Tracing B-genome chromatin in Brassica napus × B. juncea interspecific progeny
We used polymerase chain reaction (PCR) and fluorescence in situ hybridization (FISH) techniques to demonstrate the presence of Brassica B-genome chromosomes and putative B-genome introgressions in B. napus × B. juncea interspecific progeny. The B-genome - specific repeat sequence pBNBH35 was used to generate PCR products and FISH probes. The highest frequencies of viable progeny were obtained when B. napus was the maternal parent of the interspecific hybrid and the first backcross. B-genome - positive PCR assays were found in 34/51 fertile F2 progeny (67%), which was more than double the proportion found in fertile BC1 progeny. Four B-genome - positive F2-derived families and 1 BC1-derived family were fixed or segregating for B. juncea morphology in the F4 and BC1S2, respectively, but in only 2 of these families did B. juncea-type plants exhibit B. juncea chromosome count (2n = 36) and typical B-genome FISH signals on 16 chromosomes. The remaining B. juncea-type plants had B. napus chromosome count (2n = 38) and no B-genome FISH signals, except for 1 exceptional F4-derived line that exhibited isolated and weak B-genome FISH signals on 11 chromosomes and typical A-genome FISH signals. B. juncea morphology was associated with B-genome - positive PCR signals but not necessarily with 16 intact B-genome chromosomes as detected by FISH. B-genome chromosomes tend to be eliminated during selfing or backcrossing after crossing B. juncea with B. napus, and selection of lines containing B-genome chromatin during early generations would be promoted by use of this B-genome repetitive marker.