Cereal Evolution

Cereal crops are our major food source. Rice alone is the staple food of over 50% of the world's population and together with wheat and maize makes up of 60% of the world's total food supply. Other cereals (e.g. foxtail and pearl millet, oats, sorghum, sugarcane, rye, triticale) are also important in many areas of the world, particularly in developing countries Thus an understanding of the biology, genetics, evolution and history of this crop group is of fundamental social importance.

From their appearance cereals are obviously all types of 'grass'. Indeed all of our cultivated cereal crops have evolved from wild grass species. The ancestral cereal grew more than 100 million years ago, from this plant evolved the various wild species that were the starting points for the emergence of the cereals we recognise today.

The earliest wheat crops were 'farmed' by nomadic tribesmen of the Fertile Crescent (today the Middle East) some 15,000 years ago. A chance hybridisation between two wild grasses gave rise to the progenitor of modern day macaroni wheats. This was 'selected' for use by the early farmers. About 9,000 years ago another chance hybridisation occurred and the new seed was again multiplied. This natural three-way cross gave rise to a new species, Triticum aestivum, or modern day bread wheat. Wheat contains three copies of the original cereal genome.

Maize has a similar, although more ancient history. A chance hybridisation some 20 million years ago, somewhere in the American tropics, gave rise to Zea mays, which was eventually domesticated by the Indians in that region and spread throughout the continent. Maize thus carries two copies of the original cereal genome.

Rice's evolution is a little simpler. The genus Oryza originated more than 100 million years ago in Gondwanaland, and with its break-up got distributed to Africa, Asia and Australia. Asian rice, Oryza sativa, is now grown all over the world. Rice has a single copy of the cereal genome and (at 400 million base pairs) has one of the lowest amounts of total DNA of any grass.

Although 100 million years of evolution, and 10,000 years of selective breeding, have produced plants that look very different, at the level of their genetics modern cereals are very similar. Not only do they have many genes in common, their genes are arranged in similar ways. The location of genes in relation to one another can be represented on linear or circular maps. Our circular maps have helped us to understand how, during evolution, large blocks of genes have been reorganised and moved around in the different cereals. The maps allow us to visualise the relationships between all the modern cereals and their 100 million year old long-extinct ancestor cereal.

Rice has one of the smallest and simplest cereal genomes with about 40,000 genes. Wheat and maize are far more complex. They have more genes (because of the multiple genomes derived from two (maize) or three (wheat) wild ancestors) embedded in a great deal more DNA (the chemical basis of genes) that does not encode genetic information. In fact maize has 24 times and wheat 40 times as much DNA as rice. Now that we know how similar they are, when studying the genetics and biology of wheat or maize it is often easier to use the relatively simple rice plant as a model, and transfer knowledge between the different cereal.