This is the third largest family of angiosperm (650 genera and 18,000 species, Polhill and Raven 1981), contributing 27% of world food production in the form of edible oil, food and feed (Graham and Vance 2003). Biological nitrogen fixation is one of the distinctive features of the legumes (Dixon and Sumner 2003). In early genetic mapping studies, conserved syntenic blocks were observed among the various legumes (Menancio-Hautea et al. 1993; Boutin et al. 1995; Lee et al. 2001). Useful degrees of gene linkage conservation have been reported between lentil and pea genomes (Weeden et al. 1992); and between mung bean and cowpea (Menacio-Hautea et al. 1993). Use of anchor markers, designed on the basis of unique gene sequences (Choi et al. 2006), in multiple legume species revealed extensive synteny between model and crop legume species (Zhu et al. 2005; Choi et al. 2006).
Early pioneering studies suggested a simple genetic relationship between P. sativum and Lens culinaris (Weeden et al. 1992). Subsequent comparative genetic studies using sophisticated DNA markers have clearly shown macrosynteny between lentil (n = 7) and M truncatula (n = 8), and also detected syntenic relationships among important domestication traits such as pod indehiscence, flower color, seed coat pattern, and Fusarium wilt between the two species. Moreover, moderate level of chromosomal rearrangements might be the reason for differences in chromosome numbers between the two species (Phan et al. 2007).
Limited genomic studies have been conducted on the genus Arachis because of its monophyletic polyploid origin, resulting in little genetic diversity (Burow et al. 2001). Polyploid speciation triggered significantly the genome restructuring (Song et al. 1995; Chen et al. 1998), and may also have influenced Arachis species (Burow et al. 2001). Genetic variations in repetitive elements in the genomes of diploid progenitors may help to drive speciation in Arachis (Seijo et al. 2007). Genome duplication predating the divergence of the diploid progenitors (A&B) has been suggested (Burow et al. 2001).
In legumes, two model species Medicago truncatula and Lotus japonicus are being largely sequenced (Young et al. 2005), which will be of particular interest in cloning of genes involved in symbiosis (Schauser et al. 1999; Endre et al. 2002; Krusell et al. 2002; Nishimura et al. 2002; Stracke et al. 2002; Madsen et al. 2003; Ane et al. 2004; Levy et al. 2004). Partial genome sequences of 100 Mb each for L. japonicus and M. truncatula revealed a strong conservation of genome structures (Cannon et al. 2006). Similarly, syntenic blocks of as large as whole chromosome arms have been detected between the chromosomes of these two species based on genetic mapping (Young et al. 2005). Low levels of intra-genomic synteny within each of the genome indicated the occurrence of ancient whole genome duplication followed by gene loss and chromosomal changes (Cannon et al. 2006). Conservation between M truncatula and L japonicus genomes is higher than that of M truncatula and Glycine max (Ku et al. 2000; Choi et al. 2004).
In other studies, a high level of macrosynteny between the Pisum sativum and M. trunctula genomes and long tracts of macrosynteny between M. truncatula and Phaseoleae species (soybean and mungbean) enabled the construction of a genome-wide picture of legume synteny in the form of concentric circles of corresponding chromosomes anchored by M. truncatula (Choi et al. 2004).
A number of microsynteny studies between the closely related model species (M. trunctula and L. japonicus) and crop species (pea) enabled positional cloning of symbiosis related genes (Endre et al. 2002; Stracke et al. 2002), and have been stretched to the more evolutionarily distant soybean showing appreciable collinearity and microsynteny among BAC contigs (Yan et al. 2003, Mudge et al. 2005). Remnants of synteny were also identified between G. max and A. thaliana (Grant et al. 2000; Lee et al. 2001). An extended sequences comparison of the tandemly duplicated N-hydroxycinnamoyl/benzoyltransferase (HCBT) gene family derived from G. max, M. truncatula and A. thaliana revealed a network of synteny within conserved regions, interrupted by gene loss and rearrangements (Schlueter et al. 2008). Comparison of model genomes with the forthcoming soybean genome (Jackson et al. 2006) will help in further clarifying gene order and synteny among the different genomes of the family Fabaceae.
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