Maize

Maize (Zea mays L.), otherwise known as corn, is a highly unusual, economically important, and genetically well-characterized member of the grass family. It is believed to have originated some 8,000 to 10,000 years ago in the fields of the first agriculturalists of Mexico and Central America. These early farmers carefully selected traits that would ultimately transform the tiny, sparsely seeded spike of a wild grass into the large cob bearing many rows of kernels that we recognize today as an ear of corn.

The success of these early plant breeders was manifested by the spread of corn cultivation throughout the New World, long before the arrival of Europeans. Today, maize is grown in more countries than any other crop, and is a major source of food for both humans and domesticated animals throughout the world. The world production of maize in 2000 exceeded 23 billion bushels, the largest producer being the United States (43 percent).

Early Studies of Maize

As a major crop plant, maize was already the subject of study by plant breeders at the time of the rediscovery of Mendel's laws of inheritance at the beginning of the twentieth century. The inheritance patterns of readily observed traits were uncovered through controlled crosses and the examination of progeny. In many respects, maize was an ideal model system for this early period in the study of genetics. Male and female flowers are borne separately and are easily manipulated for controlled crosses. Large amounts of pollen are produced in the tassels (male inflorescence) over a period of days, and one ear (female inflorescence) contains many seeds (kernels). Large progeny arrays could be produced in one season.

The high genetic diversity of maize provided many interesting mutant phenotypes to study, many of which were recessive. These could be maintained in a heterozygous state by the outcrossed breeding system (most fertilizations are the result of pollen transfer among plants) and easily uncovered by selfing (fertilizations that result from a plant's own pollen). There was also ample scope for selection of extreme phenotypes in continuous (quantitative) traits. A drawback for maize, compared to short-lived fruit flies, is that it only produces one or two crops per year, depending on location. However, many early maize geneticists knew that kernel phenotypes, which were discernable at harvest time, often predicted phenotypes in the adult plants, and could be used to set up the following season's crosses.

One of the earliest breakthroughs in crop breeding was the detection of hybrid vigor in maize by George Harrison Shull in 1908. He found that the progeny of two inbred lines were more productive than their wind-pollinated progenitors. This discovery provided the stimulus for the commercial propagation of maize and made it one of the most productive food plants worldwide.

Later Maize Studies

Many important genetic discoveries were made in maize by a group of scientists brought together at Cornell University in the 1920s and 30s by R. A. Emerson, who is often referred to as the spiritual father of maize genetics. The Emerson group, which included the future Nobel laureates Barbara McClintock and George Beadle, laid the foundation of maize genetics. They assembled information on maize mutants and ultimately produced the first genetic map of maize, based on linkage studies, in 1935. McClintock's first major contribution occurred early in her career (1929), when she perfected the techniques used to visualize maize chromosomes under the microscope. This allowed individual chromosomes to be identified by size, form, and features such as the highly staining regions, called "knobs."

This milestone allowed McClintock and other members of the Emerson group to make major advances in cytogenetics, which combines genetic crossing data and cytological landmarks to locate genes on chromosomes. Cytological landmarks include trisomics, reciprocal translocations, and deficiencies. Another of McClintock's breakthroughs, achieved with the collaboration of her colleague Harriet Creighton, was to establish the cytological proof of crossing over, which refers to the exchange of chromosomal segments during meiosis. Of course, McClintock's most famous discovery was that genetic elements within the genome can move (transpose) from one locus on the chromosome to another. These "jumping genes" (transposable genetic elements of transposons) were later discovered in bacteria, flies, and humans and eventually resulted in McClintock receiving a Nobel Prize in 1983.

In recent years, transposable elements have been exploited as tools for understanding the function of many maize genes. If a transposon inserts into a gene, it will disrupt the function of that gene. The disruption of gene function may result in a mutant phenotype affecting tissues or developmental stages of the plant that give some indication of the function of that gene. For instance, a transposon that inserts into a gene required for chlorophyll production would result in an albino seedling. Because the DNA sequences of many transposable elements in maize are known, they provide convenient molecular tags with which to clone and further characterize the gene into which they have inserted. Corn transposons have also been adapted to mutagenize and "tag" genes in the model plant Arabidopsis thaliana.

Denise E. Costich

Bibliography

Dold, Catherine. "The Corn War." Discover (December 1997): 109-113.

Fedoroff, Nina, and David Botstein, eds. The Dynamic Genome: Barbara McClintock's Ideas in the Century of Genetics. Cold Spring Harbor, NY: Cold Spring Harbor Laboratory Press, 1992.

Kass, Lee B. "Barbara McClintock: American Botanical Geneticist (1902-1992)." In Plant Sciences for Students, vol. 3. New York: Macmillan Publishing, 2000.

Keller, Evelyn Fox. A Feeling for the Organism: The Life and Work of Barbara McClintock. San Francisco: W. H. Freeman, 1983.

Rhoades, Marcus M. "The Early Years of Maize Genetics." Annual Review of Genetics 18 (1984): 1-29.