Development of yellow seeded BRASSICA NAPUS L., through interspecific crosses
Yellow seeded Brassica napus was developed by introgressing genes for yellow seed colour from the two mustard species, B. juncea and B. carinata into rape, B. napus through interspecific crosses. This achievement was based on the hypothesis that it should be possible to incorporate genes for yellow seed colour in the A and C genomes of B. napus L. (genome AACC), from yellow seeded B. juncea Czern. (genome AABB) and yellow seeded B. carinata Braun (genome BBCC). The interspecific crossing scheme involved the following steps. Black seeded, fully pigmented B. napus (genome AACC) was crossed with yellow seeded B. juncea (genome AABB), and with yellow seeded B. carinata (genome BBCC). The objective of these two interspecific crosses was the introgression of genes for yellow seed colour from the A genome of B. juncea and the C genome of B. carinata into the A and C genomes of B. napus, respectively. The interspecific F1 generations were backcrossed to B. napus to shift the genome composition towards B. napus, thus eliminating undesirable B genome chromosomes and to improve fertility. Backcross F2 (BCF2) plants of the (B. napus x B. juncea) x B. napus cross, carrying genes for yellow seed colour from B. juncea in their A genome, were crossed with BCF2 plants of the (B. napus x B. carinata) x B. napus cross, carrying genes for yellow seed colour from B. carinnta in their C genome. The objective of this intercrossing was to combine the A and C genome yellow seeded characteristics of the two backcross populations into one genotype. The F2 generation of the BCF2 intercrosses was grown in the field, and F2 plants were individually harvested. Seed colour of F2 plants was visually rated. The hypothesis was confirmed with the identification of 91 yellow seeded plants from the 4858 inspected plants and that the interspecific crossing and selection scheme was successful in introgressing the genes for yellow seed colour from B. juncea and B. carinata into B. napus. It is concluded, from the results of this study, that genes for yellow seed colour must be present in the homozygous recessive condition in both the A and C genomes of B. napus to achieve yellow seeded forms in this species. Yellow seeded B. juncea and B. carinata were suitable sources of genes for yellow seed colour, and their introgression into the B. napus A and C genomes, respectively, was a successful strategy for developing yellow seeded B. napus. The number of yellow seeded plants, that segregated in six of the 20 F2 families studied, closely approximated a 1:15, two gene segregation ratio which supported the hypothesis of introgression of one seed colour gene each from the A genome of B juncea and the C genome of B. carinata into the A and C genomes of B. napus. It is further concluded that black seed colour in B. napus is dominant over yellow seed colour, and that yellow-brown seeded B. napus plants are the result of incomplete dominance relationships of brown over yellow in the C genome of B. carinata. The yellow seeded B. napus plants developed in this study provide a new source of yellow seededness in B. napus which may be utilized in breeding yellow seeded B. napus cultivars.