VARIABILITY IN WHEAT-BASED ON LOW-COPY DNA-SEQUENCE COMPARISONS

The chromosomes of the B genome of hexaploid wheat (AABBDD) do not pai r completely with those of any of the diploid species with genomes sim ilar to B. Various biochemical and molecular analyses have suggested t hat each of the five diploid species in section Sitopsis of Triticum a re ancestral to B. These observations have led to the hypothesis that the B genome may be polyphyletic, descending from more than one diploi d ancestor. This hypothesis may account for differences between the wh eat B genome and the diploids and also for variability that currently exists among different wheat accessions. In this study, we cloned and compared nucleotide sequences for three low-copy DNA fragments from th e B and D genomes of several wheat accessions and from diploid relativ es of the B and D genomes. Our results suggested that the amount of DN A sequence variability in wheat is low, although somewhat more variabi lity existed in the B genome than in the D genome. The B genome of whe at was significantly diverged from all the Sitopsis diploid species, a nd Triticum speltoides was closer to B than to other members of this s ection. The D genome of wheat was very similar to that of its progenit or, Triticum tauschii. No evidence for a polyphyletic origin of the B genome was found. A more parsimonious hypothesis is that the wheat B g enome diverged from its diploid ancestor after the original hybridizat ion event occurred.