Comparative genetics and cereal evolution

We now recognize that chromosome gene content (synteny) and gene order (colinearity) have been conserved among the grasses over much longer evolutionary periods than previously thought. The first indication that this was the case arose from the comparative genetic maps of the three genomes of hexaploid bread wheat. Apart from a few evolutionary intra- and inter-genomic translocations that became fixed during the recent evolution of the domesticated crop, chromosomes of the A, B, and D genomes are found to be entirely colinear. Moreover, serious disruption of the conserved gene order within chromosome segments was not observed in later comparisons with other Triticeae crop species, rye and barley, or relatives, e.g., Aegilops umbellulata Zhuk., although, in some cases, the chromosomes have been considerably rearranged by translocations. </P><P>In fact, colinearity is found to be largely maintained throughout the grasses , representing some 60 million years of evolution. Some chromosomal rearrangements are, of course, necessary to provide for the different haploid chromosome numbers, and translocations have been identified which define subfamilies of species such as the Pooideae and Panicoideae. Other results include the clear demonstration that maize is indeed an ancient tetraploid. </P><P>Comparisons have recently been made to assess the degree of colinearity retained between monocots and eudicots. This is particularly important now that the complete arabidopsis genomic DNA sequence is available. The results indicate that similarities exist even between these two groups. Sadly, however, the rice-arabidopsis DNA sequence comparisons indicate that changes have been so extensive that predictive positional cloning utility between the two model species will be minimal.