Restriction patterns reveal origins of chloroplast genomes in Brassica amphiploids

Chloroplast (ct) DNA from the three elementary Brassica species (B. nigra (L.) Koch, B. oleracea L. and B. campestris L.) and the three amphiploid Brassica species (B. carinata A. Br., B. napus L. and B. juncea (L.) Czern.) was digested with fifteen restriction endonucleases. In all species restriction sites for enzymes with GC-rich recognition sequences were less frequent and not as variable as for those with AT-rich sequences. Comparisons between species revealed two distinct groups of ct DNA fragment patterns: complex one, composed of B. oleracea, B. napus, B. campestris and B. juncea and complex two, composed of B. nigra and B. carinata. The patterns of B. carinata were virtually identical to those of B. nigra and those of B. juncea were virtually identical to those of B. campestris indicating not only where the ct genomes of B. carinata and B. juncea originated, but also how little these genomes have been altered since the origin of these amphiploids. Ct DNA in B. napus shows more homology with that of B. oleracea than with that of B. campestris, but the ct genome of this amphiploid has diverged more from that of its putative parent than have those of the other two amphiploids.