DEVELOPMENT OF SYNTHETIC BRASSICA AMPHIDIPLOIDS BY RECIPROCAL HYBRIDIZATION AND COMPARISON TO NATURAL AMPHIDIPLOIDS

In a previous study we proposed that cytoplasmic genomes have played a n important role in the evolution of Brassica amphidiploid species. Ba sed on this and other studies, we hypothesized that interactions betwe en the maternal cytoplasmic genomes and the paternal nuclear genome ma y cause alterations in genome structure and/or gene expression of a ne wly synthesized amphidiploid, which may play an important role in the evolution of natural amphidiploid species. To test this hypothesis, a series of synthetic amphidiploids, including all three analogs of the natural amphidiploids B. napus, B. juncea, and B. carinata and their r eciprocal forms, were developed. These synthetic amphidiploids were ch aracterized for morphological traits, chromosome number, and RFLPs rev ealed by chloroplast, mitochondrial, and nuclear DNA clones. The mater nal transmission of chloroplast and mitochondrial genomes was observed in all of the F1 hybrids examined except one hybrid plant derived fro m the B. rapa x B. oleracea combination, which showed a biparental tra nsmission of organelles. However, the paternal chloroplast and mitocho ndrial genomes were not observed in the F2 progeny. Nuclear genomes of synthetic amphidiploids had combined RFLP patterns of their parental species for all of the nuclear DNA clones examined. A variation in fer tility was observed among self-pollinated progenies of single amphidip loids that had completely homozygous genome constitutions. Comparisons between natural and synthetic amphidiploids based on restriction frag ment length polymorphism (RFLP) patterns indicated that natural amphid iploids are considerably more distant from the progenitor diploid spec ies than the synthetic amphidiploids. The utility of these synthetic a mphidiploids for investigating the evolution of amphidiploidy is discu ssed.