Genomics of Wheat Domestication

The review covers several issues concerning the state of molecular knowledge of the effects induced by domestication and breeding on the wheat crop. Genes at the root of the domesticationsyndrome are currently the focus of an active research which frequently uses comparative genomics approaches. Conclusions drawn on available data indicate that the domestication syndrome is originated by sudden genetic events, controlled by few major pleiotropic genes. These events were followed by the accumulation of a larger set of minor mutations, having a multifactorial mode of inheritance. Moreover the organization of nucleotide variability enables the detection of domestication-related molecular footprints, suggesting that the genomic regions more responsible for genetic variation and more related to domestication are reduced when compared to the whole genome size. The polyploidy history of the domesticated wheats is presented, making a specific mention to the origin of the wheat A, B, D and G genomes and to the molecular control of chromosome pairing in polyploids. A general presentation is also provided on the genomic changes which have accompanied the emergence of domesticated wheats. What follows is a molecular information on: i) the wheat adaptation to the environment (genomics of photoperiod, vernalization, heading date, plant height, and erect plant type); ii) the effect of domestication on seed-related yield components (genomics of seed size, grain hardness, and tillering); iii) modification of traits affecting harvestability (emergence of free-threshing seeds, rachis toughness, and presence of ear awns). Genetic bottlenecks which have been associated to wheat domestication and breeding are considered in a final section. The relatively young history of the wheat crop, the presumably small founder population of this gene pool, and the intensive long-term selection for agronomic traits did set the basis for a reduced genetic variability of the genus.