type of hybrids had 31. 9%, 45. 4% and 50% PMCs contained at least one B-genomechromosome paired with A/C chromosomes and maximally three B-chromosomes inallosyndesis per cell observed in AC.B and A.C.B combinations. Each type of hybrids had10.9%, 16.6% and 9.4% PMCs with at least one bivalent formed by B chromosomes. Amaximum of two bivalents formed by autosyndesis within B genome at diakinesisappeared in all combinations. The accurate analyses of auto- and allo-syndetic pairing forB genome in trigenomic combinations provided further evidence for the hypothesis thatthree basic genomes of the cultivated Brassica species were secondary polyploids andderived from one common ancestral genome with a lower chromosome number. 2. 30, 4 and 3 hybrid plants were produced from crosses hexaploidâ… ,â…¡,Ⅲ×O.violaceus respectively. All hybrids were different from the hexaploid or self progeniesof hexaploid on phenotype. Young leaves and stem of adult plants showed some traits ofO.violaceus. Cytology observation revealed that all hybrids contained cells with differentchromosomes which indicated the mixploidy of the F1 plants. The highest chromosomenumber was 46, and the lowest was 20. Cells with 44 chromosomes were found withhighest frequency in 32 hybrid plants, 42 in 4 hybrid plants and 36 in only hybrid plant.3. GISH analyses found no intact chromosome or chromosome fragments fromO.violaceus in all three type hybrids. According to the number and behavior of B genomechromosomes, all hybrid plants were classified into three types. All somatic cells of Typesâ… plants contained 16 B genome chromosomes which formed 8â…¡at diakinesis and shown8:8 segregation at anaphase; Typesâ…¡contained 14 B genome chromosomes whichformed 6â…¡+2â… at diakinesis and showed 6:8 or 7:7 segregation at anaphase. Typesâ…¢contained above two chromosomes number and behavior. GISH analyses also found thatone chromosome from A genome with rDNA locus was eliminated in all hybrids betweenhexploidâ… and O.violaceus. Obviously, complete O.violaceus chromosomes and partialmaternal chromosomes were eliminated.4. AFLP analyses revealed that all hybrids contained bands specific for O.violaceus,especially for hybrids between hexaploid I and O.violaceus which contained 4.8%-7.1%bands specific for O.violaceus. Deleted bands for maternal parent and novel bands werealso found in all hybrids. Possilably, introgressinon of O.violaceus chromatin disturbedthe maternal genome.5. Cytology, morphology, AFLP and cross analyses indicated that the genome ofhybrids and their self progenies resembled B.juncea. For intact B-genome chromosomeswere found in most of the hybrids, more chromosomes from C genome were eliminatedthan from A genome which indicated that the stability of three genome showingrelationships with B＞A＞C.6. RT-PCR SSCP analyses revealed that rRNA genes from A/C genome had beentranscripted in only hybrid but rRNA genes from B genome were transcripted in allhybrids. The plant contained 14 chromosomes which indicated that the deleted twochromosomes might have rDNA locus.7. Here extra divisions and nuclei fusions were observed to occur in microsporenuclei of partial hybrids between synthetic Brassica hexaploid I and O.violaceus. Abnormal spindle were formed and chromosomes were separated into several nuclei ofvariable sizes after bi-, or multi-polar divisions in the four cells of tetrads. As aconsequence, more than eight mini-microspores of different sizes were produced by onetetrad. Genomic in situ hybridization results indicated that no chromosome replicationoccurred during such divisions. In some tetrads, the four nuclei were fused to form onelarge cell with increased chromosome number. The extra divisions or fusions appearedonly in some flower buds of one plant, some anthers in the same buds, or even inindividual cells of tetrads. The possible mechanisms behind these cytological phenomenawere discussed.8. In meiosis of one hybrid between hexaploidâ…¢and O.violaceus, no chromosomepairing was found at diakinesis. Univalents then congregated which were found always atedge of the cell. Congregated chromosomes then separated randomly to two poles.However, cytoplasm did not divide. Cells with two nuclei then stopped development butformed thick ektexine which were dyed darkly. No spindle was found when specific dyefor spindle was used for those cells.Finally, chromosome behavior of hybrids between Brassica hexaploid andO.violaceus was different from that of hybrids between diploid or tetraploid andO.violaceus. Probably, zygote cells experienced chromosome double and partial parentgenome separation during development. More C genome chromosomes were divided intocells with more O.violaceus chromosomes and those cells were eliminated later. Thespecific genome rRNA gene expression might help to stabilize its chromosomes.O.violaceus chromatin introgression lead to alternation of maternal genome structure anddisturbed gene expression, such as new AFLP bands emergence and abnormal meiosis.
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