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.
Interest in the domesticated teosinte after the discovery of the mutants that allowed kernels not to be immediately released from the ‘cob’ and absence of the hard encasement around the kernel was selection for endosperm types. This part of the kernel is where the majority of carbohydrates are stored. The C4 photosynthesis process of teosinte was a major contribution of this ancestor of corn.
The endosperm of the corn kernel is composed of two major cellular structures. The outer layer of the endosperm cells, the aleurone, includes a concentration of proteins activated as enzymes to digest the starch into sugars utilized as energy driving the germination activity of the seed. The aleurone is also the site of synthesis of anthocyanins, often functioning as antitoxins that can ward off pathogens. The bulk of the endosperm are cells storing carbohydrates usually in the form of starch. These inner endosperm cells are also the site of carotene biosynthesis.
Ease of transport and potential food use of this new species thousands of years ago led to multiple selections of genetics coinciding with environments from the Andes to lowland tropics in South America. Corn had spread to much of North America before Europeans arrived 7000 years after the initial domestication of teosinte. Human emphasis on endosperm development included not only larger deposits of starch but also specific characteristics.
Carotene synthesis includes multiple steps but if the recessive mutation of the Y1 gene is present this process isn’t completed resulting in no yellow pigment and white endosperm. Aleurone anthocyanin colors are dependent upon three genes affecting that process. A dominant gene, labeled as C1 allows color to be developed, the recessive c1 form of the gene prevents colored aleurone layer. Another dominant gene R1 also allows a colored aleurone pigment. If dominant gene Pr1 is combined with R1 or C1, a purple or blue color is developed in the aleurone layer of cells. Recessive form of the gene (pr1) results in red corn if the C1 or R1 is present. White corn has the recessive y1 activity in the center part of the endosperm and recessive c1 and r1 in the outer aleurone layers. Yellow corn has the dominant Y1 combined with c1 and r1.
As corn was utilized by ancient and modern corn breeders, other endosperm modifications became emphasized. While the attention remains on endosperm characteristics, selections for the multiple genetics involved in plant development appropriate to the environments also occurred. This has led to the large diversity available to current corn breeders.
All flowering plants utilize the endosperm as storage of starch as an energy source for seed germination. Grains used for human food have been chosen for having larger endosperms than simply needed for germination energy. The variety of teosinte (Zea mayssubspeciesparviglumis) known as Balsas teosinte, believed to be the source of human domestication of maize (Zea mays subspecies mays). Mutations in the Balsa teosinte resulted in absence of the hard encasement around the kernel and absence of abscission layers leading to early release of the kernels from the central rachis (cob). Occurrence and acknowledgment of these mutations occurred to humans about 9000 years ago in the Tehuacan Valley, south of the current Mexico City. Fortunately, this occurrence coincided with the migration of people from northern Asia across the Bering Strait into North America, spreading south through Mexico to South America.
The attraction of these primitive Zea maysseeds as a food source and ease of transportation must have allowed for distribution from that original source to Peru by 6500 years ago and to the eastern base of the Amazon River by 4030 years ago. Recently published research indicates that much development into current corn occurred near the Andes (Kistler et al., Science 362, 1309–1313 (2018).
Human’s interested centered on the endosperm of the corn seed. The fact that this species has at least one generation per year, and that it was cultivated by multiple growers who essentially were corn breeders, selecting for endosperm size and characters. These people did not need to study genetics to realize that if they chose kernels with the most desirable characters in the endosperm to plant, they could increase kernels with those characters. Essentially, there were thousands of corn breeders in a huge number of environments selecting for corn genetics favoring their desired corn endosperm characters. Some preferred certain starch components that could be ground into flour for culture desirable foods. Others chose strong carotene (yellow) colors, still desired because it results in dark yellow egg yolks when fed to chickens. Others preferred the blue or red anthocyanin colors in the aleurone layer of the endosperm.
Selection for desirable endosperm in multiple environments of South and North Americas also allowed for diversity of genetics for other plant characteristics including root growth, time to flowering, leaf size and structure. Multiple internal characteristics affecting photosynthesis, disease resistance and transportation of sugars to the endosperm were also affected by the selection for desirable endosperm by these many corn breeders, across many environments for thousands of years.
