Breeding a self-pollinated crop
1. Introduction to breeding self-pollinated crops
1.1. Definition
Plant breeding, also known as crop genetic enhancement, involves creating new and improved varieties of crops for practical use. The newly developed variety may exhibit higher yields, superior grain quality, enhanced resistance to diseases, or reduced susceptibility to lodging. Ideally, it will present a novel combination of traits that significantly surpass those of existing varieties.
The methods of breeding and selection for self-pollinated crops rely on the understanding that genetic variability generated through hybridization and recombination among carefully chosen parent plants allows for the possibility of achieving more advantageous combinations of traits. Furthermore, it is feasible to produce homozygous lines that contain these recombinants through self-fertilization and selection. There are various techniques available for breeding and selecting self-pollinated plants. When selecting a specific method, the breeder takes into account the genetic control of the trait, including whether its inheritance is simple or complex, its heritability level, the extent of linkage with any undesirable traits (if known), as well as the time, labor, and space constraints of the breeding program.
1.2. Objective
The breeding methods commonly employed for self-pollinated plants were established over a century ago in Europe, with minimal significant changes since then. Essentially, the selection process for the best progenies has been refined, particularly through agricultural experimentation inspired by Fisher's publications. Over the last 25 years, advancements in molecular biology have introduced the potential for direct genotype selection. Global research has predominantly concentrated on the application of molecular DNA markers. While a wealth of foundational information has been gathered, it remains clear that precise phenotypic analysis in the field is still crucial. Evidence has shown that the traits breeders focus on are largely complex, likely governed by multiple genes. This realization has become increasingly evident, as improvements are fundamentally achieved through the accumulation of benefits across successive selection cycles, involving recurrent selection and intercrossing of the best available lines or cultivars in each cycle. Consequently, the objective is to pinpoint the plants or progenies with the highest genotypic merit during each selection cycle. Another aim is to create cultivars that can be utilized over several years. Numerous studies have been dedicated to enhancing breeding efficiency. This paper will discuss some of these studies aimed at increasing the breeding efficiency of self-pollinated plants by identifying progenies or lines with the highest genotypic merit and will also explore strategies for developing cultivars with extended lifespans.
1.3. Methodology
To complete the term paper, both a table study and online research were conducted. References were drawn from online sources as well as BD Singh's Principles of Plant Breeding.
2. Description
2.1. Types of self-pollinated cultivars
In terms of genetic structure, self-pollinated cultivars can be classified into two categories:
1. Those that originate from a single plant.
2. Those that come from a mixture of plants.
Single-plant selection may or may not follow a planned cross, although it often does. Cultivars that are derived from single plants are both homozygous and homogeneous. In contrast, cultivars that arise from plant mixtures may seem homogeneous; however, due to the differing genotypes of the individual plants and the occurrence of some outcrossing (even if minimal) in most selfing species, heterozygosity can develop later within the population. The breeding methods for self-pollinated species can be categorized into two main groups – those that are preceded by hybridization and those that are not.
2.2. Mass selection
Mass selection exemplifies the process of selecting from a biologically diverse population where the differences are genetically based. The Danish biologist W. Johansen is recognized for establishing the foundation of mass selection in 1903. This method is often regarded as the oldest technique for breeding self-pollinated plant species. Nevertheless, this does not imply that the method is outdated. As a traditional practice, farmers have historically saved seeds from preferred plants for the subsequent season’s planting, a technique that remains prevalent in the agriculture of many developing nations. This selection method is relevant for both self- and cross-pollinated species.
Applications
As a contemporary approach to plant breeding, mass selection has numerous applications:
1. It can be utilized to preserve the purity of an existing cultivar that has become contaminated or is segregating. Off-types are simply removed from the population, and the remaining material is bulked. Over time, existing cultivars can become contaminated through natural processes (such as outcrossing and mutation) or human error (for instance, unintentional seed mixing during the harvesting or processing stages of crop production).
2. This method can also be utilized to create a cultivar from a foundational population established through hybridization, following the procedure outlined next.
3. It can serve to maintain the identity of an existing cultivar or a new cultivar that is about to be released. The breeder selects several hundred (200–300) plants (or heads) and arranges them in individual rows for evaluation. Rows that exhibit significant phenotypic variations from the others are eliminated, while the remaining rows are combined as breeder seed. Before this bulk process, sample plants or heads are collected from each row and preserved for future reproduction of the original cultivar.
4. When a new crop is introduced to a different production area, the breeder may adapt it to that region by selecting for essential factors necessary for successful cultivation (e.g., maturity). This process thus enhances the new cultivar for the specific production area. 5 Mass selection can be employed to incorporate horizontal (durable) disease resistance into a cultivar. The breeder introduces low levels of disease inoculum (to encourage moderate disease development) so that the genetic effects of minor genes (rather than major genes) can be evaluated. This approach ensures that the cultivar is non-specific to races and moderately tolerant to disease. Additionally, crop yield remains stable, and the disease resistance is long-lasting. 6 Some breeders incorporate mass selection into their breeding programs to eliminate undesirable plants, which helps to streamline the materials advanced, saving time and reducing breeding costs.
Procedure
The overall approach in mass selection involves removing off-types or plants exhibiting undesirable characteristics. Some researchers refer to this as negative mass selection. The specific methods for preserving representative individuals within the population differ based on species, traits of interest, or the breeder's ingenuity in enhancing the breeding program. While roguing out and bulking is generally considered the fundamental strategies of mass selection, some breeders may prefer to select and advance a significant number of plants that are both desirable and uniform concerning the trait(s) of interest (positive mass selection). When applicable, individual pods from each plant may be collected and bulked for planting. In the case of cereal species, the heads may be harvested and bulked.
