Abstract
In Brassica, interspecific hybridisation plays an important role in the formation of allopolyploid cultivars. In this study, the ploidy of F1 and F2 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 F1 hybrids of head cabbage × rapeseed (S3) and kale × 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 F2 generation, which were derived after self-pollination of F1 hybrids (FS3, FS20) or by open crosses between F1 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 F2 hybrids had many superior agronomic traits compared to parental B. napus and B. oleracea lines and triploid F1 hybrids. In the generative stage, they were characterised by larger flowers and flower elements, such as anthers and lateral nectaries. F2 hybrids were male and female fertile. The pollen viability of F2 hybrids was comparable to parental genotypes and varied from 75.38% (FS3) to 88.24% (FC320), whereas in triploids of F1 hybrids only 6.76% (S3) and 13.46% (S20) of pollen grains were fertile. Interspecific hybrids of the F2 generation derived by open crosses between plants of the F1 generation (FC320, FC230) had a better ability to set seed than F2 hybrids generated from the self-pollination of F1 hybrids. In the vegetative stage, F2 plants had bigger and thicker leaves, larger stomata, and significantly thicker layers of palisade and spongy mesophyll than triploids of the F1 generation and parental lines of B. oleracea and B. napus. The allohexaploid F2 hybrids analysed in this study can be used as innovative germplasm resources for further breeding new vegetable Brassica crops at the hexaploid level.
Highlights
Interspecific hybridisation plays an important role in the origins of polyploid cultivars, and it is acknowledged as the main source of genetic variation in cultivated plants
The F1 hybrids of head cabbage × rapeseed (S3) and kale × rapeseed crosses (S20) were allotriploids with 2n = 28 chromosomes; the nuclear DNA content was intermediate to the genome size of progenitors (1.97 and 1.99 pg, respectively)
In interspecific hybrids of the F2 generation, which were derived after self-pollination of F1 hybrids (FS3, FS20) or by open crosses between plants of the F1 generation (FC320, FC230), chromosome numbers were similar (2n = 56 or 2n = 55), whereas genome sizes varied between 3.81 ± 0.10 pg 2C for FS20 and 3.95 ± 0.091 pg 2C for FC230
Summary
Interspecific hybridisation plays an important role in the origins of polyploid cultivars, and it is acknowledged as the main source of genetic variation in cultivated plants. Polyploidy may result from the functioning of 2n gametes (meiotic doubling), distant hybridisation followed by chromosome doubling, or induced through artificial chromosome doubling (mitotic polyploidisation) [3,4]. Polyploid plants originating from the process of meiotic polyploidisation are superior to those produced from somatic doubling due to enhanced meiotic recombination, which leads to the rapid creation of genetic diversity [7]. Allopolyploid crops that contain two or more genomes derived from distant species usually benefit from combining their traits and exhibit improved agronomic characteristics, better generative reproduction, and are useful as material for the breeding of new polyploid crop species [11]
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