Genome comparisons reveal a dominant mechanism of chromosome number reduction in grasses and accelerated genome evolution in Triticeae

Abstract

Single-nucleotide polymorphism was used in the construction of an expressed sequence tag map ofAegilops tauschii, the diploid source of the wheat D genome. Comparisons of the map with the rice and sorghum genome sequences revealed 50 inversions and translocations; 2, 8, and 40 were assigned respectively to the rice, sorghum, andAe. tauschiilineages, showing greatly accelerated genome evolution in the large Triticeae genomes. The reduction of the basic chromosome number from 12 to 7 in the Triticeae has taken place by a process during which an entire chromosome is inserted by its telomeres into a break in the centromeric region of another chromosome. The original centromere–telomere polarity of the chromosome arms is maintained in the new chromosome. An intrachromosomal telomere–telomere fusion resulting in a pericentric translocation of a chromosome segment or an entire arm accompanied or preceded the chromosome insertion in some instances. Insertional dysploidy has been recorded in three grass subfamilies and appears to be the dominant mechanism of basic chromosome number reduction in grasses. A total of 64% and 66% ofAe. tauschiigenes were syntenic with sorghum and rice genes, respectively. Synteny was reduced in the vicinity of the termini of modernAe. tauschiichromosomes but not in the vicinity of the ancient termini embedded in theAe. tauschiichromosomes, suggesting that the dependence of synteny erosion on gene location along the centromere–telomere axis either evolved recently in the Triticeae phylogenetic lineage or its evolution was recently accelerated.