In a previous study we proposed that cytoplasmic genomes have played an important role in the evolution of Brassica amphidiploid species. Based on this and other studies, we hypothesized that interactions between the maternal cytoplasmic genomes and the paternal nuclear genome may cause alterations in genome structure and/or gene expression of a newly 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 reciprocal forms, were developed. These synthetic amphidiploids were characterized for morphological traits, chromosome number, and RFLPs revealed by chloroplast, mitochondrial, and nuclear DNA clones. The maternal transmission of chloroplast and mitochondrial genomes was observed in all of the F1 hybrids examined except one hybrid plant derived from the B. rapa x B. oleracea combination, which showed a biparental transmission of organelles. However, the paternal chloroplast and mitochondrial 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 fertility was observed among self-pollinated progenies of single amphidiploids that had completely homozygous genome constitutions. Comparisons between natural and synthetic amphidiploids based on restriction fragment length polymorphism (RFLP) patterns indicated that natural amphidiploids are considerably more distant from the progenitor diploid species than the synthetic amphidiploids. The utility of these synthetic amphidiploids for investigating the evolution of amphidiploidy is discussed.
Although conceptually simple, in practice construction of an integrated map from diverse sources (populations and types of markers) is a non-trivial exercise. This is particularly true where genetic maps have been generated from different populations or sub-populations with different subsets of informative genetic markers. The situation is exacerbated where multiple paralogous loci may exist as a result of chromosomal segmental duplication over relatively recent evolutionary time, which in the case of B. napus is compounded by amphidiploidy. This may lead to a low number of shared (bridge or anchor) markers between maps. Moreover, the quality of genotype data may vary across studies, thus hampering the progress of genetic map integration. 041b061a72