Genetic uniformity paradox
Crops such as soybean, wheat, rice and sorghum that are normally mostly self-pollinated show significantly less variety improvement in production of their harvested product than easily cross-pollinated crops like corn. The physical distance between the male and female structures of a corn plant plus the ease of aerial distribution of pollen from the extended anthers on the tassel further assures that most pollen will likely not land on the silk of the same plant, even during a day without wind. The other crops have both male and female structures within the same plant, making it much easier for selfing.
This ease of cross pollination in maize has provided the species with great diversity historically, as it became adapted to a huge range of environments world-wide. The enigma of this phenomenon is the advantage of genetic variability can also lead to disadvantages in grain production within a field. Diversity can allow selection of preferred traits such as grain type, harvest features, germination, disease resistance and mineral uptake, but each of these inherited characters require the genetic consistency from the controlled crosses of specific homozygous parents.
Obtaining the homozygous parents, in which a member of each chromosome has identical genetic code as the other member of the chromosome pair is the goal of placing pollen on the silk of the same plant for several generations or made by crossing pollen from haploid inducer onto the breeding source plant. A small percentage of the plants kernels will have only a single member of each chromosome. Chemicals applied of these haploid kernels cause the chromosomes to duplicate, forming homozygous plants. Regardless of the method of obtaining homozygosity, some of the genes have negative effects on the plants growth. Inbreds of any species carry expression of negative genes where-as crossing with other inbreds that have a dominant form of the gene can block that expression. Identifying that potential for two inbreds to express this hybrid vigor does not require complete homozygosity in the parents but reproduction of the parents for future hybrid consistency requires at least near-homozygosity for ultimate development of hybrid parents.
Inbreds used as hybrid parents often express negative characters for efficient seed production. Those used as females in a seed field, must have the capacity of dependable germinations, characters partly determined by the non-chromosomal DNA within mitochondria. Male parents must have adequate and dependable pollen production and release. Dependable extension of silks when plants are stressed, and large number of kernels are important female inbred characters. Seed producers make considerable effort to overcome the weaknesses of the inbreds and assure maximum advantage for the hybrid corn grower.
Inbreeding causes the successful, repeatable production of hybrid seed. Inbreeding also exposes the negative genes that detract from the inbred performance.
Essential to reproducing identical hybrid seed corn is use of homozygous parent seed. This requires inbred seed production isolated from outside pollen, which is no easy task given that corn’s basic advantage for genetic variability is its tendency for cross fertilization. Even if the parent seed is relatively clean of contaminants, the problem continues into hybrid seed production. Traditionally, outcrosses have been often defined as plants taller than the majority of plants in a field. This has been misleading. Several years ago, we intentionally made outcrosses by pollinating a female hybrid parent with pollen from hybrid plants. That seed was planted along with the correct hybrid seed for comparison. Some of the outcrosses were taller than the correct hybrid and some were much shorter. Outcross plants varied greatly in timing of pollen and silk production as well and, significantly, were all different from each other. This is consistent with what we see in seedlings (Seedling Growouts®), each outcross plant is different from the correct hybrid and different from each other.
Outcrosses that originate with pollen coming from hybrid plants are different because that pollen is the result of meiosis in the hybrid plant. Meiosis results in only one member of each of the 10 pairs of chromosomes to be represented. Minimum number of possible combinations of genetics is 2log10 or 1024 combinations of chromosomes among the pollen grain. Actually, more than that are probable because of chromosome crossovers and mutations that also occur during meiosis. Evaluation based upon taller plants is also influenced by plant height of the correct hybrid versus that of contaminating hybrid. If the correct hybrid is produced by a tall female inbred crossed with a short male inbred, it is more likely that pollen from a tall commercial hybrid will cause more ‘talls’. Even in that case, the pollen that happen to include more of the contaminating hybrid male chromosomes are likely to be smaller than the correct hybrid.
Hybrid seed with less than 1% outcrosses, tall or short, have an insignificant effect on yield. Outcrosses have become more important to those with GMO interests. If the contaminating hybrid included a gene for a GMO from one of its parents, half of the resulting pollen would include that gene. If the seed production field was intended to be non-GMO, then half of the outcrosses would include that gene. If the contaminating pollen included multiple traits on separate chromosomes then the math becomes more complicated, with each of those chromosomes having a 50% chance of being included in each pollen grain. If the trait genes were closely linked on the same chromosome then they likely would be carried together but not always because some of the linkages may break.
