As a PhD student, I once scared some work colleagues by making popcorn during a tea break (in the kitchen, not the lab, I hasten to add). They did not expect to hear the microwave making a series of “popping” noises, and for a few moments, I think they were genuinely worried that the microwave might explode.
Popcorn is made from heating the grains (kernels) of the plant maize until they explode, or “pop”. Also known by its latin name Zea mays ssp. mays, maize is the second most important crop plant in the world behind rice and is widely used in many human foods, as well as for animal feed and to make biofuels. It has many characteristics that make it useful to humans. Maize produces large kernels with high starch content (except for the varieties that are grown to make sweetcorn). The case surrounding a kernel is firm, but soft enough to allow us to grind these kernels to make cornflour. Also, harvesting the crop is relatively easy because the kernels stay on the cob even when ripe.
Maize belongs to the grass family (Poaceae) of flowering plants. The seeds of most of the grasses typically fall off the plant when they ripen to allow them to be scattered in the environment. Since maize kernels stay on the cob, it relies on humans to harvest the grains and sow them for the following season. So how did maize come to be so dependent on us?
To answer this question we must look to the origins of this crop in South America. Maize was domesticated from a wild grass called teosinte (Zea mays ssp. parviglumis). Archeological and genetic evidence suggests that farmers in the Balsas river valley of Mexico were the first to selectively breed teosinte about 9000 years ago. The people would have had to work hard to produce food from teosinte as each cob produces fewer kernels than modern maize. Also, these kernels are much smaller, fall off the plant when ripe, and have hard cases that would have made grinding difficult.
The domestication of teosinte also led to some other changes in physical characteristics. For example, teosinte plants have many branches, but in maize these branches are shortened and the leaves wrap around the cob to protect the kernels from birds, insects and other pests. Teosinte is adapted to life in tropical regions where the length of the day and night vary little throughout the year. If it is grown in more temperate regions – where the days become longer in summer – the plants flower later. Flowering time in modern maize is less sensitive to day length, which has allowed maize to be grown in much wider areas (including the UK).
In the 1970’s, George Beadle – who won a Nobel prize in 1958 for his work on the fungus Neurospora – became the first person to cross breed modern maize and teosinte. He then cross bred the offspring (known as the F1 generation) with each other and studied the characteristics of the next (F2) generation. Most of the characteristics of these F2 plants were intermediate between maize and teosinte, but some plants had features that were more like the teosinte or maize parent. Based on the frequency of these features in the F2 plants, Beadle argued that most of the differences between teosinte and modern maize may be accounted for by just 5 gene positions (loci). Further experiments by other scientists have backed up this idea and identified the roles some of these loci have played in domestication. For example, the gene locus tga is responsible for the harder coat of teosinte kernels, which appears to be due to differences in the proteins produced by this locus in teosinte and maize. Another locus, called tb1, is involved in stem branching and its activity is regulated differently in teosinte and maize.
The five gene loci suggested by Beadle illustrate how dramatic changes in a plant can happen with relatively few genetic changes. However, these loci are not the only regions of the genome that differ between teosinte and maize. Analysis of whole genome sequences from multiple teosinte and maize plants suggests that nearly 500 regions of the genome have been subject to selection. Some of these regions do not contain genes, and many of the genes identified in these regions show differences in their levels of activity. This suggests that changes in gene regulation have played an important role in the development of modern maize.
Hake, S and Ross-Ibarra, J (2015) The natural history of model organisms: Genetic, evolutionary and plant breeding insights from the domestication of maize. eLife 4:e05861. DOI: http://dx.doi.org/10.7554/eLife.05861