Genome relationships: the grass model in current research.

Ten years ago, with the advent of comparative mapping, a new tool became available to plant geneticists. Comparative genome analyses demonstrated that gene orders among related plant species remained largely conserved over millions of years of evolution. This finding has revolutionized our thinking and formed the basis of a new science—comparative genomics. Since the first comparative mapping experiments, studies of genome relationships have included an ever wider range of plant species, and the focus has shifted from comparisons at the gross map level to studies of gene organization in small chromosomal regions and finally to the DNA sequence itself. To date, the most comprehensive data set comes from the grasses. The Poaceae family includes the staple cereals rice, maize, wheat, barley, sorghum, and the millets. For most of the past century, research has proceeded on each of these species individually. Mutants were produced, genetic maps were generated, and biochemical pathways were unraveled. Comparative genomics has provided the basis and stimulus to integrate this knowledge for application to all cereal crops. The realization of the power of a comparative approach has led to several plant genome initiatives and has promoted rice as a model for cereal crops. The rice genome, with only 400 million DNA base pairs (Mb), is the smallest among the major cereal crops and only approximately four times larger than that of the eudicot model plant Arabidopsis. Dense genetic maps, carrying some 2500 markers, have been constructed, and physical maps cover most of the genome. Additionally, large public collections of expressed sequence tags (ESTs) have become available, and it is clear that sequence data covering the entire rice genome will add a further vital dimension to comparative genomics in the near future. Views on the prospective applications of comparative genomics have developed as new insights have been gained into genome relationships and genome organization. At the same time, new opportunities have been envisaged and exploited. Below, we present an overview of the status of comparative genomics, with examples of ongoing research projects, focusing on wheat.