Searching for Ancestral Chromosomes Fusions in Triticeae: In Case Study on Hexaploid Bread Wheat

The explosion of genomes sequencing in the past decade has highlighted whole genome duplications (WGDs) as a major evolutionary driving force shared by multiples branches of the tree of life. Thus WGDs have shaped ancestral vertebrates genomes and have occurred frequently in plants. 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. These two mechanisms have been found through genomes sequences comparisons and ancestral karyotype reconstruction in plants (Salse, Curr. Plant Biol., 2012). Models have been proposed explaining CNR by chromosomes fusions where two Chromosomes can merge by their extremities or by centromeric/telomeric interaction leading to nested chromosome fusions (NCFs). However, molecular mechanisms driving ancestral chromosome fusions that have led to the present day diversity of plant are still largely unknown.

Following this evolutionary mechanisms modern Triticeae (wheat, barley) with a basic number of 7 chromosomal groups have been shaped from a monocot ancestral genome structured in 12 protochromosomes following 5 NCFs (Murat et al. Genome Biol. Evol., 2014, Curr. Plant Biol., 2014). Hexaploid bread wheat, consisting in three sets of the 7 Triticeae chromosomal groups (21 modern chromosomes), has been used to clone one of the 5 NFCs based on BAC libraries screening and sequencing of the targeted BACs contigs using long reads sequencing technologies. Ultimately the annotation and the comparison of such sequences will be valuable to unveil mechanisms of DNA interaction that drove plant chromosomes repatterning during evolution through NFCs.