Genes and mutations underlying domestication transitions in grasses.

Over the past several years, our understanding of plant domestication has advanced substantially at the gene and genome levels. This is due largely to the rapid accumulation of genomic resources that provided genome-wide markers for population and molecular genetic analyses of crops and their wild relatives. A number of recent reviews captured some general aspects of these advances (Doebley et al., 2006; Ross-Ibarra et al., 2007; Vaughan et al., 2007; Burger et al., 2008). The primary goal of this article is to update the recent progress made in the grass family (Poaceae). Grass domestication had a unique importance in the history of human civilization. Cereal crops, domesticated from wild grasses thousands of years ago, have provided food security for the human society. Of the modern cereals, maize (Zea mays), rice (Oryza sativa), and wheat (Triticum spp.), domesticated in different regions of the world 7,000 to 10,000 years ago, are the top three food crops with much higher annual production than others. Barley (Hordeum vulgare), arguably the fourth important cereal crop used as human food, animal feed, and brewing grains, is also among the earliest domesticated cereals. Sorghum ranks next in the annual cereal production, followed by oat (Avena sativa), rye (Secale cereale), and millets (http:// faostat.fao.org/). The number of studies published on cereal domestication appears to be correlated with the agronomic importance of the crops, with especially a wealth of data generated for maize, rice, wheat, and barley. The amount of genomic resources that became available over the past several years also seemed to have an impact on publications. The completion of rice genome sequencing (International Rice Genome Sequencing Project, 2005) has greatly accelerated the progress of studying rice domestication, resulting in a number of recent reviews on this subject (Kovach et al., 2007; Sang and Ge, 2007a, 2007b; Sweeney and McCouch, 2007; Vaughan et al., 2008). Of the four top cereals, maize represents a rather unique case. It differs from barley, rice, and wheat by having undergone much more drastic morphological modifications during domestication. Considerable effort undertaken to investigate maize domestication has yielded classic literatures on the molecular basis of morphological evolution, which was thoroughly reviewed recently (Doebley, 2004; Doebley et al., 2006). For these reasons I will focus on updating recent progress made in barley, rice, and wheat, in which the studies of phenotypic transitions that allowed effective harvesting, such as reduction in shattering and improvement of threshing, will be the main theme of the article.