Bridging Model and Crop Legumes through Comparative Genomics

The Fabaceae, or legumes, constitute the third largest family of flowering plants, comprising more than 650 genera and 18,000 species (Polhill and Raven, 1981).Economically, legumes represent the second most important family of crop plants after Poaceae (grass family), accounting for approximately 27% of the world's crop production (Graham and Vance, 2003).On a worldwide basis, legumes contribute about onethird of humankind's protein intake, while also serving as an important source of fodder and forage for animals and of edible and industrial oils.One of the most important attributes of legumes is their unique capacity for symbiotic nitrogen fixation, underlying their importance as a source of nitrogen in both natural and agricultural ecosystems.Legumes also accumulate natural products (secondary metabolites) such as isoflavonoids that are beneficial to human health through anticancer and other health-promoting activities (Dixon and Sumner, 2003).The legumes are highly diverse and can be divided into three subfamilies: Mimosoideae, Caesalpinioideae, and Papilionoideae (Doyle and Luckow, 2003).Of these, the Papilionoideae subfamily contains nearly all economically important crop legumes, including soybean (Glycine max), peanut (Arachis hypogaea), mungbean (Vigna radiata), chickpea (Cicer arietinum), lentil (Lens culinaris), common bean (Phaseolus vulgaris), pea (Pisum sativum), and alfalfa (Medicago sativa).With the notable exception of peanut, all these important crop legumes fall into two Papilionoid clades, namely, Galegoid and Phaseoloid, which are often referred to as cool season and tropical season legumes, respectively (Fig. 1).Despite their close phylogenetic relationships, crop legumes differ greatly in their genome size, base chromosome number, ploidy level, and selfcompatibility (Table I).Nevertheless, earlier studies indicated that members of the Papilionoideae subfamily exhibited extensive genome conservation based on comparative genetic mapping (Weeden et al., 1992;Menancio-Hautea et al., 1993).To establish a unified genetic system for legumes, two legume species in the Galegoid clade, Medicago truncatula and Lotus japonicus,