If the intended hybrid has the gene on the male parent only, then some or all the outcrosses from a non-trait hybrid would lack the gene. Selfing of the female parent in the seed production field would also lack the gene. These cases are most relevant to herbicide resistant traits and for that reason it is preferred that these genes are in the female parent genetics.
Grain production involves similar dynamics except that hybrid plants tend to produce sufficient pollen to reduce the probability of contamination. It does present apprehension about strict non-gmo rules, including sampling and testing procedures.
Separation of male and female flowers in corn and natural aerial distribution of pollen allowed corn to have broad genetics during the domestication of the species and adaptation of many environments. It also carries with that a small problem when we try to maintain specific trait purity such as in specific grain characteristics (ie. white or amylose corn) or GMO traits.
Genetic contamination in corn
Seed producers attempt to make genetically pure hybrid seed by using homozygous parents. This is not easy. Inbreeding theoretically makes gene have the identical code on the other member of the chromosome pair and thus be homozygous dominant or homozygous recessive. Small mutations can occur along the way of developing these inbreds that can interfere with the process. A larger potential source of problems can come as the inbred in increased when unintended pollen invades the seed increase field.
Corn pollen viability can last only a few hours if the weather permits. Most corn pollen falls within a few feet to a few hundred feet, depending upon wind, although there is evidence of viable corn pollen found ½ mile from the source if wind and humidity is favorable. Fields for increasing parent seed inbreds are consequently isolated from other corn fields at long distances to minimize the contamination.
Hybrid production using pure inbred parents also must struggle with the same problem but with a few more dynamics. Both parents are inspected visibly looking for off type plants in the male and female rows, removing those that are obvious. Most off types will show some hybrid vigor over the inbreds and thus can be easily identified and removed before pollination. Hybrid seed is produced by preventing pollen from the designated female plants by either removing the tassels or use of male sterile female inbreds. The intent of the seed producer is to have adequate pollen from the male parent to cover all exposed female silk as they emerge. The small presence of foreign pollen in the air at the same time always causes the potential for contamination, making the timing of male parent pollen production essential. Environments greatly affect the success of this endeavor. Dry field conditions tend to delay silk elongation whereas it has little effect on pollen production. This can cause most male parent pollen to be released before silks are exposed. Wet field conditions allow more elongation of silks causing the risk of silks exposed before male inbred pollen is released. Exposure of silks when little intended pollen is present increases the probability of the wrong pollen landing the on the silk and thus fertilizing the egg cell in the ovule at the end of the silk. The first viable pollen grain to arrive ‘wins’!
Unintended genetics in production of hybrid seed production because of potential contamination is nearly impossible to avoid. If foreign pollen comes from a commercial hybrid field, all of the off types will not be identical. Genetics of that hybrid will include a strand of DNA from that hybrid’s male parent and a strand from the female parent. With meiosis occurring in the production of the hybrids pollen resulting in a minimum of 1024 new combinations of the 10 chromosomes in the pollen, contaminating pollen leads to many new genetics in new seed. Consequently, the outcrosses among intended hybrid seed vary greatly in appearance, some being taller than the canopy and some being shorter, along with a wide range of other morphological features. Usually contamination is associated with timing and production of intended male parent pollen thus the outcrosses often are more common within a seed size with the seed lot. First silks to emerge are from the base of the ear and thus the larger sizes. Poorly pollinated ears, perhaps from those in which silks emerged after most intended male pollen was exhausted and thus more likely to be pollinated by outside pollen results in more large, round seed. Best purity usually is in those from the middle of the ear of well pollinated ears (the medium flats).
Although a few contaminants in hybrids are common and have practically no effect on hybrid yield, they gain significance in special trait hybrids. That is the topic of the next blog.
Homozygosity in corn
More than 100 hundred years ago, it was evident that crossing corn varieties independently selected for performance in their environments would result in performance boosts. Academics realized that this boost could be increased and more consistent if the parents derived from the unrelated varieties were made genetically consistent by making genes homozygous as opposed to the heterozygous state of most plants in these populations.