An interesting read on the internet on the teosinte and maize relationship can be found at
The meristem in at least one of the lateral buds of a corn plant develops into an ear. This meristem includes 500-1000 lateral meristems with mother cells with diploid sets of chromosomes, 10 chromosomes from each of that plant’s parents. Meiosis occurs in this diploid cell, resulting in 4 haploid cells, each cell having only a single set of 10 chromosomes consisting of a random mix of the two parent’s chromosomes. Three of the 4 haploid cells degenerate, leaving a single megaspore. This megaspore nucleus undergoes mitosis three times, resulting in 8 cells within the megaspore structure now called the embryo sac. One cell at the bottom of the embryo sac becomes the egg cell while two of the haploid cells fuse in the center of the embryo sac.
The embryo sac (ovule) is enclosed in an ovary, at part of the female part of the parent plant. Part of this female flower is the silk., extending from a single ovary and attached to its ovule. The male flower also produces pollen via meiosis followed by a single mitosis, resulting in two haploid nuclei. A pollen grain adhering to the silk, germinates and extends down the silk to the ovule. Upon entrance of the ovule, one nucleus fuses with the haploid egg cell forming a diploid nucleus to become the seed embryo. The other pollen haploid nucleus fuses with the two ovule nuclei in the center of the ovule resulting in a triploid nucleus, having two sets of chromosomes from the female parent and one from the male. This triploid nucleus undergoes mitosis to become the endosperm of the seed.
Whereas the inheritance of the embryo, and its resulting mature plant, is determined equally by the genetics of the male and female parents, characteristics of the endosperm is slanted towards the genetics of the female parent. If the female parent has the recessive Y1 gene, and thus a white endosperm, but the pollen is from a parent with dominant gene and thus has yellow endosperm pigment, the resulting endosperm will be lemon white in color. The female genetics has the major affect on endosperm function in the maize seed because of contributes two of the three sets of chromosomes in endosperm cells.
A single layer of cells immediately inside the pericarp is the aleurone. Contrary to the pericarp’s origin from the female plant’s ovary, the aleurone is derived from the pollinated ovule. It develops from the endosperm portion and thus from the contribution of one sperm nucleus combining with two haploid egg nuclei. Cells of the endosperm, including the aleurone cells is triploid, having three sets of chromosomes in their nuclei. It originates with other endosperm cells as they multiply after pollination. These cells surrounding the inner, starch filled endosperm cells, have an important role in seed germination, providing the enzymes to digest the starch in the rest of the endosperm into the glucose molecules.
Although only a single layer of cells, it can include 30% of the total proteins of the endosperm. It is also the location of pigment molecules influencing the color of the corn kernel. Pigment expression in corn kernels is mostly influenced by the carotenoid and anthocyanin pathways. A dominant gene (Y1) codes for synthesis of a protein in the carotenoid pathway for production of yellow pigment all of the endosperm, including the aleurone layer. If the female and male parents of a hybrid have the recessive version of this gene, and lacks recessives for the anthocyanin genes, the hybrid kernels will be white. However, if the male parent is yellow and the female parent white, the result will be the intermediate color between white and yellow. If both parents have the recessive gene of Pr1 the kernel will have purple anthocyanin pigment in the aleurone cells. The color will show as more blue pigment if the recessive carotenoid gene gives a white endosperm. Red kernels likewise are more intense with the absence of carotenoid pigments and presence of the recessive version of R1 gene for anthocyanin production.
Mutations of genes influencing pigments produced in the aleurone layer of cells was utilized a few thousand years ago by local cultures as corn was moved from that original Teosinte base. We use it today to make those colored corn chips.
The toxin fumonisin causing severe disease in horses and other mammals is produced in corn by the fungal species Fusarium verticillioides. This species, once known as Fusarium moniliforme, if a commonly associated with corn worldwide. Whereas other Fusarium species associated with corn are frequent in soil, this species is more dependent on existence is corn debris between crop seasons. The fungus does invade corn seedling roots, especially if root growth is slow due to low temperatures and/or injured by insect feeding.
