| Summary: | Interspecific hybridization between <i>B. oleracea</i> inbred lines of head cabbage, Brussels sprouts, kale and <i>B. taurica</i> and inbred lines of rapeseed (<i>B. napus</i> L.) were performed aiming at the development of the new sources of genetic variability of vegetable Brassicas. Using conventional crossings and the embryo-rescue techniques the following interspecific hybrids were developed: 11 genotypes of F<sub>1</sub> generation, 18 genotypes of F<sub>2</sub> and F<sub>1</sub> × F<sub>2</sub> generations (produced after self- and cross-pollination of interspecific F<sub>1</sub> hybrids), 10 plants of the BC<sub>1</sub> generation (resulted from crossing head cabbage cytoplasmic male-sterile lines with interspecific hybrids of the F<sub>2</sub> and F<sub>1</sub> generations) and 8 plants of BC<sub>1</sub> × (F<sub>1</sub> × F<sub>2</sub>). No viable seeds of the BC<sub>2</sub> generation (<i>B. oleracea</i>) were obtained due to the strong incompatibility and high mortality of embryos. The morphological characteristics during the vegetative and generative stages, pollen characteristics, seed development and propagation, nuclear DNA contents and genome compositions of interspecific hybrids were analyzed. All the interspecific F<sub>1</sub> hybrids were male-fertile with a majority of undeveloped and malformed pollen grains. They showed intermediate values for morphological traits and nuclear DNA contents and had nearly triploid chromosomal numbers (27 to 29) compared with parental lines. The F<sub>2</sub> generation had a doubled nuclear DNA content, with 52 and 56 chromosomes, indicating their allohexaploid nature. F<sub>2</sub> hybrids were characterized by a high heterosis of morphological characteristics, viable pollen and good seed development. F<sub>1</sub> × F<sub>2</sub> hybrids were male-fertile with a diversified DNA content and intermediate pollen viability. BC<sub>1</sub> plants were male-sterile with an intermediate nuclear DNA content between the F<sub>2</sub> and head cabbage, having 28 to 38 chromosomes. Plants of the BC<sub>1</sub> × (F<sub>1</sub> × F<sub>2</sub>) generation were in majority male-fertile with 38–46 chromosomes, high seed set, high heterosis and intermediate values for morphological traits. The obtained interspecific hybrids are valuable as new germplasm for improving <i>Brassica</i>-breeding programs.
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