Morphology and fertility were characterized for 22 intersubspecies hybrids within B. campestris L. Nine subspecies, representing crop types from different geographical areas, were used as pollen donors on three different seed parents. Stability of scored morphological characters was divided into four classes based on their appearance in F1 hybrids; i) constant (present in all hybrids when the character was present in one of the parents, e.g., enlarged hypocotyl, divided leaf), ii) variable (present in some hybrids when the character was present in parent types, e.g., petiole color, pubescence), iii) novel (appearing in hybrids but not present in parents, e.g., anther tip spot, self-compatibility), and iv) reciprocal differences. Constant characters are assumed to have a strong genetic component, variable characters may result from heterozygosity in parents, an allelic series, or polygenic inheritance, and novel characters may arise through mutation or altered gene or physiological interactions. Reciprocal crosses revealed morphological components controlled by the maternal parent, and were most striking in pak-choi (ssp. chinensis) by turnip [ssp. rapifera (Metzg.) Sinsk.] hybrids. Pollen and seed fertility of hybrids was generally reduced when Indian oilseeds [ssp. dichomata (Roxb.) Olsson; ssp. trilecularis (Roxb.) Olsson] were used as parents. Inheritance of the enlarged hypocotyl character was tested in one F2 population. Segregation of the enlarged hypocotyl trait was consistent with a hypothesis of a dominant Mendelian locus. Various novel characters appeared in this F2 population that were not evident in the parents of the hybrid, some of which also showed Mendelian segregation. Genetic differentiation of nuclear or plastid genomes may account for these observations.
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