The Biology of Zea mays (L.) (Maize)
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Biology Document BIO1994-11: A companion document to the Directive 94-08 (Dir94-08), Assessment Criteria for Determining Environmental Safety of Plant with Novel Traits
Table of Contents
Part A - General Information
Part B - The Biology of Z. mays
- B1. General Description, Cultivation, Use as a Crop Plant and Hybrid Production
- B2. The Centres of Origin of the Species
- B3. Summary of the Ecology of Z. mays and its Progenitors
Part C - Z. mays Related Species
- C1. Inter-Species/Genus Hybridization
- C2. Potential for Introgression of Genetic Information from Z. mays into Relatives
- C3. Occurrence of Z. mays and Related Species in Canada
- C4. Summary of the Ecology of Relatives of Z. mays
Part D - Potential Interactions of Z. mays with Other Life Forms
Part E - Acknowledgments
Part F - Bibliography
Part A - General Information
Since 1988, Agriculture and Agri-Food Canada has been regulating the field testing in Canada of agricultural and horticultural crop plants with novel traits (PNT's). "Plants with novel traits" are defined as a plant variety/genotype possessing characteristics that demonstrate neither familiarity nor substantial equivalence to those present in a distinct, stable population of a cultivated species of seed in Canada and that have been intentionally selected, created or introduced into a population of that species through a specific genetic change. "Familiarity" is defined as the knowledge of the characteristics of a plant species and experience with the use of that plant species in Canada. "Substantial equivalence" is defined as the equivalence of a novel trait within a particular plant species, in terms of its specific use and safety to the environment and human health, to those in that same species, that are in use and generally considered as safe in Canada, based on valid scientific rationale.
The PNT's can either be derived from recombinant DNA technologies, or from traditional plant breeding. Regulated field testing is necessary when the PNT's have traits of concern, i.e., the traits themselves, their presence in a particular plant species or their use are: 1) considered unfamiliar when compared with products already in the market; 2) not considered substantially equivalent to similar, familiar plant types already in use, and regarded as safe.
Before PNT's may be authorized for unconfined release, they must be assessed for environmental safety. The CFIA's Directive 94-08 (Dir94-08), entitled "Assessment Criteria for Determining Environmental Safety of Plants with Novel Traits", has been developed to define criteria and information requirements that must be considered in the environmental assessment of PNT's to ensure environmental safety, in the absence of confinement conditions.
The present document is a companion document to Dir94-08 and is intended to provide background information on the biology of Zea mays L., its centres of origin, its related species, and the potential for gene introgression from Z. mays into relatives, and details of the life forms with which it interacts.
Such species-specific information will serve as a guide for addressing some information requirements of Part D of Dir94-08. Specifically, it will be used to determine whether there are significantly different/altered interactions with other life forms, resulting from the PNT's novel gene products, which could potentially cause the PNT to become a weed of agriculture, become invasive of natural habitats, or be otherwise harmful to the environment.
The conclusions drawn in this document about the biology of Z. mays only relate to plants of this species with no novel traits. Novel traits of concern might confer new characteristics to the plant, that could impact on the environment pursuant to their unconfined release.
Part B - The Biology of Z. mays
B1. General Description, Cultivation, Use as a Crop Plant and Hybrid Production
Zea is a genus of the family Graminae (Poaceae), commonly known as the grass family. Maize (Z. mays L.) is a tall, monecious annual grass with overlapping sheaths and broad conspicuously distichous blades. Plants have staminate spikelets in long spike-like racemes that form large spreading terminal panicles (tassels) and pistillate inflorescences in the leaf axils, in which the spikelets occur in 8 to 16 rows, approximately 30 long, on a thickened, almost woody axis (cob). The whole structure (ear) is enclosed in numerous large foliaceous bracts and a mass of long styles (silks) protrude from the tip as a mass of silky threads (Hitchcock and Chase, 1971).
Pollen is produced entirely in the staminate inflorescence and eggs, entirely in the pistillate inflorescence. Maize is wind pollinated and both self and cross pollination are usually possible. Shed pollen usually remains viable for 10 to 30 minutes, but can remain viable for longer durations under favorable conditions (Coe et al., 1988).
Cultivated maize is presumed to have been derived from teosinte (Z. mexicana) and is thought to have been introduced into the old world in the sixteenth century. Maize is cultivated worldwide and represents a staple food for a significant proportion of the world's population. No significant native toxins are reported to be associated with the genus Zea (International Food Biotechnology Council, 1990).
