Variety parent seed is identical to the final seed product. Parents of hybrid seed are not identical to the hybrid. Single cross corn hybrids are inbreds selected primarily for favorable performance when combined as a hybrid but each with homozygosity for repeatable genetics. This inbreeding process does result in some genetic expression of negative characters in one parent that are covered up by the other when combined as hybrids. After identifying such a combination of inbreds, selection of which parent will become the source of the seed and which will become the pollinator becomes significant to the commercial production of hybrid seed.
The corn kernel is a fruit. The outer layer, the pericarp, is a structure of the female plant. It does not include any genetics of the inbred chosen to be the pollen source. The bulk of the corn seed within the kernel is the endosperm where storage of starch is made available as energy utilized for germination of the seed. Cells within the endosperm include 2 copies of female plant chromosomes and one of the pollen parent. Only the embryo has equal genetics from both parents of hybrid seed.
Commercially acceptable female parents of hybrids need to have reliable and consistent elongation of silks even when under some moisture stress. Silks need to be receptive to fertilization after pollenated. High number of ovules is favored. Pericarp structure must be inclined to withstand stress with minimal cracking. The most important character of the female parent is consistently high percentage of germination. A major factor linking this to the female seed parent is the genetics of the mitochondria within the embryo cells. Mitochondrial genetics originate only from the ovule. These sources of transforming energy stored as carbohydrates into that needed for cell metabolism are full of membranes that can be damaged by rapid swelling when water infuses into dry seed. Maintenance of the integrity of these membranes become essential to the germination process. Tolerance of natural stresses on emergence of silks, of pathogens and stresses on pericarps and of function of mitochondria are all associated with the female parent of a corn hybrid.
Pollen sources for hybrid seed production do have some responsibility as well. Most critical is reliable and timely production of live pollen grains. Release of pollen grains from the anthers is affected by genetics, as the anther chambers must dehisce as the relative humidity drops. Timing with the presence of receptive silks on the female parent is essential. It is probable that part of the pressure for selecting parents that increase grain yield involves shifting the genetics for energy needed to produce pollen to that of more grain results in less pollen.
Commercial hybrid corn breeding programs identify which hybrid parent is best as the female or male based upon quantity and germination of the seed. These are determined by the genetics affecting the biology within the corn seed.
Breeding corn as a variety, the seed selected from open-pollinated plants displaying the traits preferred for grain production is relatively simple compared to breeding parents for hybrids. The advantages of single cross hybrids coming from uniformity of crossing 2 homozygous inbreds that have genetics resulting in heterosis in traits that provide superior performance for grain production, standability and disease resistance drive the incentive for the more difficult process of hybrid breeding.
Experience and experimentation showed that crosses of inbreds developed from different general backgrounds have a high probability of expressing heterosis. The inbreeding process, however, sorts those 30-40000 genes, in each generation of inbreeding from the beginning population with randomness. Although each inbred from a base population may share some characteristics, and many can be immediately discarded by the breeder visually with each generation of selfing, the ultimate test of acceptability comes after evaluation of performance after crossed with potential heterotic partners. Hybrid corn breeding programs devise methods to solve the conundrum of selecting preferable inbred characters while considering heterotic performance efficiently.
Traditional methods of selfing heterozygous seeds for several generations before crossing with potential hybrid partners to evaluate hybrid performance has the frequent disappointment of considerable effort over several generations because of hybrid performance. Making hybrids from potential, but not completely, homozygous inbreds has been used to more quickly and efficiently select desirable inbreds but carries the risk of genetic drift after further selfing.
Dihaploid corn breeding involves crossing heterozygous plants with pollen from a haploid inducer, resulting in up to 10% of resulting embryos only having the one set of chromosomes from the female plant and none from the male. Treatment with specific chemicals can cause up to 50% of these embryos to double this single set of chromosomes, resulting in completely homozygous two sets of chromosomes, a dihaploid. These dihaploid plants can be selfed to produce a small quantity of distinct inbreds in three generations. These dihaploids are still a random set of genes from the parent stock, the value of which must be identified in hybrid performance.