With 30,000-40,000 genes located on 10 pairs of chromosomes, getting the same set of nucleic acid codes within each gene on both members of each chromosome pair whether through continuous selfing or use of double haploids includes some random selection of phenotypes. It results in multiple phenotypes, including many not related to the ultimate performance of the inbred or hybrid. We try to evaluate the presence of homozygosity by morphological consistency by looking at characters such as tassel branches, plant height, silk color, leaf features traditionally in samples grown after hybrid seed production. Later methods involved evaluating the protein products of the genes in a process called electrophoresis. PSR offered a different approach in 1987 after realizing the process of inbreeding caused consistent seedling characters unique to each inbred and hybrid. As with the other methods of purity evaluation, the characters displayed may not have significant effect on performance but indicate a slight level of heterozygosity. Slight variability of seedling characters distributed among seedlings of a sample reflects a small lack of homozygosity, but drastic differences indicate unintended genetics such as from pollen from outside the seed field. If all the off types are identical to each other a seed mix is indicated, either within the seed field or after harvest. The Seedling Growout® method (by PSR) offers the advantage of larger sample sizes and shorter time for evaluations than the field methods.
A small amount of variability in hybrids is not necessarily detrimental to hybrid performance unless it involves lack of a gene for resistance to a herbicide. Obtaining and maintaining complete homozygosity in inbred parents is more significant if the inbred is to be maintained for many generations. Independent maintenance of standard public inbreds such as B73 have been shown to have slight differences among them either from accidental contamination or mutations. Seed companies need to consider the long-term prospect for an inbred versus the time and effort involved in obtaining complete homozygosity in a new inbred or an inbred recently introgressed with a trait.
Self-pollinating corn plants promotes the genetic uniformity, including both the dominant and recessive genes. Recessive genes with negative effects not evident when the other member of the paired chromosomes has a dominant form of the gene, show the negative trait when homozygous for the recessive. Accumulation of these negative homozygous recessives does result in a reduction of plant size of inbreds. Hybrid vigor is expressed when crossed with another inbred that has the dominant matching gene. This becomes evident in all aspects of the hybrid versus inbred parent plants.
Obtaining and maintaining genetic homozygosity in hybrid parents is one of the contributors to uniformity in corn hybrids.
Disease and young corn plants
Infection of young corn plants by pathogens affects the disease development in older plants. Exserohilum turcicum, cause of northern corn leaf blight, spreads spores from infected corn leaf debris to the whorl of young plants. About two weeks later, lesions develop producing more spores spread by the wind to other plants. If the spores land in the whorls, infection is likely because of the moisture. Spores of this fungus are most often spread within a field or to nearby fields because of the size of the spores. The gray leaf spot fungus, Cercospora zeae, on the other hand has lighter spores that are more likely to be spread over greater distance.
Puccinia sorghiand Puccinia polysora, causes of common and southern rust respectively, have small urediniospores that are carried great distances by wind. These fungi requiring a living host plant, are maintained. Infection of corn in USA Midwest occurs after spores are carried from winds from the Mexico and Caribbean Islands to be deposited in corn whorls after rain storms. These fungi quickly produce more urediniospores that spread and infect more plants.
Goss Wilt bacteria, Clavibacter michiganensissubsp. nebraskensis, mostly need plant tissue injury to enter the corn plant. This pathogen is spread from diseased leaves of the previous season to young plants during storms, especially with hail, that causes injury in the leaf tissue. The bacteria can spread within the plant, eventually killing young plants.
Young corn plants are also vulnerable to damage from virus infections. These diseases are usually associated with transmission of the virus by an insect vector. Maize dwarf mosaic virus, vectored by aphids, causes significant damage if infected by V2-V6 stage. Aphids, thrips, beetles and mites are often the vector associated a specific corn virus. Severity of the disease often depends upon the timing of the vector presence and the maturity of the corn plant. Severity of the disease often does not become apparent until the plant reaches the flowering stage.
Environments of the early season that favor good growth can make a corn field look beautiful. That same environment can also favor pathogens. Resistance to the pathogens not obvious until the plants approach maturity. Dynamics of diseased debris from the previous season, intensity of the disease in corn far away, origin of storms, and presence of pathogen vectors all affect the eventual disease development in the corn field.
Corn and pre-flowering rain
Water has many effects on the early growth of corn. Cell elongation in roots and leaves is the main contributor to expanding root volume and plant height during the first two months after planting. Cell elongation in young cells occurs as water moves via osmosis through cell membranes, expanding the cells before cell wall solidification occurs. Much of the plant upward growth witnessed during this time is from elongation of cells in the leaf sheaths.
Water supplied to plants during early pre-flowering stage affects plant height. As the apical meristem later begins to expand into producing more stem tissue cells, water has the same interaction ultimately determining how tall the corn canopy will be during the post-flowering period.
