Maize is a tall, monoecious, annual grass varying in height from 1 to 4 meters (Watson & Dallwitz 1992). The main stem is made up of clearly defined nodes and internodes. Internodes are wide at the base and gradually taper to the terminal inflorescence at the top of the plant. Leaf blades are found in an alternating pattern along the stem. Maize is a unique grass as both male and female flowers are borne on the same plant but are located separately. The tassels or staminate (male) inflorescence form large spreading terminal panicles that resemble spike-like racemes. Pollen is shed from the tassel and is viable for approximately 10 to 30 minutes as it is rapidly desiccated in the air (Kiesselbach 1980). Maize plants shed pollen for up to 14 days. The reproductive phase begins when one or two auxiliary buds, present in the leaf axils, develop and form the pistillate inflorescence or female flower (Purseglove 1972). The auxiliary bud starts the transformation to form a long ‘cob’ on which the flowers will be borne. From each flower a style begins to elongate towards the tip of the cob in preparation for fertilization. These styles form long threads, known as silks. The base of the silk is unique, as it elongates continuously until fertilization occurs (Purseglove 1972). Styles may reach a length of 30 cm, the longest known in the plant kingdom. Individual maize kernels, or fruit, are unique in that mature seed is not covered by floral bracts (glumes, lemmas, and paleas) as in most other grasses, but rather the entire structure is enclosed and protected by large modified leaf bracts, collectively referred to as the ear (Hitchcock and Chase 1951). The mature female flowers will remain ready for fertilization for up to two weeks, at which point if fertilization has not occurred, the nucleus will de-organize and fertilization will no longer be possible (Hitchcock & Chase 1951). The pollen of maize, a protandrous plant, matures before the female flower is receptive (Purseglove 1972). This may have been an ancient mechanism to ensure cross-pollination, but is no longer considered conducive to modern agricultural practices. However, decades of conventional selection and improvement have produced many maize varieties with similar maturities for both male and female flowers, to ensure seed set for agricultural proposes.
Under natural conditions, maize reproduces only by seed production. Pollination occurs with the transfer of pollen from the tassels to the silks of the ear. About 95% of the ovules are cross-pollinated and about 5% are self-pollinated (Poehlman 1959), although plants are completely self-compatible. Maize, as a thoroughly domesticated plant, has lost all ability to disseminate its seeds and relies entirely on the aid of man for its distribution (Stoskopf 1985). The kernels are tightly held on the cobs and if ears fall to the ground, so many competing seedlings emerge that the likelihood that any will grow to maturity is extremely low. Maize cannot reproduce asexually by natural means. It is possible to reproduce maize using tissue culture techniques, however, it has proven extremely difficult with a low rate of success (Hoisington et al. 1998).
Classification and Phylogeny of Maize
Zea is a genus belonging to the grass family, Poaceae, in the Andropogoneae tribe, but is commonly cited in literature to belong to the tribe Maydeae. Zea (zeia) was derived from an old Greek name for a food grass. The genus Zea consists of four species of which only Zea mays ssp. mays L. is economically important. The other Zea species, referred to as teosintes, are largely wild grasses native to Mexico and Central America (Doebley 1990). The number of chromosomes in Zea mays is 2n=20, 21, 22, 24 (FAO 2000c). The tribe Maydeae consists of seven genera: two of American origin, Zea and Tripsacum; and five of Eastern origin which extend from India through Southeastern Asia to Australia and include Coix, Sclerachne, Polytoca, Chionachne and Trilobachne (Watson & Dallwitz 1992). It is generally agreed that maize phylogeny was largely determined by the American genera Zea and Tripsacum, however it is accepted that the genus Coix contributed to the phylogenetic development of the species Z. mays (Radu et al. 1997). The genus Manisuris, from the tribe Andropogoneae, may also have contributed to the evolution of Zea mays (Eubanks 1997a; Radu et al. 1997).
Center of Origin and Progenitors of Maize
The center of origin for Zea mays ssp. mays has been established as the Mesoamerican region, now Mexico and Central America (Watson & Dallwitz 1992). The exact period of domestication and the ancestors from which maize arose are unclear. Archaeological records suggest that domestication of maize began at least 6000 years ago, occurring independently in regions of the southwestern United States, Mexico, and Central America (Mangelsdorf 1974). The origins of domesticated maize have been difficult to trace as hybridization events in its evolution are thought to have involved a now extinct wild maize ancestor of which little evidence has been found (Eubanks 1997a).
A study in 1992 concluded that maize landraces occupied 42% of the land under production in less developed countries (México 1994). Mexico and Central America are the only regions where ancient maize lines, such as pod corn, are found (Mink & Dorosh 1985). The survival of maize landraces has been attributed to the integral role maize sustains in small communities where it has very specific uses for food and other special functions. These landraces have been well characterized in Latin America, and germplasm not yet evaluated is known to be present in the region. There is a growing trend in developing countries to adopt improved maize varieties, primarily to meet market demand. In Mexico, only 20% of the corn varieties grown 50 years ago remain in cultivation (World Watch Institute 2000). The narrowing of genetic diversity in modern maize varieties emphasizes the importance of conserving genetic traits for future plant breeding. Leading the way in preserving maize germplasm is CIMMYT (International Maize and Wheat Improvement Centre), which has the world’s largest collection of maize accessions, with over 17,000 lines (CIMMYT 2000). Collections of distant maize relatives native to Asia have been recommended by conservation experts to ensure their preservation for future research (Sharma 1998).
