Corn Breeding: Types of Cultivars
Overview
This is the third in a series of lessons specifically designed to instruct individuals without any formal training in genetics or statistics about the science of corn breeding. Individuals with formal training in genetics or statistics but without any training in plant breeding also may benefit from taking these lessons.
Overview and Objectives
Associate Professor, Department of Agronomy and Horticulture at University of Nebraska Lincoln, USA
Leah Sandall
Graduate Student, Department of Agronomy and Horticulture at University of Nebraska Lincoln, USA
This is the third in a series of lessons specifically designed to instruct individuals without any formal training in genetics or statistics about the science of corn breeding. Individuals with formal training in genetics or statistics but without any training in plant breeding also may benefit from reading these lessons.
A cultivar is a plant variety that has been developed for a specific use, such as production of grain. The most common type of corn cultivar grown today and since the middle of the 20th century by U.S. farmers is the single-cross hybrid. In the 1930’s and 1940’s, double-cross hybrids were more prevalent. Before double-cross hybrids, farmers raised open-pollinated varieties. In this lesson, the development and production of these and other types of corn cultivars will be described. The advantages and disadvantages of types of cultivars other than single-cross hybridsthat could be grown by farmers also will be discussed. The lesson begins with a brief description of genes and alleles, which will be helpful in understanding some of the differencesamong types of cultivars.
Objectives
At the completion of this lesson you will be able to
Genes and Alleles
A gene is a unit of molecular information. This information, which controls cell development and other cellular activities, is coded in a linear sequence of different types of deoxyribonucleic acids (DNA). Genes are passed (or inherited) from parent to offspring. This is the main reason that children have similar appearances to their parents. In corn, the number of genes is at least 10,000. These genes are located on chromosomes,which are found in the nucleus of each plant cell. The specific location on the chromosome of a particular gene is referred to as that gene’s locus (plural is loci).
A corn plant has two copies of each gene, one obtained from the female parent and the other from the male parent. An organism, such as corn, with two copies of each gene is called a diploid (“di” is from the Greek word, dyo, meaning two; “ploid” means set of genes or chromosomes). The reproductive cells (egg and pollen cells) produced by a diploid corn plant have only one copy of each gene and thus are said to be haploid (one set of genes or chromosomes).
The two gene copies inherited from the female and male parents may be functionally the same or different. Different forms of the same gene are called alleles. Often, alleles are designated by letters. Thus, if there are two different alleles at a locus, then the three possible genetic constitutions (i.e., genotypes) at that locus could be designated as:
Inbreds
In the lesson, Corn Breeding: Lessons from the Past of this series, an inbred (sometimes called an inbred line or simply a line) was described as a pure-breeding strain of corn. When pollen from an inbred plant is placed on the silks either of the same plant (this is a self-pollination) or of another plant of the same inbred (this is cross-pollination) and the resulting seed is grown, all the progeny will be genetically identical to the parental inbred plant(s). The reason for this is that at every locus an inbred is hom*ozygous (i.e., A1A1 or A2A2, etc.). An A1A1 parent (or two A1A1 parents) can generate only A1A1 progeny. (See pictures below)
Click here to view animation of corn crossing
Single-cross Hybrids
A hybrid plant results from a cross of two genetically different plants. The two parents of a single-cross hybrid, which is also known as a F1 hybrid, are inbreds. Each seed produced from crossing two inbreds has an array (collection) of alleles from each parent. Those two arrays will be different if the inbreds are genetically different, but each seed contains the same female array and the same male array. Thus, all plants of the same single-cross hybrid are genetically identical. At every locus where the two inbred parents possess different alleles, the single-cross hybrid is heterozygous.
Plants of a single-cross hybrid are more vigorous than the parental inbred plants. In Figures 2a and 2b, the single-cross hybrid plant and ear are shown with the plants and ears of the parental inbreds. Clearly, the hybrid plant is taller and the hybrid ear is larger. The increase in vigor of a hybrid over its two parents is known as hybrid vigor.
