Exploring the Role of x = 7 Species in Brassica Evolution: Hybridization with B. nigra and B. oleracea

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Abstract
The role of the x = 7 species, B. adpressa and Diplotaxis erucoides in Brassica evolution, was investigated by hybridizing them to the cultivated species B. nigra (x = 8) and B. oleracea (x = 9). In general, the hybrids displayed a low frequency of bivalents and unbalanced reductional division during meiosis, resulting in poor fertility. This cytological behavior was also observed in hybrids between the x = 7 species, indicating strong genome divergence of both genomes. Among all the hybrids, those of D. erucoides × B. nigra had the highest level of fertility, permitting the generation of D. erucoides–B. nigra alien addition lines. One of the hybrid plants variegated for yellow-white-red petal color and red pigmentation for other organs. The same enzyme activity zones were found to have multiple bands in all the species including those with x = 7 chromosomes, suggesting possible gene duplications. These multiple banded patterns persisted in pollen leachates and were transmitted to the hybrids. The hybrids reported in this study provide additional information on intergenomic relationships among Brassica and related species of allied genera.