Structural and Evolutive Analysis of an Ancestral Chromosomes Fusion Point within the Hexaploid Wheat Genome

The explosion of genomes sequencing has highlighted whole genome duplications (WGDs) as a major evolutionary driving force shared by multiples branches of the tree of life. However, the genome sizes and chromosome numbers have not increased exponentially over time because WGDs have been balanced by chromosome number reduction (CNR) and gene losses. In plants these two mechanisms have been found through genomes sequences comparisons and ancestral karyotype reconstruction. For monocots models have been proposed explaining CNR by nested chromosome fusions (NCFs). However, molecular mechanisms driving ancestral chromosome fusions that have led to the present day monocots karyotypes are still largely unknown. Based on this ascertainment, we aimed at characterizing a wheat genomic locus corresponding to ancestral chromosomes fusion point (FP).

To obtain a high quality sequence of a FP we implemented a strategy based on the reconstruction of the ancestral grass karyotype, the definition of the synteny relationships between wheat and rice (as the modern representative of the Ancestral 12 chromosomes grass karyotype) and the screening of wheat chromosome specific BAC libraries.

We focused on the FP located on the long arm of chromosomal group 1. We investigated this region on chromosomes 1AL, 1BL, 1DL obtaining sequences for each of them. The annotation of genes and transposable elements allowed us to established orthology relations chromosome 5 and 10 of rice (images of ancestral chromosomes A5/A10). The origin and the order of the genes neighboring the FP were confronted to models of chromosomes fusion of Triticeae proposed in the literature.