| Figure. 2a: | Figure. 2b: |
| Corn Plants: Inbred B73 (left), Inbred Mo17(middle), Single cross B73 x Mo17 (right) (UNL, 2004) | Corn Ears: Inbred B73 (left), Inbred Mo17(right), Single cross B73 x Mo17 (middle) (UNL, 2004) |
Breeders often measure the degree ofhybrid vigorof a trait with the following formula:
where Hyb = the value of the trait in the hybrid and
MP = the average (mid-parent) value of the trait in the two parents. For example, in Figure 2a the height of the single-cross hybrid is 3.0 m (this equals Hyb), the average height ofthe inbreds is 2.0 m (this equals MP), and the value ofhybridvigoris 50%. Hybrid vigorcalculated in this way is called mid-parent hybrid vigor. Another type is high-parent hybrid vigor. This is the superiority, expressed as a percentage, of the hybrid over the parent with the better or highervalue of the trait. Corn breeders will be successful in increasing hybrid performance ifthe hybridvigorof a new hybrid compared to an older hybrid is increased and the two sets of parents have equal performance and/or if hybrid vigoris unchangedbut the mid-parent value of the parentsof the newer hybrid is superior to that of the parents of the older hybrid.
The genetic basis of hybrid vigoris not completely understood. However, experience has shown that a hybrid produced by crossing two inbreds that are closely related usually will exhibit less hybrid vigorthan a hybrid produced by crossing inbreds that are more distantly related.
If a single-cross hybrid is allowed to open-pollinate (pollen is despersed freely), each of the plants grown fromthe resulting seed will be genetically unique. Tounderstand why this is so, first consider a single locus. All plants of a single-cross hybrid are genetically identical, so at a single heterozygouslocus any cross- or self-pollination occuring with open-pollination can be represented as:
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Use the diagram to predict the kinds of offspring produced from this cross. Click here for the answer.
The A1A2 and A2A1 genotypes are functionally identical, so three unique genotypes can be produced at a single heterozygous locus when open-pollination occurs.
With two heterozygousloci, A and B, the situation can be illustrated as follows:
In the two-locus case, nine unique genotypes are produced. This occurrence of multiple genotypes among progeny arising from the self- or cross-pollination of parents that all have the same heterozygous genotype at one or more loci is known as genetic segregation.
This segregation occurs at hundreds or even thousands of gene loci. The number of unique genotypes resulting from genetic segregation at n loci is given by 3n. Thus, if n=1 (i.e., one locus), then the number of unique genotypes is three, and if n=2 the number of unique genotypes is nine. But, if n=20, the number of unique genotypes balloons to 3,486,784,401 (=320) Any commercial single-cross hybrid of corn is likely heterozygous at many more than 20 loci. That is why open-pollination of such a single cross results in progeny that are all genetically unique.
The progeny produced from self-pollination of a F1 single-cross hybrid are known as F2 plants. On average, F2 plants will have vigor that is approximately half-way between the single-cross parental plants and the average of the two inbred grandparents; that is, half of the hybrid vigoris lost. This is illustrated in Figure 3. The F2 ears on the bottom row vary in size, but on average are larger than the ears from their inbred grandparents and smaller than the ear from their single-cross parent. That is why farmers have an incentive to purchase new single-cross hybrid seed each year.
| Figure. 3: Top row of corn ears: Inbred B73 (left), Single cross B73 x Mo17 (middle), Inbred Mol7 (right) Bottom row of corn ears: From F2 plants derived from B73 x Mo17 (UNL, 2004) |
When single-cross hybrid seed is commercially produced, one inbred is the male parent and the other the female parent. Either the female parent must be male-sterile (pollen is not produced or is not functional) or the tassel on each female plant must be removed (this is called detasseling) prior to any pollen production (Figure 4). In either case, all the seed produced on the female parent will be single-cross hybrid seed.
| Figure. 4:A single-cross hybrid production field with female inbred parent detasseled) and male inbred parent (not detasseled)(UNL, 2004) |
Developing an inbred from a single-cross hybrid requires approximately seven generations of repeated self-pollinations (Figure 1). Each year in the United States, commercial seed companies produce hundreds of new inbreds and test in field trials many thousands of new single-cross hybrids obtained by crossing these inbreds. Compared to existing commercial hybrids, the vast majority of these new hybrids will be poorer or no better in performance. Only the hybrids that have superior performance in these trials are produced in mass quantities and sold as commercial hybrids to farmers.
Considerable time and inputs are required to develop, select, and produce single-cross hybrids. Achieving a high level of cost efficiency of these processes typically requires large-scale operations.
Double-cross Hybrids
The most prevalent type of hybrid that wasgrown in the United States in the 1930’s and 1940’s is known as a double-cross hybrid. As the name implies, producing a double-cross hybrid requires two stages of crossing involving two pairs of inbreds (see diagram below). In Step 1, two pairs of inbreds, A and B and Yand Z, are crossed to produce single-cross hybrids, AB and YZ. In Step 2, the two single-cross hybrids produced in Step 1 are crossed to produce the double-cross. Typically, A and B are closely related and Y and Z are also closely related, but neither A nor B is closely related to Y or Z. Unlike a single-cross hybrid, plants of a double-cross hybrid are not genetically uniform.