PSR's Rapid Inbreeding® (RI), a system of shortening the time to developing corn inbreds, utilizes the natural homozygosity that occurs in any genetic segregating population. PSR utilizes its skills of evaluating seedling phenotypes to pick out the individual plants that display characters most identifiable with near homozygosity. These seedlings are transplanted, grown to flowering and selfed. This seed is then crossed with potential heterotic inbreds for hybrid evaluation. These near-inbred plants are sufficiently homozygous to assure that future increases from these plants will show similar hybrid performance to that shown in the initial tests.
Breeding for hybrids is more complex than variety breeding but the final reward can be greater as well.
As it became apparent to some corn breeders in the early 1900’s that consistency and repeatability of genetics in corn required creating homozygous inbreds from populations frequently expressing heterosis. The enigma was that inbreeding greatly reduced the volume of hybrid seed to be planted but the production of grain from those hybrid seed was greater than produced by indigenous varieties. A few academic corn breeders pushed the idea of using the hybrids as parent seed to make double cross hybrids to overcome the seed volume problem. They encouraged several farmer seed producers to adapt this concept in the 1930s. The significance of heterosis resulting from crossing specific inbreds became obvious to many during the 30’s, stimulating investigation into the genetics and botany of corn in academia and entrepreneurship among farmer breeders.
As more farmers switched to using hybrid seed, public and private corn breeders increased inbred breeding programs. New synthetic populations were created by breeders by crossing seed from existing varieties, selecting for heterosis by crossing with opposing inbreds, recycling the best, testing again and repeating the cycle to create new improved populations from which new inbreds could be created. Stiff stalk synthetic population created at Iowa State University became and continues to be a powerful source of new inbreds that commonly used as female parents of hybrids. Populations derived from varieties with origin in Eastern USA and grossly identified as Lancaster often became sources of inbreds expressing heterosis with stiff stalk derived inbreds.
Breeding efforts to select more productive seed parents, improved seed production methods and economics of corn grain led to the introduction of single cross hybrids in the USA in the late 1960s. A similar pattern developed in the multiple environments on other continents as well. Continual selection by humans from the diverse genetics selected by previous human generations has led to continual improvement of grain productivity of this species. Its biological features of separation of male and female flowers, C4 photosynthesis, easily transported seed and 30-40000 genes has served us well.
A ‘Compilation of North American Maize Breeding Germplasm’ published in 1993 by Crop Science Society of America includes a list of more than 500 distinct, open-pollinated dent and flint varieties that existed in North America. Their locations of origin and use illustrate the varied environments corn occupied in one temperate zone continent.
Adaptation of corn to a wide range of environments included a range of length of growing seasons. This included selections by people for varieties that would flower with minimal accumulation of heat, such as the Gaspe Flint variety used by indigenous people on the Gaspe peninsula in north Eastern Quebec at least as early as the year 1524. This variety, growing in relatively cool environment, manages to mature with minimum heat units. If the same variety is grown in Central United States, it matures in only 40-50 days after planting at a height of only 30 inches (76cm). Tropical hybrids respond to short day lengths rather than heat accumulation to trigger flowering and thus cannot complete a generation in Central USA unless artificially given short days.
Diversity encouraged by the world-wide selection of desirable genetics adapted to local conditions not only affected factors affecting time to maturity in corn, but also multiple metabolic and structural aspects of the species. It is probable that size of vascular systems in stems, affecting movement of minerals and photosynthates, root structures and efficiency of absorption of individual minerals, number and reactivity of stomata, number and efficiency of chloroplasts and mitochondria must have been among the multiple inadvertent selections made by those early corn breeders as they selected for adaptation to their environments. This variability is offered to current corn breeders as sources for continual adaptation to human needs.