Fumonisin production by the fungus is linked to a few genes in the fungus leading to some variation among the isolates of F. verticilloidesin production of fumonisin. There is evidence that fumonisin production by the fungus assists in overcoming the plant’s resistance system by causing plant cell death. Fumonisin produced in roots has been shown to be transported to the leaves of the corn plant.
Growth of this fungus in seedling roots is linked to slow growth of the seedling, mostly due to low temperatures. After initial infection, the fungus produces conidia small enough to be carried in the xylem through the complex first node separating the mesocotyl from the coleoptile. It has been noted that low light conditions are associated with more rapid spread from the root to the above ground-portions of the corn plant. This implies that plant physiological condition associated with photosynthesis affects the ability of the fungus to spread within the corn plant.
Fusarium verticilloidesalso infects the kernels through the silk (style). There is evidence that corn genetics associated with the size of the small opening at the tip the corn ovary allowing the pollen entrance is associated with successful early invasion of hyphae of this fungus. Environmental stresses during kernel formation, including insect feeding is also associated with invasion by this fungus.
Corn’s association with this fungus is complex and seemingly ubiquitous. The fungus often appears to be an endophyte, causing no visible harm. Not all variants of the fungus produce fumonisin. Seedlings growing in good environments infected with F. verticilloidesmay express limited damage. Not all kernels infected with this fungus produce fumonisin. It is one of the inhabitants of corn’s environment that can produce a toxin.
Fungi, like the rest of us non-photosynthesizing organisms, are dependent on plants for existence. Species of the fungal genus Fusarium are ubiquitous, often feeding on dead and living plant materials. They are identified by microscopic observation of their asexually produced spores (conidia) abundantly produced from the filaments (mycelium) growing in and on plant tissue. When these fungi are stimulated to sexually reproduce, they develop spores in microscopic ‘sacs’ called asci. This means of sexual reproduction places Fusarium in a class of fungi called ascomycetes. The genus of ascomycetes that have Fusarium as a conidial form is Gibberella. Consequently, a fungus frequently found in corn stalks, leaves and ears may be identified by its conidia shape as Fusarium verticilloides but if stimulated to form sexual bodies it would be called Gibberella fujikuroi. Another species of Fusarium (Fusarium graminearum) more frequently is found on corn stalks and ears as the sexual stage, Gibberella zeae. This dual naming system, tolerated by mycologists and plant pathologists, occurred because the fungus was initially only known and named by the asexually produced conidia.
These fungi on corn are not aggressive pathogens but mostly invade dead or weakened cell tissue. Infection of corn ears by Fusarium species is often through old silk tissue. Fusarium or Gibberella stalk rot occurs after the stalk tissue has been weakened by desiccation due to roots rot. Fusarium mycelium is easily found in leaf tissue, as if it is an occupant, perhaps feeding on weakened or dead cells within living leaves. Its widespread presence in corn seeds, seedlings, stalks, leaves and ears often leads to difficulty in determining its significance. Was it an aggressive pathogen killing the tissue or was it an invader of weakened tissue? Is seedling blight due to a Fusarium species attacking a vigorous, corn seedling or was it simply infecting a corn seedling weakened by environment? Gibberella stalk rot occurs when the plant-environment- genetics interaction results in roots dying from insufficient carbohydrate to sustain metabolism. Stalk cells die because of consequential wilting and shortage of carbohydrates as well. Fusarium graminearum feeds on the weakened and dead tissue, eventually producing the sexual reproduction bodies of Gibberella zeae and thus allowing us mere humans to call it Gibberella Stalk Rot.
Carbohydrates stored in the endosperm of a corn kernel is a potential source of nutrition for fungi and insects. The pericarp can be a major barrier to attack to these potential invaders. Studies concerning the tropical corn storage insect, Maize Weevil (Sitophilus zeamais), showed that the cross-linked structural components of the pericarp cell walls were highly correlated with resistance to this insect. Other factors included phenols (Afr. Crop Sci. J. 9:431–440) produced by the pericarp cell metabolism and even endosperm hardness (flintiness) contributed to reduced susceptibility to this storage insect (Crop Sci. 44:1546–1552 (2004)).