Compared to single-cross hybrid production, production of double-cross requires an extra step. During the early history of the hybrid seed industry in the United States, this extra step was necessary because the inbreds available at that time produced so little grain that making commercial quantities of seed of single-cross hybrids was difficult. Even though the inbreds of each pair of a double-cross hybrid were related, the resulting single-cross hybrids exhibited sufficient vigor to allow those single crosses to be used successfully as parents in mass production of commercial seed. In most environments, the best single-cross hybrid will have superior performance to the best double-cross hybrid. As breeders gradually improved the performance of inbreds through selection, it became possible to commercially produce the more desirable single-cross hybrids.
Double-cross hybrids may become important in the organic corn market. In production of organic hybrid seed corn, herbicides cannot be used. Therefore, seed producers desire parents that have good vigor and can compare successfully against weeds. The single-cross parents of double-cross hybrids have this desired vigor.
Populations, Open-pollinated Varieties, and Synthetics
A population is a group of plants that are mating with each other (i.e., intermating). At many loci, more than one kind of allele will be present in the population. Thus, unlike an inbred or a single-cross hybrid, each plant in a population will have a unique set of genes and will be genetically different from all other plants in the population. Collectively, however, each new generation of a population will share certain characteristics it inherits from the previous generation. That is, each population will have a set of characteristics that distinguishes it from other populations. An example of a popluation in nature would be all the mice living in a barn. They mate among each other, but rarely or not at all with mice living elsewhere.
Open-pollinated varieties and synthetics are types of corn populations. Until the introduction of double- and then single-cross hybrids, farmers grew open-pollinated varieties. Unlike hybrids, seed production of these varieties requires no controlled pollinations. In fact,open-pollinated varietieswere so named becauseseed for the following year’s crop was obtained by saving seed from the current year’s crop. This seed was from open-pollinated plants.
Hundreds of different open-pollinated varieties were developed by farmers in the United States during the 19th and early 20th centuries by selecting for different plant characteristics in different environments. Some of the more famous of these were Krug, Lancaster Sure Crop, Leaming, Midland, and Reid (Figure 5).
| Fig. 5: Lancaster Sure Crop (left) and Reid (right) are open-pollinated varieties of corn. (UNL, 2004) |
These and other open-pollinated varieties were the source material from which the first inbreds were developed. Typically, these inbreds were developed by successive generations of self-pollination (Figure 1).
Most inbreds today are developed not from open-pollinated varieties but rather from F2 populations. Usually, the two inbred grandparents of these F2’s are related, so the typical single-cross hybrid from which a F2 is developed is not one that is sold as a commercial product.
Synthetic is a term often used to describe a population that is developed by allowing cross-pollination to occur for several generations among a number of different cultivars, such as inbreds. A F2 is a two-line synthetic. Probably the most famous corn synthetic is the Iowa State Stiff-Stalk Synthetic. This synthetic was developed at Iowa State University in the 1930’s and 1940’s by crossing together for several generations 16 inbreds that at the time were considered to have above-average stalk strength. Using various selection methods, this population has been undergoing improvement at Iowa State University for many decades.
A population may be genetically improved in several ways. First, the performance of the population itself may be improved (this is referred to as per se performance). Also, the performance of ahybrid in which the population is one of the parents could be enhanced (this is called cross or testcross performance). Finally, the probability of obtaining an elite inbred by repeated self-pollination of individual plants from the population may increase.
Public corn breeders have developed many improved populations. Some were improved primarily for grain yield, whereas for others a different trait, such as lodging resistance or resistance to a particular insect pest or disease organism, was the primary trait improved by selection. In some cases the focus was on increasing per se performance, whereas other times the focus was on cross performance.
Populations are expected to have vigor that is better than inbreds but less than single-cross hybrids. However, in low-input environments in which grain yields are lower, the actual difference in performance between populations and single-cross hybrids becomes less. Also, for some traits other than yield, such as nutritional quality of the grain, some populations may be superior to most commercial hybrids.
Seed of many populations developed at public universities is available to other plant breeders as well as to farmers. The populations improved by selection for per se performance should be better than open-pollinated varieties that have undergone little or no selection. No special pollination is required to produce seed of a population. Also, a farmer or group of farmers could use the same principles as used by university corn breeders to continually improve a population.