A Germplasm Enhancement of Maize (GEM) project, a collaborative effort by USDA-ARS and public and private research scientists cross USA adapted maize germplasm with exotic germplasm in attempt to expand adaptation of traits selected under diverse environments. These new combinations are intended to be sources of new gene combinations for future corn hybrids available to all potential corn growing environments. More information about GEM can be found at https://usda-gem.public.iastate.edu/GEM_Project/GEM_Project.htm
Corn spread from its origin in central Mexico to much of North and South America over an 8000-9000 year period. Its annual life cycle, transportability and large endosperm favored its use as a food source. Cross pollination biology of corn favored genetic variability and thus adaptation to multiple environments as it coincided with movement of humans across these two continents. As people settled in different areas, they tended to select desired genetics favored by that location and their culture. Over time, distinct types, or races, of corn developed. These races varied in time to flowering, size of kernels, types of starch stored in the endosperm and pigments in endosperm. Obtaining these features also required genetics for resistance to local corn pathogens and insects, root structures to fit the soil types and moisture levels, leaf shapes to capture maximum sunlight, and hormone systems to stimulate timely flowering for both male and female flower parts.
This pattern spread to other parts of our earth after the European immigrants arrived, not only getting introduced to this new food source but also bringing livestock that would thrive on corn. This species was then spread to other continents where it became adapted to other environments.
The culture of identifying ‘varieties’ of corn that was perceived as best fit for their livestock and food needs was adapted by American farmers a few hundred years ago. These farmers saved the seed from the plants with the most favorable features, especially emphasizing the amount of clean grain from single plants growing in their fields. As result they continued the isolation of hundreds of individual, genetically-diverse populations. These populations experienced some inbred depression, as some recessive genes becoming homozygous would cause a negative effect on yield performance.
Occasional mixing of populations occurred however with unexpected results. The Reid family in Ohio had a poor field emergence with their soft starch variety (Southern Dent) that they had traditionally grown. They filled in the bare spots with an earlier variety of a hard-starched new England Flint variety. Saved seed from that field produced much larger yields than their original seed. This principle of hybrid vigor after crossing varieties with distinct genetics became evident in the late 1800’s, leading to university and USDA geneticists, and eventually others to develop methods to utilize the historical genetic development of this species to expand the use of its diverse history. That pattern continues as science and experience with corn advances.
The unique history of origin and humans’ interactions with corn are presented in many writings available in journals, books and the internet. This interaction is probably closer than that of any other between humans and a plant species. Humans have been dependent upon some plant species; the history of human migration and unique biology of this plant species has led to a distinct relationship.
Humans’ interest in the corn endosperm use for food, ease of transport of kernels and annual reproduction of corn caused corn to gain diverse genetics. As cultures developed local uses for corn, they selected endosperm characters to fit their needs. Flinty endosperm types with hard endosperm have Compact cells especially towards the outer layers of the endosperm. Perhaps favored initially because of having less moisture upon maturity and therefore less damage from freezing and perhaps easier to store. Popcorn is a variant of flint corn in which the concentrated of compact cells in the outer portions of the endosperm surround softer cells with more moisture. Heating causes expansion of the inner cells leading to the explosion exposing the outer cells.
Human selection of flinty corn led to specific food uses thousands of years ago and to preferences in current dry milling processes. Similar pressures also led to the softer starch types such as the floury corns in which the endosperm composition led to extremely easy grinding into flour. This was favored by the Aztecs and Incas because of this characteristic.
Dent corns are an intermediate for the flint and floury corn types.
Most of the corn endosperm types are greatly associated with recessive genes influencing the starch metabolism. A recessive gene (su) reduces and delays the metabolism in endosperm that results in sugar to be transformed into starch. The gene sh2 results in even more sugar. Most corn endosperm starch is composed of a branched, poly-carbohydrate called amylopectin and a non-branched one called amylose. A recessive gene results in all amylopectin and is called waxy. A different recessive gene results in most amylose starch.
Ancient selections for favored endosperm types often included isolation of corn populations probably encouraged by humans’ realization that these types required freedom of contamination from other types. The consequence of isolation and endosperm selections over multiple environments allowed the selection of diverse genetics for other characteristics of corn. Today we benefit from this genetic diversity worldwide.
About Corn Journal
The purpose of this blog is to share perspectives of the biology of corn, its seed and diseases in a mix of technical and not so technical terms with all who are interested in this major crop. With more technical references to any of the topics easily available on the web with a search of key words, the blog will rarely cite references but will attempt to be accurate. Comments are welcome but will be screened before publishing. Comments and questions directed to the author by emails are encouraged.