|Keywords (Extracted from title, table of contents and abstract of thesis)
brassica species, amphidiploid-amphidiploid, amphidiploid-diploid, diploid-diploid, digenomic triploids, trigenomic haploids, induced colchiploids, b. nigra, b. campestris, b. juncea, b. napus, b. carinata, hybridization, cytogenetic procedures, crossing procedures
Studies based upon; 9012 Interspecific sexual crosses (attempted in 42, 96 and 18 amphidiploid-amphidiploid, amphidiploid-diploid and diploid-diploid combinations respectively), and chromosomal associations in 5 species, 8 digenomic triploids, 3 trigenomic haploids and 2 induced colchiploids are presented here. Crosses were attempted among fourteen genotypes, three each of B. nigra, B. campestris, B. juncea, B. napus and two of B. carinata. Sum of 2645, 4823 and 1544 pollinations were made at the amphidiploid-amphidiploid, amphidiploid-diploid and diploid-diploid levels, respectively. The crossibility was very high at amphidiploid- amphidiploid ploidy level. Brassica napus and B. juncea among the species and B. juncea X B. napus among the combinations/direction of cross proved highly crossible. Brassica juncea crossed freely with Brassica napus and B. campestris, and fairly with B. carinata and B. nigra. Brassica napus neither proved as a good seed parent or as a good pollinator with B. carinata. None out of 267 pollinations attempted In 6 combinations of B. campestris x B. carinata produced any hybrid seed. Hence Brassica juncea among the species and Jun 9 among its genotypes proved the highly crossible and can be used as bridge for genetic transgression across the genomes. The cross-compatibility at every level demonstrated the monophyletic origin of Brassica species. The highly crossible amphidiploids however determine their relative phylogenetic closeness as compared to the diploids.
Chromosomally B. campestris (2N) proved as ideal diploids with 1/1 synapsis and 1-1 disjunction. Neither any secondary association nor any B chromosomes was observed in its PMC's, at Ml. Autoploidy was induced through applying 0.02 70 aqueous colchicine to the seedlings of Brassica campestris and it was concluded that, its 6 hours treatment is threshold for colchiploidy induction in Brassica. In autotetraploids however, the chromosomes were oriented into asyndetics, one or two-chiasmatic bivalents, trivalents and quadrivalents. The anomalous chromosomal behavior predicts the imbalance chromosomal disjunction; resulting into viable aberrant gametes the union of such gametes could give rise to individuals, with a variety of genome constitutions. Meiotic analysis of autotetraploids that, autoploidy and the union of their aberrant gametes has probably played an important role in the evolution of the present Brassica species.
Meiotic analysis in 8 digenomic triploids, 4 each containing genomes AAC and AAB, and 3 trigenomic haploids containing genomes ABC revealed univalent, bivalent and multivalent chromosomal configurations of various orientations. Variation in pollen fertility was so evident even within a plant and tetrads ranging from all the fertile to all the sterile gametocytes with variety of all the possible intermediates were available. Though variation among chromosomal association was present at the combinations and association levels uniformity in the frequency of the synaptic and asynaptic chromosomes was uniforl1l i.e. 70 and 30 percent respectively either in the case of digenomic triploids or trigenomic haploids. Ml associations both in digenomic triploids or trigenomic haploids showed that, no one among the genomes B or C had got any genetic factor for suppressing the homoeologous pairing among genomes A, from various sources. Besides autosyndesis among the genomes A, allosyndesis was also evident among the genomes A, C and B from the same or different sources, respectively. Furthermore meiotic regularity was higher in the digenomic triploids ABA and ACA as compared to the trigenomic haploids ABC. The studies also concluded that, whatsoever the source or origin of genome A was, it has retained chromosomal homology resulting frequently into the autosyndetic bivalents in the digenomic triploids.