Pericarp tissue also is a barrier to entrance into the seed by multiple kernel rotting fungi. Most enter the ovary through the silk channel immediately before pollination. This becomes most evident when silks are left exposed for several days in an environment favoring the pathogen. After invasion, the fungus can spread cell-to-cell within the pericarp through small holes (pits) in the cell walls that allows movement of metabolites between cells. Integrity of the pericarp is a significant factor in avoiding invasion by many potential fungal species.
The phenomenon known as silk-cut can expose the seed to fungal infection. After the pollen tube grows down the silk channel and dumps the pollen nuclei into the ovule, silk tissue deteriorates and detaches from the ovary. Not all silks are pollenated even under ideal conditions, leaving some attached to their ovary while adjacent pollenated ovaries grow. These remaining silks interfere with normal contiguous growth of the pericarp cells in the adjacent ovary wall (Plant Disease 81 (5):439-444). This can result in a break in pericarp as the kernels enlarge and thus an opening for invasion by fungi. Genetics and environments influence the occurrence of silk-cut. Stresses that delay silking beyond pollen availability can be an important factor but genotypes vary in vulnerability both to reaction to the stress and probably the tendency of this phenomenon.
The corn kernel is a fruit. Grains are fruits with a single seed enclosed. The ovary wall, part of the female plant, grows after pollination results in enlargement of the single embryo it encloses. The ovary wall thus becomes the pericarp. Genetics of the pericarp cells are those of the female plant and therefore the genetics of the female inbred parent in hybrid seed production or both hybrid parents in the grain field.
These genetics influence the important cell wall components that give both strength and resistance against insect and fungal invaders. Cell walls get their strength from various polymers that are cross-linked. Whereas non-grass species tend to have more lignin chemistry in cell walls than those in grasses and especially the pericarp features less lignin and more of a class of complex compounds called xylan. A major component of these xylan compounds is feruloyic acid. Feruloyic acid is associated with resistance to Maize weevil damage to corn kernels (Crop Sci. 44:1546–1552 (2004). Varieties differ in chemical components of pericarp cell walls that probably influence many aspects of corn grain storage. https://www.frontiersin.org/files/Articles/219955/fpls-07-01476-HTML/image_m
The female parent of a corn seed is the sole genetic source for the pericarp, mitochondria and chloroplasts as well as half of each diploid chromosome.
Mitochondria are not the only membrane source in corn seed cells. Nearly all cell functions are carried out on membranes. Endoplasmic reticulum (ER) is a major component of the cell which acts as transporter of the enzymes and proteins being produced in the cells. It is also the transporter of the messages from the DNA in the nucleus of the cell. Virtually all cell functions are dependent upon the structure of cell membranes.
Development of membranes in seed is dependent upon a combination of the genetics of the variety and the environmental stresses during seed development. Drying the seed also significantly affects the membranes, as the membranes collapse with drying. Corn seed producers are very aware of this potential problem and develop methods to assure that seed drying is carefully monitored so handling is gentle. Seed membrane deterioration can occur in the field, especially if seed is allowed to dry slowly after fully developed at about 35% moisture when the abscission layer (black layer) cuts off nutrition from the plant. Rain preventing harvest at this critical time is one cause of loss of seed viability because it begins an aging process in corn seed.
A combination of drying temperatures below 100°F and quick drying by high air movement is critical to maintaining membrane integrity in corn seed. Although all cellular membranes are probably affected by drying conditions, the fact that germination deterioration is mostly linked to the female parent of a hybrid, it is likely that the mitochondrial membranes are affected the greatest. Each corn genotype varies somewhat in tolerance to these factors, but the principles of drying temperatures and speed of drying appear to mostly involve membrane deterioration. There is also some effect on the pericarp of the grain if it causes breakage. Seed producers apply this knowledge and a lot of art to balance all the variables involved in producing seed with high field emergence rates the following spring.
