Differential Selection of Grain and Embryo Weight during Wheat Evolution under Domestication

Wheat (Triticum spp.) is one of the Neolithic founder crops, domesticated in the Near-Eastern Fertile Crescent ~10,500 years ago. Following the domestication episode, its evolution under domestication has resulted in various genetic modifications. In the current study, we investigated the genetics and morphological changes associated with grain weight, embryo weight and their interactions, among domesticated durum wheat and its direct progenitor, wild emmer wheat (T. turgidum ssp. dicoccoides). Experimental data show that grain weight has increased over the course of wheat evolution without any parallel change in embryo weight, resulting in a significantly reduced (30%) embryo weight / grain weight ratio in domesticated wheat. The genetic factors associated with these modifications were further investigated using a population of recombinant inbred substitution lines that segregated for chromosome 2A. A cluster of loci affecting grain weight and shape was identified on the long arm of chromosome 2AL. Interestingly, a novel locus controlling embryo weight was mapped on chromosome 2AS, on which the wild emmer allele promotes heavier embryos and greater seedling vigor. To the best of our knowledge, this is the first report of a QTL for embryo weight in wheat. The results suggest a differential selection of grain and embryo weight during the evolution of domesticated wheat. We argue that conscious selection by early farmers favoring larger grains and smaller embryos appears to have resulted in a significant change in endosperm weight / embryo weight ratio in the domesticated wheat. The identified genetic factors underscore the potential benefit of introducing wild alleles into modern cultivars and is likely to be useful for the selection of high vigor lines in future breeding programs.