Exotic varieties of maize are collected to add genetic diversity
when selectively breeding new domestic strains
Maize is an annual grass in the family Gramineae, which includes such plants as wheat, rye, barley, rice, sorghum, and sugarcane. There are two major species of the genus Zea (out of six total): Zea mays (maize) and Zea diploperennis, which is a perennial type of teosinte. The annual teosinte variety called Zea mays mexicana is the closest botanical relative to maize. It still grows in the wild as an annual in Mexico and Guatemala.
Many forms of maize are used for food, sometimes classified as various subspecies related to the amount of starch each has:
- Flour corn: Zea mays var. amylacea
- Popcorn: Zea mays var. everta
- Dent corn : Zea mays var. indentata
- Flint corn: Zea mays var. indurata
- Sweet corn: Zea mays var. saccharata and Zea mays var. rugosa
- Waxy corn: Zea mays var. ceratina
- Amylomaize: Zea mays
- Pod corn: Zea mays var. tunicata Larrañaga ex A. St. Hil.
- Striped maize: Zea mays var. japonica
This system has been replaced (though not entirely displaced) over the last 60 years by multivariable classifications based on ever more data. Agronomic data were supplemented by botanical traits for a robust initial classification, then genetic, cytological, protein and DNA evidence was added. Now, the categories are forms (little used), races, racial complexes, and recently branches.
Maize is a diploid with 20 chromosomes (n=10). The combined length of the chromosomes is 1500 cM. Some of the maize chromosomes have what are known as "chromosomal knobs": highly repetitive heterochromatic domains that stain darkly. Individual knobs are polymorphic among strains of both maize and teosinte.
Barbara McClintock used these knob markers to validate her transposon theory of "jumping genes", for which she won the 1983 Nobel Prize in Physiology or Medicine. Maize is still an important model organism for genetics and developmental biology today.
The Maize Genetics Cooperation Stock Center, funded by the USDA Agricultural Research Service and located in the Department of Crop Sciences at the University of Illinois at Urbana-Champaign, is a stock center of maize mutants. The total collection has nearly 80,000 samples. The bulk of the collection consists of several hundred named genes, plus additional gene combinations and other heritable variants. There are about 1000 chromosomal aberrations (e.g., translocations and inversions) and stocks with abnormal chromosome numbers (e.g., MaizeGDB, the Maize Genetics and Genomics Database.
In 2005, the US National Science Foundation (NSF), Department of Agriculture (USDA) and the Department of Energy (DOE) formed a consortium to sequence the B73 maize genome. The resulting DNA sequence data was deposited immediately into GenBank, a public repository for genome-sequence data. Sequences and genome annotations have also been made available throughout the project's lifetime at the project's official site.
Primary sequencing of the maize genome was completed in 2008. On November 20, 2009, the consortium published results of its sequencing effort in Science. The genome, 85% of which is composed of transposons, was found to contain 32,540 genes (By comparison, the human genome contains about 2.9 billion bases and 26,000 genes). Much of the maize genome has been duplicated and reshuffled by helitrons—group of rolling circle transposons.