Genetics and Genomics of the Triticeae pp 685-711
Triticeae Genome Structure and Evolution
Repeated nucleotide sequences are by far the largest component of Triticeae genomes, accounting for about 90% of the nuclear DNA. Tandem repeated sequences play an important role in chromosome function during mitosis and meiosis. Interspersed repeated sequences fill the intergenic regions. The most remarkable attribute of this component is its unprecedented rate of turnover, which is in stark contrast to the stability of gene content. The term “gene order paradox” is coined to reflect this dichotomy. A model is proposed postulating the existence of two strata in Triticeae genomes, “conservative” and “dynamic” to account for this paradox and its evolutionary causes. Numerous aspects of gene content in Triticeae genomes, such as the location of single-copy genes, multi-gene loci, gene deletions and duplications, gene density, restriction fragment length polymorphism and single nucleotide polymorphism, location of novel and lineage-specific genes, and the level of synteny, correlate with recombination rate and gene location on the centromere-telomere axis. Special attention is devoted to the discussion of gene distribution along chromosomes. It is pointed out that evidence for the existence of gene-rich islands is weak. A model accounting for correlation between gene density and recombination rate is proposed. It is suggested that the vast amounts of repeated sequences in Triticeae genomes play a role in the evolution of new genes and in adaptation.