Maize has been cultivated by the indigenous peoples of North America, including Canada, for thousands of years. The modern era of maize hybrid production in Canada began in the U.S. where research conducted in the early part of this century proved that hybrid maize could produce a yield superior to open-pollinated varieties (Sprague and Eberhart, 1977). Gradually, hybrid-derived varieties replaced the open-pollinated types in Canada in the 1930's and 1940's. In Canada, maize is grown primarily in Ontario and Quebec. Farmers must purchase new supplies of hybrid seed each season.
Maize is planted when soil temperatures are warm (greater than or equal to 10° C) usually early to mid-May in southern Ontario (OMAF, 1994) and Quebec (MAPAQ, 1984). Optimum yields occur when the appropriate hybrid maturity and population density are chosen. In addition, exogenous sources of nitrogen fertilizer are generally applied and weed and insect control measures are generally recommended. Choice of the appropriate hybrid for the intended growing area helps to ensure that the crop will mature before frost halts the growth of the plant at the end of the season; hybrids are categorized according to the amount of "heat units" that will be required for maturity. The maize-growing areas of Canada are illustrated on maps that indicate the number of heat units that they receive (e.g., OMAF, 1994; MAPAQ, 1984). Therefore, a hybrid developed for a specific heat unit zone, will not mature in (cooler) areas that receive fewer "heat units". Traditional cultivation practices in maize often result in bare soil' which is susceptible to erosion by wind or water; increasingly, "no till" maize is being grown in an effort to reduce this soil loss. For more information on specific cultivation practices, please contact the appropriate Canadian provincial agricultural authorities.
In 1993, there were more than 24 million ha planted to the crop in North America. Maize grown in North America is predominantly of the yellow dent type, a commodity crop largely used to feed domestic animals, either as grain or silage. The remainder of the crop is exported or processed by wet or dry milling to yield products such as high fructose maize syrup and starch or oil, grits and flour. These processed products are used extensively in the food industry, for example, maize starch serves as a raw material for an array of processed foods, and in industrial manufacturing processes. Since the early 1980's a significant amount of grain has also been used for fuel ethanol production. The by-products from these processes are often used in animal feeds. For a full discussion of the uses of maize see Watson (1988).
The production of hybrid seed requires the development and maintenance of inbred lines and subsequent controlled crosses to produce commercial seed. Self pollination is essential for inbred development while controlled cross pollination is mandatory for hybrid seed production. Mechanisms have been developed to ensure the correct form of pollination for each process and to prevent genetic contamination of seed stocks (Wych, 1988).
In breeding nurseries, receptive ear shoots are protected from unwanted pollination by ear shoot bags that cover the silks. Pollen is contained and collected in bags that cover the tassels. Controlled hand pollinations are then made by exposing the ear shoot on the selected female parent and covering it with the bag containing pollen from the selected male parent.
Breeder or foundation seed is produced from self pollinated seed after the eighth or ninth generation of inbreeding. A high degree of self pollination is assured by planting in blocks that are isolated by a distance of at least 200 meters from any other contaminating source of pollen.
Hybrid seed production is accomplished by interplanting rows of the male and female inbred parents (e.g., one row of male to four female rows). Hybrid seed production requires isolation similar to that for foundation seed. Self pollination of the female parent is prevented through detasseling prior to pollen shed or by the use of male sterile females.
Genetic conformity of inbreds and hybrids is monitored and assured through grow-outs of representative seed lots and laboratory screening using such criteria as isozyme profiles.
B2. The Centres of Origin of the Species
It is generally agreed that teosinte (Z. mexicana) is an ancestor of maize, although opinions vary as to whether maize is a domesticated version of teosinte, (Galinat, 1988).
Teosinte is an ancient wild grass found in Mexico and Guatemala. Because it has differentiated into various races, species and plant habits, taxonomic classification is still a matter of controversy. Doebley and Iltis (1980) and Iltis and Doebley (1980) classified the annual teosintes into two subspecies of Z. mays: ssp. mexicana (including races Chalco, Central Plateau and Nobogame) and ssp. parviglumis-var. parviglumis (race Balsas) and var. huehuetenangensis (race Huehuetenango) and the species Z. luxurians (race Guatemala). The perennial teosintes from Jalisco, Mexico are separated into two more species according to ploidy, Z. perennis and Z. diploperennis.
B3. Summary of the Ecology of Z. mays and its Progenitors
During its domestication from teosinte, maize has gained many agronomically significant attributes but it has lost the ability to survive in the wild. It has become so domesticated that seeds cannot be separated from the cob and disseminated without human intervention. Maize seeds show poor dormancy, especially in the northern ranges of the crop's distribution. Plants occasionally grow in uncultivated fields and by roadsides or occur as volunteers in cultivated crops in the year following cultivation of a maize crop. However, maize is incapable of sustained reproduction outside of domestic cultivation and is non-invasive of natural habitats (Gould, 1968). Some Zea species are successful wild plants in Central America, but they have no pronounced weedy tendencies (Galinat, 1988).
