A genome-wide analysis of single nucleotide polymorphism diversity in the world's major cereal crops

Over 3.5 million expressed sequence tags from the major cereal taxa were used to electronically mine over 176 000 putative single nucleotide polymorphisms (SNPs). The density, distribution and degree of linkage between these SNPs were compared among the different taxa. The frequency of sequence polymorphism was lowest in diploid taxa (rice, barley and sorghum), intermediate in tetraploid maize and highest in allohexaploid wheat and octoploid sugarcane. SNPs were further categorized as either intravarietal (differences between gene family members and homoeologues) or varietal (differences between two varieties), and as either co-segregating or non-co-segregating with neighbouring polymorphisms. Varietal co-segregating SNPs represent the best candidates for molecular markers as they show variation between varieties and have a high probability of being validated, as sequencing errors are unlikely to co-segregate with one another. This elite class of SNPs was most abundant in barley and least abundant in wheat and rice. Despite the large number of observed sequence polymorphisms in allohexaploid wheat, only a fraction of those available are likely to make good molecular markers. In addition, we found that rice SNPs up to 10 kb apart were in linkage disequilibrium (LD), but that high levels of LD attributable to population structure confounded the tracking of LD over greater distances.