Genome Structure of Triticeae Species

Abstract

This chapter describes characteristic features of the chromosomes and genomes of Triticeae species. Centromeres contain typical CENH3 nucleosomes, but these are associated with repeats that are larger than in other plant species. The sub-telomeric ends are rich in transposable elements and contain diverse repeats and recombination hotspots. The nucleolar organizer regions contain hundreds or thousands of ribosomal genes, rDNA repeats, arranged in tandem arrays that form a constriction known as the nucleolar organizer (NOR). We describe their mapping as well as the phenomenon known as Nucleolar dominance. Genome sizes in the Triticeae are large, with 1C values ranging in diploids from 4.0–9.4 pg, compared to related grasses such as rice (1C = 0.5 pg). These size differences are mostly due to a large amount of repetitive DNA, in particular of transposable elements, with retroelements as the most prominent repeats. In hexaploid bread wheat, genome size reaches 1C = 16 pg, with ~ 108,000 high-confidence protein-coding genes, and a high number of pseudogenes and RNA genes. The wheat transcriptome shows complex expression patterns for homoeologous loci. We discuss gene organization in islands as well as the high synteny between the different species and the role of introgression in shaping genomes.