Relatives of Maize
The closest wild relatives of maize are the teosintes which all belong to the genus Zea. The teosintes are wild grasses native to Mexico and Central America and have limited distribution (Mangelsdorf et al. 1981). Teosinte species show little tendency to spread beyond their natural range and distribution is restricted to North, Central and South America (Watson & Dallwitz 1992). The nearest teosinte relative to Z. mays ssp. mays is Z. mays ssp. mexicana (Schrader) Iltis (previously classified as Euchlaena mexicana, Zea mexicana) (2n = 20). This Central Mexican annual teosinte is a large flowered, mostly weedy grass with a broad distribution across the central highlands of Mexico. It does not spread readily. It has limited use as a forage and green fodder crop, but can be problematic due to weedy tendencies (Doebley 1990, Watson & Dallwitz 1992). The closest wild relatives to maize outside the Zea genus are from the genus Tripsacum. The genus Tripsacum is comprised of about 12 species that are mostly native to Mexico and Guatemala but are widely distributed throughout warm regions in the USA and South America, with some species present in Asia and Southeast Asia (Watson & Dallwitz 1992). All species are perennial, warm season grasses.
Gene flow from maize can occur by two means: pollen transfer and seed dispersal. Seed dispersal can be readily controlled in maize as domestication has all but eliminated any seed dispersal mechanisms that ancestral maize may have previously used (Purseglove 1972). The kernels are held tightly on the cobs and if the ear falls to the ground, competing seedlings limit growth to maturity (Gould 1968). Pollen movement is the only effective means of gene escape from maize plants. Pollen is released from the tassels at the top of the plant and transported by wind to the female flowers located on the stalk. Pollen shed occurs over a 14 day period, with a peak during the first 5 days of shed (Sears 2000). The stigmas are receptive for a large part of this two week interval (Kiesselbach 1980). Insects, such as bees, have been observed to collect pollen from maize tassels, but they do not play a significant role in cross-pollination as there is no incentive to visit the female flowers (Rayor et al. 1972).
Wind speed and direction affect pollen distribution
Differences in flowering dates among commercial maize varieties are small. Cross-pollination between varieties may occur if grown in adjacent fields. The limited viability of maize pollen reduces the risk of cross-pollination, as a receptive host must be found within the 30 minutes that the pollen remains biologically active. Cross-pollination is also affected by the concentration of maize pollen released; pollen produced by a maize crop will successfully compete with foreign pollen sources when present in higher concentrations (Rayor et al. 1972). The isolation of crops using separation distances and physical barriers are common techniques for restricting gene flow and ensuring seed purity for maize seed production. Cross-pollination is controlled in seed lots by separating different lines. The OECD standards require a minimum 200 m (660 ft) isolation distance for maize seed production. This distance has been estimated to maintain inbred lines at 99.9% purity. Physical barriers, such as trees or barrier rows, are also effective in reducing cross-pollination. Detasseling or bagging the tassel effectively prevents pollen movement and limits gene flow.
All forms of Zea mays spp mays, such as dent, sweet, and pop corn, freely cross pollinate forming fertile hybrids (Pursglove 1972).
All teosintes can be crossed to maize and form fertile hybrids, except for the tetraploid Z. perennis. Maize-teosinte hybrids exhibit low fitness and have little impact on gene introgression in subsequent generations (Galinat 1988). The tendency to form natural hybrids differs among teosintes: Z. luxurians rarely hybridizes with maize, whereas Z. mays ssp. mexicana frequently forms hybrids (Wilkes 1977). Molecular data confirms that there is gene flow between maize and teosinte and suggests that introgression of maize and teosinte occurs in both directions but at low levels (Doebley 1990).
Intergeneric hybridization Tripsacum species (T. dactyloides, T. floridanum, T. lanceolatum, and T. pilosum) have been successfully hand crossed with maize to form hybrids. The hybrids generally have 28 chromosomes, 10 from maize and 18 from Tripsacum, and are pollen sterile with limited female fertility (Mangelsdorf & Reeves 1939, Mangelsdorf 1974). This infertility is common in such wide crosses because of differences in chromosome number and lack of pairing between chromosomes (Eubanks 1997b). Maize readily crosses with hexaploid wheat (Triticum aestivum) with high frequencies of fertilization and embryo formation (plant breeders use maize pollen to develop double haploids of hexaploid wheat). However, maize chromosomes are eliminated from the genome during the initial stages of meiosis and result in haploid embryos. There is little evidence to suggest fertile hybrids between maize and hexaploid wheat could be produced in nature. There have been unsubstantiated reports of hybridization between maize and sugar cane (Saccaharum sp.).