Respiration of dried seed is needed to maintain membrane integrity while being stored but low temperature and dry atmosphere is needed to minimize the activation of excessive seed metabolism. The combination of seed moisture no greater than 13% and the temperature of 50°F and 50% relative humidity is usually considered adequate for maintaining seed germination quality for several months. Genotypes vary in maintenance of seed quality even in this environment. Stresses during the growing season also can cause some seed to deteriorate even with careful storage monitoring.
Production and maintaining high quality corn seed requires corn genetics, non-stressful growing environment, careful handling at harvest, drying and shelling and proper storage before the next planting season. There is an art to being a good seed producer.
Most new crop seed is tested after drying and shelling and before bagging. Systematic sampling is done after sizing. These samples are tested for genetic purity, warm germination and cold germinations. These results are used for bagging decisions. After sizing and seed treatments are made seed is moved to bagging. Again, systematic sampling is made. Those samples are then tested at least in a warm and cold test. The warm test result is used for the reported germination printed on the bag tag. Germination percentage is within a statistically acceptable range of the published percent germination on the tag and within the month as published.
Not all individual seed within the bag are at exactly the same state of quality- some deteriorating faster than others. This could be related to position on the ear, or within the seed field or handling of seed after harvest. If 5% of the seed did not germinate in a December warm test, a few more percentage may (or may not) be deteriorating in the next few months. Seed companies attempt to estimate this rate by considering the cold test results or perhaps special tests. Suspect seed lots may be retested a few months later.
Each corn seed is a living organism, vulnerable to ageing from membrane deterioration including within the mitochondria, that transfer the energy from stored carbohydrates into usable forms for metabolism needed for cell growth and multiplication. Damage to membranes because of partial metabolic activation prior to artificial drying can affect mitochondrial activity when germination is encouraged. Physical injury to the pericarp can allow imbibition to be too rapid for cell membranes of the dormant cellular components, resulting in breakage. These membranes can self-repair but do this best at warmer temperatures. Leakage from injured kernels attracts micro-organisms that can further inhibit the seed metabolism.
It is a challenge to all involved to correctly predict the percentage of seed that will emerge in the field. Warm tests are often done at 70°F (21°C) as a measure of viability for a week after moistened. Successfully germinated seedlings have clear development of the primary root and shoot. Percentage of seeds showing these structures become the warm test result. Some seed will be slow to push both structures within that time. Those showing only a root are generally called non-germinated and not included in the percent germinated. There is some judgement needed for classification of those that are slow to push out both structures. The cold germination test in which the moistened seeds are kept at 50°F (10°C) for one week before moving to the warm test environment further amplifies the effect of the seed deterioration on germination. Some seed lot samples will express high germination percentages in both warm and cold tests. Usually those showing marginally acceptable warm test results usually have much lower cold test results, assumedly because the individual seeds with membrane damage could not self-repair after imbibition at low temperatures.
Professional Seed Research, Inc. plants corn seed under about ½ inch of artificial soil mix for warm and cold tests. Seedlings are counted when most plants show the third leaf. Individual seedlings that only show the ‘spike’ emerging are counted as not germinated on the assumption that these individuals, if they do emerge in the field, will be non-competitive plants. Uniformity of emergence of the 400 seed sample is also scored in the warm germination report.
The ultimate objective for all involved in corn seed is to allow full expression of the genetics on grain yield and not allow seed germination to be a detraction from that potential.
Use of homozygous inbreds as corn hybrid parents is important to producing new hybrid seed with identical parents’ genes successful in past seasons. Traditional method of obtaining homozygosity has been done by self-pollinating selected plants from a segregating population for 5-8 generations. Studies from the 1950’s showed that basic heterosis with another parent could be expressed after 3 selfing generations, but the remaining selfing generations are needed to assure repeatability of minor traits. Corn breeders have tried various methods to try to reduce the time and testing expense to this process.
