Effect of Meiotic Polyploidisation on Selected Morphological and Anatomical Traits in Interspecific Hybrids of Brassica oleracea x B. napus

In Brassica, interspecific hybridisation plays an important role in the formation of allopolyploid cultivars. In this study, the ploidy of F-1 and F-2 generations resulting from interspecific hybridisation between B. oleracea inbred lines of head cabbage (B. oleracea L. var. capitata) (2n = 18) and kale (B. oleracea L. var. acephala) (2n = 18) with inbred lines of rapeseed (B. napus L.) (2n = 38) was examined by flow cytometry analysis and chromosome observation. Furthermore, the effect of meiotic polyploidisation on selected phenotypic and anatomical traits was assessed. The F-1 hybrids of head cabbage x rapeseed (S3) and kale x rapeseed crosses (S20) were allotriploids with 2n = 28 chromosomes, and nuclear DNA amounts of 1.97 (S3) and 1.99 pg (S20). These values were intermediate between B. oleracea and B. napus. In interspecific hybrids of the F-2 generation, which were derived after self-pollination of F-1 hybrids (FS3, FS20) or by open crosses between F-1 generation hybrids (FC320, FC230), the chromosome numbers were similar 2n = 56 or 2n = 55, whereas the genome sizes varied between 3.81 (FS20) and 3.95 pg 2C (FC230). Allohexaploid F-2 hybrids had many superior agronomic traits compared to parental B. napus and B. oleracea lines and triploid F-1 hybrids. In the generative stage, they were characterised by larger flowers and flower elements, such as anthers and lateral nectaries. F-2 hybrids were male and female fertile. The pollen viability of F-2 hybrids was comparable to parental genotypes and varied from 75.38% (FS3) to 88.24% (FC320), whereas in triploids of F-1 hybrids only 6.76% (S3) and 13.46% (S20) of pollen grains were fertile. Interspecific hybrids of the F-2 generation derived by open crosses between plants of the F-1 generation (FC320, FC230) had a better ability to set seed than F-2 hybrids generated from the self-pollination of F-1 hybrids. In the vegetative stage, F-2 plants had bigger and thicker leaves, larger stomata, and significantly thicker layers of palisade and spongy mesophyll than triploids of the F-1 generation and parental lines of B. oleracea and B. napus. The allohexaploid F-2 hybrids analysed in this study can be used as innovative germplasm resources for further breeding new vegetable Brassica crops at the hexaploid level.