Part C - Zea mays Related Species
C1. Inter-Species/Genus Hybridization
Maize and other species and subspecies of teosinte are sexually compatible and can produce fertile hybrids (Wilkes, 1977). Related Zea species are geographically restricted and occur only in Mexico and Guatemala. The closest known relative of Zea is Tripsacum, a genus of eleven species, widely distributed between 42°N and 24°S latitude (de Wet et al., 1981). Three species occur in the United States, two of which, Tripsacum floridanum (Florida Gamagrass) and Tripsacum lanceolatum (Mexican Gamagrass), are confined to the southernmost states of the United States. Only one, Tripsacum dactyloides (Eastern Gamagrass), has a distribution that includes the northern (U.S.) maize belt (Gould, 1968).
C2. Potential for Introgression of Genetic Information from Z. mays into Relatives
An examination of the literature prior to 1980 would lead to the conclusion that there is constant gene flow between maize and teosinte, and that the weedy teosinte (Z. mays ssp. mexicana) is a hybrid of the two sub-species, and functions as a genetic bridge between the two (de Wet and Harlan, 1972; de Wet, 1975; Galinat, 1973).
However, this premise has been re-evaluated using techniques of gene mapping, which failed to show any evidence of recent introgression between maize and teosinte (Smith et al., 1985).
Moreover, Z. mays ssp. mexicana seems not to be a hybrid of the wild and cultivated forms of Zea and therefore probably does not serve as a genetic bridge; physical similarities being due to parallel adaptation to the same habitat (Doebley, 1984).
There is evidence of highly restricted gene flow between Zea spp. that apparently occurs predominantly from teosinte into maize (Doebley et al., 1987).
Tripsacum and Zea have different chromosome numbers (n = 9 versus n = 10). Crosses between Z. mays and T. dactyloides can be made, but only through human intervention and, even then, only with extreme difficulty. Moreover, the progeny are frequently sterile or genetically unstable (Manglesdorf, 1974). The process of transferring Tripsacum germplasm into maize is technically difficult. The transmission rate of the single extra Tripsacum chromosome added to the genome is so low and the rate of maize Tripsacum crossing over so reduced, as to practically exclude the general use of experimentally-introduced Tripsacum germplasm in maize improvement (Galinat, 1988).
C3. Occurrence of Z. mays and Related Species in Canada
Of the Zea species, only Z. mays is common to Canada and is known only from cultivation (Gould, 1968). T. dactyloides occurs from Massachusetts to Michigan in the United States (Hitchcock and Chase, 1971), but it has not been reported in Canada.
C4. Summary of the Ecology of Relatives of Z. mays
With the possible exception of T. dactyloides, relatives of Z. mays do not occur in Canada. T. dactyloides is not weedy nor invasive of natural habitats and is extremely unlikely to form fertile hybrids with cultivated Z. mays. T. dactyloides is not regarded as a noxious weed.
In the absence of sexually compatible species related to Z. mays in Canada, there is no potential for interspecific gene flow to occur.
Part D - Potential Interactions of Z. mays with Other Life Forms
Table 1 is intended to guide applicants in their considerations of potential impacts the release of the PNT in question may have on non-target organisms, but should not be considered as exhaustive. Where the impact of the PNT on another life form (target or non-target organism) is significant, secondary effects may also need to be considered.
Table 1: Potential interactions of Z. mays with other life forms during its life cycle
|Other life forms |
|Interaction with Z. mays
(Pathogen; Symbiont or Beneficial Organism; Consumer; Gene Transfer)
Anthracnose stalk rot and leaf blight
Bacterial leaf blight
Fusarium seedling blight, root, stalk and kernel rot
Gibberella stalk and ear rot
Pythium seedling blight and root rot
Rhizoctonia seedling blight and root rot
Northern corn leaf blight
Corn flea beetle
European corn borer
Corn leaf aphid
(e.g., deer, raccoon)
|Consumer, Symbiont or Beneficial Organisms|
|Beneficial insects||Symbiont or Beneficial Organisms|
|Earthworms||Consumer, Symbiont or Beneficial Organisms|
|Soil microorganisms||Consumer, Symbiont or Beneficial Organisms|
|Soil-borne insectes||Consumer, Symbiont or Beneficial Organisms|
|Other Z. mays||Gene Transfer|
This document was written with the assistance of the Canadian Seed Trade Association.
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