A quicker method of obtaining homozygous inbreds by inducing haploids to be doubled was known since 1959, but the method became more common after the year 2000. Pollen from haploid inducer is crossed to a prospective heterozygous genetic source. From 2-10 percent of the resulting seed will only have the genes from the female ovule although only one member of each pair of chromosomes of the female plant. These seed can be identified visually with pigments if the inducer differs from the female plant in these pigments or perhaps by molecular methods. Haploid chromosomes are doubled by either submersion or by injection with chemicals such as colchicine or specific herbicides. It is estimated that, on average, to obtain 100 new inbreds by DH system, requires initial pollination of 100 - 200 plants by the inducer to obtain 1000 haploid plants to be grown in field, after doubling chemical treatment. It is intended to obtain 100 homozygous, diploid inbreds as a result.
Professional Seed Research, Inc. (PSR) began experimenting with another method of quickly reaching near homozygosity by utilizing their Seedling Morphology Fingerprint (SMF™) technology used in their other services. This utilizes the fact that because corn has only 10 pairs of chromosomes, there is are a notable percentage of near-homozygous individuals among a segregating population. That percentage increases if the F2 population is between related parents. PSR selects these near-homozygous plants based upon seedling characters. These plants are transplanted to the field and selfed. Resulting plants range in homozygous levels equivalent to 4-8 selfing generations. This is sufficient to evaluate heterosis with prospective parents and yet may have sufficient heterozygosity to select for simple inherited characters such as ear height, pollen production or disease resistance. PSR refers to this system as Rapid Inbreeding®.
The objective of all three systems is to reach a level of homozygosity in productive hybrid parents that can be repeated with each increase. Traditional method of selfing each generation allows for selection of desirable characters in each generation but costs time. Dihaploid breeding is quicker to reach complete homozygosity than traditional methods but requires specialized efforts and genetic backgrounds affects the success rate. Rapid Inbreeding® (RI) offers the speed of dihaploid breeding to reach level of sufficient homozygosity for hybrid selection, works with any segregating population and allows further selection of minor characters in final inbred. PSR Global Genetics uses RI technology to develop thousands of inbred lines each year.
The nucleus of every living cell of the corn plant has 30-40 thousand genes on each chromosome. Each gene is composed of a string of four nucleic bases (adenine, cytosine, guanine and thymine) the order of which is the DNA code for that gene. The DNA structure is a double strand of these nucleic bases wound around a sugar molecule (deoxyribose) and a phosphate molecule. The strands are held together by the attraction of bonds of adenine to thymine and cytosine to guanine. A single gene may include a string of hundreds to thousands of these 4 nucleic bases.
When a gene is transcribed, the double helix strand is separated, and the single strand copied onto a different sugar (ribose) and phosphate, moved from the cell nucleus to the ribosome in the cytoplasm. Amino acids from the cytoplasm are attached to each other according to the sequence of the nucleic acids. Three consecutive nucleic bases code for a single amino acid. The string of amino acids become a protein once the genetic code for a single gene is read in the ribosome.
Amino acids are compounds of nitrogen, hydrogen carbon and oxygen atoms attached in 20 distinct patterns, including differing side branches. A single protein molecule often includes hundreds of amino acids combined in complex shapes because of the different charges of those side branches in each amino acid. Many of the proteins function in cells as enzymes affecting all cell metabolism.
The complexity of thousands of genes each composed of strings of 4 bases translated into strings of amino acids to form a single protein that may affect the process of producing a single cell compound is overwhelming. The stability of the system is responsible for corn plants looking like corn plants. Probably 95-99% of the genes in all corn plants are similar.
Genetic variability allowing the differing expression of favored traits affecting timing of flowering, silk extension, water and mineral uptake, leaf size and shape, disease resistance may involve only 1-5% of the DNA code. Selection of random assortments of the DNA codes has led to the variation among varieties within the corn species. Combining the inbreds to maximize the expression of favored traits is the job of the corn breeder. Providing a dependable system of genetics is the responsibility of the cell nucleus and translation of the gene is up to the cell cytoplasm.
The vast majority of genes in corn must relate the basic structures and functions of all corn plants. Mutations of these genes tend to be recessive and therefore are usually not expressed if the dominant form of the gene is present in the matching area of its paired chromosome. On the other hand, recessive genes with potentially beneficial (to us) traits need to have both members of the paired genes be in the recessive form. The enigma of attempting to benefit from hybrid heterosis and yet obtaining repeatable genetics by making homozygous parents is that the process of reaching homozygosity allows expression of the recessive traits.
