Recombination between chloroplast genomes of Trachystoma ballii and Brassica juncea following protoplast fusion

We document here the presence of a recombinant plastome in a cytoplasmic ma le sterile (CMS) line of Brassica juncea developed from the somatic hybrid Trachystoma ballii + B. juncea. Restriction endonuclease digestion of the c hloroplast (cp) DNA has revealed that the recombinant plastome gives rise t o novel fragments in addition to the parent-specific fragments. Analysis of the 16S rRNA region by Southern hybridization shows no variation between B . juncea, T. ballii and the CMS line. The rbcL gene region of the recombina nt plastome is identical to that in T. ballii. Analysis with probes for psb A and psbD using single and double DNA digests indicates that the hybridiza tion patterns of the recombinant plastome are identical to those of the par ents in digests obtained with some restriction enzymes, while novel bands h ybridize to probes in other digests. In the psbA region, a B. juncea-specif ic PstI site and a T. ballii-specific EcoRI site are found in the recombina nt plastome. The psbD region of the recombinant plastome contains a B. junc ea-specific HindLII site and T. ballii-specific BamHI and HyaII sites. Thes e results indicate the occurrence of intergenomic recombination between the chloroplasts of T. ballii and B. juncea in the somatic hybrid from which t he CMS line was developed. The recombined plastome appears to be a mosaic o f fragments specific to both parents and the recombination event has occurr ed in the single-copy regions. These recombinational events have not caused any imbalance in the recombinant plastome in terms of chloroplast-related functions, which have remained stable over generations.