Broadening the avenue of intersubgenomic heterosis in oilseed Brassica.

Accumulated evidence has shown that each of the three basic Brassica genomes (A, B and C) has undergone profound changes in different species, and has led to the concept of the "subgenome". Significant intersubgenomic heterosis was observed in hybrids between traditional Brassica napus and first generation lines of new type B. napus. The latter were produced by the partial introgression of subgenomic components from different species into B. napus. To increase the proportion of exotic subgenomic components and thus achieve stronger heterosis, lines of first generation new type B. napus were intercrossed with each other, and subjected to intensive marker-assisted selection to develop the second generation of new type B. napus. The second generation showed better agronomic traits and a higher proportion of introgression of subgenomic components than did the first generation. Compared with the commercial hybrid and the hybrids produced with the first generation new type B. napus, the novel hybrids showed stronger heterosis for seed yield during the 2 years of field trials. The extent of heterosis showed a significant positive correlation with the introgressed subgenomic components in the parental new type B. napus. To increase the content of the exotic subgenomic components further and to allow sustainable breeding of novel lines of new type B. napus, we initiated the development of a gene pool for new type B. napus that contained a substantial amount of genetic variation in the A(r) and C(c) genome. We discuss new approaches to broaden the avenue of intersubgenomic heterosis in oilseed Brassica.