Some of the expression of homozygous recessive traits may be drastic, such as albino plants. In some cases, it is expressed as susceptibility to a pathogen, such as race 1 of Bipolaris zeicola (Helminthosporium carbonum), resulting in an occasional inbred homozygous recessive for susceptibility. The majority of negative recessive genes affect the chain of biochemical pathways resulting in a reduction in effectiveness of those products. Highly inbred plants are less vigorous than hybrid plants because of this overall reduction in the physiological processes contributing to cell and plant structures. It is assumed that this is caused by the expression of recessive genes that are not expressed when paired with dominant forms of these genes.
Corn breeders attempt to overcome the negative effects of inbreeding by mating inbreds with compatible inbreds to cover the deficiencies of each parent inbred. Considering the large number of genes in corn, and the diversity of its history, identifying the best combination for maximum desired performance is not easy. It is known that inbreds developed from certain ‘families’ are most likely to combine well to make productive hybrids in most environments. A broad group of varieties generally grown in the western part of central USA were intercrossed for several generations to create a breeding population known as Iowa Stiff Stalk. Inbreds developed from this population tend to create vigorous hybrids when crossed with certain other populations originating from the Eastern USA. This principle of crossing inbreds originating from distinct populations applies to hybrid corn breeding in all regions where corn is grown.
The challenge for corn breeders is to not only select the appropriate breeding source based upon desired history of agronomic characters, but also identify prospective hybrid parents with desired characters and identify the appropriate other parent for maximum hybrid performance. Despite our gains in ability to analyze DNA, physiology and environments, the variables are multiple. There has never been the perfect corn hybrid for multiple years and locations- and the complexity of plant and environments will maintain this history for many years as well.
Corn’s history and biology has resulted in diversity beyond what most of us see in any single season. Advantages of hybrid plant uniformity for yield, harvestability, disease and pest resistance and genetic repeatability requires development of homozygous inbred parents. Each of many seed companies produce multiple hybrids each year and there are about 40,000 genes in each corn plant that are available to influence something, whether needed or not.
Corn researchers in 1920’s became aware of a need to collect and share many of the genetic sources in corn, forming a Maize Genetics Cooperation Stock Center- it’s history is summarized at http://maizecoop.cropsci.uiuc.edu/mgc-info.php. This collection started and continues to emphasize mutants affecting some identified trait, such as those involved in sweet corn, waxy corn or amylose corn and many that may not have a specific economic advantage but are useful in understanding some biochemical pathway in the corn metabolism. Study of these mutants contributed to location of genes on each chromosome and add to growing knowledge of corn DNA codes for many traits.
Despite these efforts, corn’s genetic diversity is large due to selection by humans over diverse environments. Our experiences with ‘new’ diseases as a pathogen such as the bacterium causing Goss’ wilt suddenly appears, with a previously unknown susceptibility gene in corn became part of popular corn hybrids, or susceptibility of race T of southern corn leaf blight associated with mitochondrial gene in t-cytoplasm male sterile corn. Resistance to Maize Chlorotic Mottle Virus was found in corn genetics in USA after it occurred in Kansas in the 1970’s and in Africa in 2016.
Often the strong resistance to these diseases are associated with single genes already present in corn apparently without intended human selection and without known selection pressure in absence of the disease. Perhaps there was exposure somewhere in its history where the gene was favored but also it is likely that randomness of mutations, segregation of genes during miosis, cross pollination and historic diversity of corn’s environments have provided many genes for characters that we have yet to identify. These genes must be influencing multiple internal aspects of absorption of light wavelengths, translocation of carbohydrates, absorption and movement of minerals, water uptake and conservation, and structures of leaves. Among this diversity is the future adaptation needed for changing environments.
Breeders witness diversity within their nursery as they see differences in plant structures and growers see differences among hybrids in performance each year. At Professional Seed Research Inc., we see differences among hybrids in structures of seedlings (Seedling Growout®). Genetic diversity will continue to be an important contributor to this crop as it interacts with changing environments.
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.