Other Types of Hybrids
Some farmers in the United States and many farmers in some parts of the world either want or need to produce their own corn seed. Reasons could include non-availability of single-cross hybrid seed, the high cost of single-cross seed, or the farmer may be producing corn for a small niche market that requires special traits that are not available in any single-cross hybrids. Growing populations is one alternative for these farmers, but this option does not allow the farmer to take advantage of hybrid vigor that could result from crossing two cultivars to form a hybrid.
Producing their own single-cross or double-cross hybrids likely is not economically feasible for most farmers. Are there alternatives?
Both single-cross and double-cross hybrids are produced from inbreds. However, hybrids can be produced by crossing types of cultivars other than inbreds. For example, a population hybrid is produced by crossing two populations. How much hybrid vigor would be realized in a population cross would depend on the populations used.
Producing any type of hybrid requires controlled pollinations to achieve complete cross pollination. Often this is accomplished by detasseling one of the parents. Detasseling a population would be difficultfor two reasons. First,plants of a population usually are relatively tall.Secondly, removal of all tassels would require continual walking of the fields over a period of a week or more because the plants inmost populations are quite variable for occurrence of flowering.
Both of these difficulties (tall plants and variable maturity for flowering) could be overcome by crossing a population (the male parent) to an inbred (the female parent) or to a single-cross of two related inbreds. Most inbreds have relatively short plant height. Also, because all plants of an inbred or single-cross are genetically identical, flowering occurs over a relatively short period of timeand complete detasseling could be accomplished in less time than for a population.
Although most of the best inbreds are privately owned, public inbreds that could be used by farmers to produce population x inbred hybrids are available. Furthermore, a farmer could genetically improve a population x inbred hybrid by conducting selection in the population for enhanced performance when crossed to the inbred. Procedures for doing this type of testcross selectionare outlined in subsequent lessons.
Corn Breeding: Types of Cultivars - Summary and Definitions of Key Terms
Important types of corn cultivars are inbreds, single-cross hybrids, double-cross hybrids, and various kinds of populations. Differences among these types can be understood in terms of their genetics. For example, among these types only inbreds are pure-breeding. Any inbred has this characteristic because it is hom*ozygous at all loci.
A hybrid is a cross between two genetically different plants. The most prevalent type of cultivar grown by U.S. farmers today is the single-cross hybrid. A single cross is produced by crossing two inbreds.
Corn hybrids generally are more vigorous than their parents. This increase in vigor is called hybrid vigor. When two inbreds are crossed that are not closely related, considerablevigor in plant and ear size relative to the parental inbredsis often observed in the single-cross hybrid. Usually a lesser amount of hybrid vigoris observed in other types of hybrids, such as a population x population hybrid or a population x inbred hybrid.
Key Terms
Gene - a unit of molecular information that controls cellular development and/or activity and is passed (inherited) from parent to offspring.
Locus - the specific site of a gene on a chromosome
Diploid - an organism with two copies of each gene
Haploid - an organism with a single copy of each gene
Allele - one of two or more alternative forms of the same gene
hom*ozygous - the descriptive term for a single-locus genotype of a diploid in which the two alleles are the same
Heterozygous - the descriptive term for a single-locus genotype of a diploid in which the two alleles are different
Inbred - a pure-breeding strain of corn
Single-cross hybrid - the type of hybrid that is produced when two different inbreds are cross-pollinated
Double-cross hybrid - the type of hybrid that is produced when two different single-cross hybrids are cross-pollinated
Population - a group of plants that are mating with each other and as a consequence have certain common characteristics
Hybrid vigor - the phenomenon of a hybrid plant having greater vigor than its parents
Further Reading
Fehr, W.R. 1987. Principles of cultivar development. Macmillan Publ. Co., New York.
Hain, P. and D.J. Lee. 2002. Backcross Breeding 1 – Basic Gene Inheritance [Online]. Available at
http://elkhorn.unl.edu/croptechnology2005/pages/index.jsp?what=topicsD&idinformationmodule=957885794&topicorder=1&maxto=6&minto=1 (verified 21 June 2004)
Lee, D.J. and P. Hain. 2002. Just the facts [Online]. Available at
http://elkhorn.unl.edu/croptechnology2005/pages/index.jsp?what=topicsD&idinformationmodule=976049084&topicorder=1&maxto=11&minto=1(verified 21 June 2004)
