Phylogenetic reconstruction based on low copy DNA sequence data in an allopolyploid: The B genome of wheat

Study of bread wheat (Triticum aestivum) may help to resolve several questi ons related to polyploid evolution. One such question regards the possibili ty that the component genomes of polyploids may themselves be polyphyletic, resulting from hybridization and introgression among different polyploid s pecies sharing a single genome. We used the B genome of wheat as a model sy stem to test hypotheses that bear on the monophyly or polyphyly of the indi vidual constituent genomes. By using aneuploid wheat stocks, combined with PCR-based cloning strategies, we cloned and sequenced two single-copy-DNA s equences from each of the seven chromosomes of the wheat B genome and the h omologous sequences from representatives of the five diploid species in sec tion Sitopsis previously suggested as sister groups to the B genome. Phylog enetic comparisons of sequence data suggested that the B genome of wheat un derwent a genetic bottleneck and has diverged from the diploid B genome don or. The extent of genetic diversity among the Sitopsis diploids and the fai lure of any of the Sitopsis species to group with the wheat B genome indica ted that these species have also diverged from the ancestral B genome donor . Our results support monophyly of the wheat B genome.