GENETICS OF MAIZE DOMESTICATION
The morphological differences between maize and its wild relative teosinte (Zea mays ssp. parviglumis) are quite extreme. While teosinte produces only 6 to 12 kernels in two, interleaved rows protected by a hard, outer covering (below left), modern maize boasts a cob consisting of as many as 20 rows or more, with numerous exposed kernels (below right). In fact, teosinte is so unlike maize in the structure of its ear that 19th-century botanists failed to recognize the close relationship between these plants, placing teosinte in the genus Euchlaena rather than in Zea with maize.
Maize Domestication Images
Due to such dramatic morphological differences between parent and offspring, maize is perhaps the most impressive feat of domestication and genetic modification ever. The ability of Native Americans to transform a wild grass into the world's largest production crop is not only the product of skillful breeding but also a tribute to the tremendous diversity of the teosinte genome.
So how was this dramatic transformation accomplished?
Domesticated maize was the result of repeated interaction with humans within the last 10,000 years. Early farmers selected and planted seed from those plants with beneficial traits while eliminating seed from those plants with more undesirable features. As a result, alleles at those genes controlling favored traits increased in frequency within the population, while more “deleterious” alleles decreased. Such selection was made possible due to the tremendous natural variation present in Z. mays ssp. parviglumis (nucleotide diversity at silent sites has been measured as high as 2 to 3%).
Over time, these ancient agriculturists were able to select the combination of major and many minor gene mutations that now distinguish maize from its wild ancestor. In a series of studies, it has been shown that relatively few genes are responsible for the transformation. Research by George Beadle and John Doebley has shown that as few as 5 genes may be responsible for these dramatic morphological changes. Caution, however, must be exercised when advocating a "one gene, one trait" model. Although a small number of genes clearly have a striking effect on ear and plant morphology and represent major steps in maize evolution, the vast majority of genes have only modest effects. It is likely that hundreds or even thousands of genes were necessary to complete the transformation, including those involved in steps such as increasing the size of the ear, adapting maize to modern agricultural fields, and modifying the nutrient content of the maize kernel.
What traits were modified during domestication?
1. Glume: The teosinte kernel is surrounded by a hard coating called a glume. In the wild, this glume helps to protect the seed when passing through the digestive tract of animals or when sitting in the ground during the winter. Since this tough glume is difficult to chew and digest by humans, those plants with a softer glume were conceivably targeted during domestication. In today's maize, the glume is very reduced (you may have encountered it when eating corn-on-the-cob as it's the part that often gets stuck in your teeth). A single major locus, teosinte glume architecture (TGA) has been identified that controls much of this reduction in glume size.
2. Plant Architecture: In a modern maize field, you see a single stalk and ear per plant. This is not the case in teosinte, where many stalks, called tillers, are found per plant, along with many inflorescences (female ears and male tassels). By concentrating energy resources into a single stalk and ear, it was probably possible to create larger ears that were easier to harvest. John Doebley's research group has cloned this domestication gene and named it teosinte branched 1 (tb1). Pictures of its effect can be seen below:
Plant Architecture Image
3. Other Distinguishing Traits: QTL at genes responsible for three more distinguishing traits--shattering vs. solid cobs, single vs. paired spikelets, and distichous vs. polystichous condition--are the subject of current investigation. While several regions of the genome have been identified, we have yet to definitively find those genes responsible for these traits.
4. Starch: While changes in plant shape and ear morphology were the initial focus of domestication research, many additional traits have been the target of human selection over the last few thousand years. Some traits of particular significance were yield, the size of the ear (which has increased from 2cm to 30cm), and the quality of the grain. Starch was—and is—the key product of maize, accounting for 73% of the kernel’s total weight. As such, the genes involved in starch synthesis are among the most important for grain production, critical to both the yield and the quality of the grain. To date, our lab has identified three starch loci (su1, bt2, and ae1) as targets of selection during maize domestication and improvement.
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Research on maize domestication has attracted a great deal of interest over the last century, including the attention of two Nobel laureates. Of primary interest is the evolutionary question of how maize was domesticated from a wild relative that differs so dramatically in ear and plant morphology.
The study of maize domestication can be divided into two main periods
1. Competing Hypotheses: The first period began in the 1930s when George Beadle and Paul Mangelsdorf proposed two contrasting hypotheses for the origin of maize. Beadle proposed the "Teosinte Hypothesis" in which maize is simply a domesticated form of teosinte. He believed that, through artificial selection by ancient populations, several small mutations with relatively large effects could have transformed teosinte into maize. In contrast, Mangelsdorf suggested maize was the product of a hybridization between undiscovered wild maize and Tripsacum, known as the "Tripartite Hypothesis." From its debut in 1938 until the 1960s, the Tripartite Hypothesis was widely accepted. Through productive collaborations with prominent archaeologists of his day and widespread efforts targeting maize germplasm conservation, Mangelsdorf was able to publicize his theory among a wide audience. Upon his "retirement" in 1968, however, George Beadle rejoined the maize controversy and vigorously championed the Teosinte Hypothesis. His own genetic research, followed by chromosomal studies by McClintock and Kato and morphological systematics by Iltis, would eventually convince the majority of biologists of the Teosinte Hypothesis. Although revisions of the Tripartite Hypothesis were suggested by Mangelsdorf in the 1980s and again by Eubanks in the 1990s, no convincing molecular genetic evidence has ever been found to support these revised Tripsacum hypotheses.
2. The Genome: The second period began in the late 1960s when researchers focused their attention on describing the diversity and evolutionary relationships within the genusZea, and on determining those genes involved in domestication and the evolution of the genome. Garrison Wilkes published the first thorough monograph on teosinte in 1967. Hugh Iltis, John Doebley, Rapheal Guzman, and B. Pazy invigorated teosinte research by discovering and describing the perennial species Z. diploperennis. In 1980, Iltis and Doebley established an organized taxonomy that considered the probable evolutionary relationships between taxa. McClintock and Kato synthesized and published chromosomal knob diversity data that had been collected over the previous 30 years in 1981. Throughout the 1980s and 1990s, Charles Stuber and Major Goodman's research groups produced comprehensive analyses of isozyme diversity in over 1000 maize races and in almost all known teosinte populations. With the advent of DNA analysis and sequencing in the 1980s, John Doebley, Ed Buckler, and Brandon Gaut refined the phylogenetic relationships among grasses and the genus Zea, contributing to our understanding of how the genome has evolved. Beginning in the 1990s, John Doebley's research group began to discover some of the genes involved in maize domestication.