Rain in the early season also affects corn disease development for the season. Many potential pathogens survive in the previous seasons corn debris. Moisture to that debris stimulates the pathogens to activate. Colletotrichum graminicola, the fungus causing anthracnose, is one of those that infects the first few leaves on young corn plant. This will cause small lesions in leaves and may even spread to upper leaves in a few varieties. It rarely causes significant yield damage in most varieties and appears to have no relationship to the occurrence of anthracnose stalk rot. Mostly saprophytic fungi like Fusarium species produce spores on the wet debris to at least feed on any dead or weakened corn plant material in the new plants. Senescing initial seedling leaves almost always are invaded by Fusarium.
Corn plants at the V6-to V10 growth stage have a leaf whorl that is constantly moist from transpiration in the new leaves. This makes a nice inoculation chamber for spores of many pathogens to germinate and invade the new leaf tissue. The moisture in corn whorl may be enhanced with rain but probably moisture from transpiration is adequate for most spores to germinate. The first line of defense against the invading pathogen occurs in that new leaf tissue. The first sign of infection that had occurred in the whorl is often a band of small yellow spots on leaves a few days after emerging from the whorl. These chlorotic spots are the result of the plant’s defense system reacting to the invaders at it attempts to stop further spread. Effective resistance limits the number of lesions developing from this initial infection. Pathogen success allows further spread within the field.
The pre-flowering rain is good, expanding root and leaf growth but does come with the disadvantage of also promoting spread of potential pathogens.
Glucose use in corn leaves
Glucose molecules are the immediate product of photosynthesis. They represent the transfer of light energy into a form of molecular energy holding carbon, oxygen and hydrogen atoms together, available for release during cellular respiration in the mitochondria as ATP. This energy is useful to drive further metabolism resulting in construction of more plant structures.
Glucose, and atom components, does more than only supplying energy for plant growth. Cellulose and its chemical relatives such as hemicellulose, lignin and pectin are mostly long chains of glucose molecules. Cellulose may be composed of 2000 glucose molecules held together in tight chemical bonds. These become the main component of cell walls, giving strength to plant structure. These chemical bonds are strong, allowing specific enzymes or considerable energy, such as from fire, to break up the bonds into its components. Humans cannot directly break down the cellulose to retrieve the energy locked up in the glucose components. It is the enzymes in certain microbes such as bacteria in animal guts. Many fungi thrive by feeding on the complex cell wall components of living or dead plants. Evolution favored construction of complex carbohydrate molecules to expose more tissue to light with plant growth and it also favored the production of enzymes in bacterial and fungi to capture the energy locked up in these structures.
Carbon, hydrogen and oxygen from the glucose molecules also become major components of proteins used as part of plant structure and as enzymes. Actual structure of these proteins is guided by the nucleic acid pattern in DNA. This is ‘read’ when a gene is turned on as it is duplicated into RNA, moved to ribosomes resulting in the hooking amino acids in a specific pattern. Amino acids are composed of nitrogen, carbon, hydrogen and oxygen atoms. The order of specific amino acids in the protein affect its enzymatic effect on construction of other cellular parts, including cellulose.
Photosynthesis resulted in energy transfer from light into glucose but also allowed a series of construction processes to make a corn plant.
Corn thrives in full sunlight
Zea mays is among the few species that avoid wasting much of the energy provided by full sunlight. Most plant species have an energy wasteful process called photorespiration in which oxygen molecules actually become consumed instead of released from the plant. This not only uses up energy in the form of ATP but also actually releases CO2. This process is more intense when under high temperatures. It apparently evolved as a mechanism to protect from damage from high light intensity. Soybeans, wheat and rice are among the crop species with this photorespiration system and are designated as C3 because they feature 3-carbon rings on the way to make glucose.
Corn, sorghum and sugar cane have another way of handling high temperatures and high light intensity without potential cell damage. Unique enzymes in the chloroplasts near the vascular bundles tie up the 3-carbon rings by adding a carbon atom, thus avoiding most of the photorespiration reaction. It is significant that these C4 plants evolved this system in tropical, drier environments.
The net effect of C4 photosynthesis in corn is increase in glucose production as the light intensity increases to full sunlight, and a reduction on glucose production with cloudy weather occurs. Leaves fully exposed to sunlight have higher rates of photosynthesis than those shaded by other leaves. Individual plants may have reduced photosynthesis because of shading by adjacent plants but total photosynthesis per land area may be increased because more leaf area is exposed to sunlight. Leaf architecture and leaf width affect total leaf area of a corn plant receiving maximum light intensity.
Further discussion of C4 photosynthesis in Corn Journal can be found by entering C4 in search on this page.
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.