Abstract:

Artificial intergeneric amphidiploids Rapahnobrassica (2n=36, RRCC) between Raphanus sativus L.(2n=18, RR)and Brassica oleracea L. (2n=18, CC) have been studied for more than eighty years as potential new fodder crop or bridge crop, for their disease resistance, cytoplasm male sterility and vegetative vigor. However, their use as one new crop was mainly limited for the low seed productivity. Amphidiploids synthesized from different crossing combinations showed different levels of seed fertility, which could be improved to some extent by repeated selections. To exploit the useful R. sativus genes for Brassica breeding, one new amphidiploid Raphanobrassica was produced by doubling the chromosome number of the F₁ hybrids R. sativus cv. Heqing×B. alboglabra Bailey with colchicine treatment. As previous reports, the developments of siliqua and seeds were very difficult in the early generations of the present amphidiploid (PlateⅠ-A, B), most siliquas stopped growth and became shriveled after about 30 days of fertilization, and no seeds were visible in pods. A small part of short and thick siliquas formed and contained one or several seeds per pod, but most of seeds were small, shriveled and morphologically un-regular and only a minority was of full size. Selections were made from F₄ to F₁₀ with special attention for the improvement of seed fertility, and consequently siliqua number per plant and seed maturation level increased generation by generation, on the other hand the frequency of small and shriveled seeds decreased. F₁₀ plants showed good seed fertility (PlateⅠ-E, F, G), for the mean seed yield reached 32.3 g per plant, the number of siliqua per plant was about 700, the number of seed per siliqa was 14.9 (Table 1), approximately 80% of the seeds were fully developed (PlateⅠ-F). Genomic in situ hybridization (GISH) investigations on the meiosis of F₁₀ pollen mother cells (PMCs) showed that these plants had the expected chromosome complements of the 18 chromosomes from R. sativus and the other 18 ones from B. alboglabra. The normal meiotic pairings and segregations occurred in their PMCs, for 18 bivalents were formed at diakinesis and chromosomes only paired with their homologs (PlateⅠ-H, I), segregations at anaphaseⅠ were also very regular with 18 chromosomes in each polar and without bridges and fragments (PlateⅠ-J, K). These results indicated that diploid-like meiotic behavior was established in the amphidiploid. Fertility factors analysis showed that available fertile pollen grains in anthers were enough to produce seeds in every generation, and the shriveled seeds observed frequently in the early generations might be attributable to ovule abortion. The application potential of the Raphanobrassica was discussed as a fertile genetic bridge for gene transfer from R. sativus to Brassica crops.

Key words: Raphanus sativus, Brassica alboglabra Bailey, Amphidiploid, Fertility, Genomic in situ hybridization