Steps
The breeder establishes the heterogeneous population in the field and identifies off-types for removal and disposal. This process helps maintain the original genetic structure as much as possible. A mechanical tool (for instance, a sieve to determine which grain size should be advanced) may be utilized, or selection may rely solely on the breeder's visual assessment. Additionally, selection can be based on targeted traits (direct selection) or indirectly by choosing a trait that correlates with the trait intended for improvement.
Year 1: If the goal is to purify an existing cultivar, seeds from selected plants may be progeny-rowed to verify the purity of the chosen plants before bulking. This would extend the mass selection cycle to a duration of 2 years instead of 1 year. The original cultivar should be planted alongside for comparison.
Year 2: Assess composite seed in a replicated trial, using the original cultivar as a control. This evaluation may take place at various locations and over multiple years. The seed is bulk harvested.
2.3. Pure-line selection
The concept of pure line selection was introduced in 1903 by Danish botanist Johannsen. While examining the seed weight of beans, he illustrated that a mixed population of self-pollinated species could be differentiated into genetically pure lines. However, these lines later proved to be unresponsive to selection within each group. Selection is considered a passive process as it removes variation without generating it. The essence of pure-line theory can be summarized as follows:
1. Genetically distinct lines can be effectively isolated from a population of mixed genetic types.
2. Any variation observed within a pure line is not inherited but is solely a result of environmental influences. Thus, as demonstrated in Johansen’s bean study, further selection within the line is ineffective.
Lines play a crucial role in various breeding initiatives. They serve as cultivars or as parental lines in hybrid production (inbred lines). Additionally, lines are utilized in the creation of genetic stock (which includes specific genes such as disease resistance or nutritional quality) as well as in the development of synthetic and multiline cultivars.
Procedure
The process of pure-line selection in breeding involves multiple cycles of selfing, following the initial selection from a mixture of homozygous lines. Natural populations of self-pollinated species are composed of mixtures of homozygous lines, with transient heterozygosity arising from mutations and outcrossing.
Steps
Year 1: The initial step involves acquiring a variable base population (for instance, introductions, segregating populations from crosses, or landraces) and planting it in the first year, followed by the selection and harvesting of desirable individuals (Figure 16.3).
Year 2: Grow progeny rows from the selected plants. Remove any variants. Harvest the selected progenies individually. These represent experimental strains.
Years 3–6: Carry out preliminary yield trials of the experimental strains, including suitable check cultivars.
Years 7–10: Conduct advanced yield trials across various locations.
Release the highest-yielding line as a new cultivar.
Genetic issues
Pure-line breeding results in cultivars with a limited genetic base, making them less likely to maintain stable yields across diverse environments. These cultivars are also more susceptible to being decimated by pathogenic outbreaks.
Since outcrossing occurs to some degree in most self-pollinated cultivars, along with the potential for spontaneous mutations, variants may emerge in commercial cultivars over time.
It can be tempting to select from established cultivars to create new lines, a practice that some consider to be unprofessional and unacceptable.
As previously mentioned, pure-line cultivars rely mainly on phenotypic plasticity for their production response and stability in various environments.
2.4. Pedigree Selection
Pedigree selection is a commonly employed technique for breeding self-pollinated species, as well as cross-pollinated species like corn and other hybrid crops. A significant distinction between pedigree selection and methods such as mass selection or pure-line selection is the use of hybridization to create variability in the base population, which is not a characteristic of the other methods. This technique was initially introduced by H. H. Lowe in 1927.
Procedure
The essential steps involved in the pedigree selection process are:
1. Create a base population by crossing selected parent plants.
2. Space the progenies of the selected plants appropriately.
3. Maintain precise records of selections across generations.
Steps
Year 1: Identify desirable homozygous parents and perform approximately 20–200 crosses (see Figure 16.4).
Year 2: Cultivate 50–100 F1 plants, including the parents, to verify their hybridity.
Year 3: Grow around 2,000–5,000 F2 plants, ensuring they are spaced to facilitate individual examination and documentation. Include check cultivars for comparison. Select and harvest desirable plants separately while keeping accurate records of their identities. In certain situations, it may be beneficial not to space plant F2s to promote competition among the plants.
Year 4: Seeds from superior plants are progeny-rowed in the F3–F5 generations, ensuring that the rows are spaced for easy record-keeping. Selection at this stage occurs both within and between rows, starting with the identification of superior rows and selecting 3–5 plants from each progeny for the next generation.
Year 5: By the conclusion of the F4 generation, there should be between 25–50 rows with documented records of the plants and rows. Grow progeny from each selected F3.
Year 6: Family rows are planted in the F6 generation to produce experimental lines for preliminary yield trials in the F7 generation. The benchmark or check variety is a locally adapted cultivar, and multiple checks may be included in the trial.
Year 7: Advanced yield trials are carried out across various locations, regions, and years during the F8–F10 generations, with only the best experimental materials progressing to the next generation. The ultimate aim is to pinpoint one or two lines that outperform the check cultivars for the purpose of releasing a new cultivar. Therefore, evaluations in the advanced stages of the trial must encompass a superior expression of traits considered agronomically significant for the successful cultivation of the specific crop (for instance, lodging resistance, shattering resistance, and disease resistance). If a superior line is identified for release, it undergoes the standard cultivar release procedure (which includes seed increase and certification).
2.5. Bulk Population Breeding
Bulk population breeding is a crop improvement strategy that leverages natural selection more directly in the early generations by postponing strict artificial selection to later stages. This method was developed by H. Nilsson-Ehle. In the 1940s, H. V. Harlan and his team further established a theoretical basis for this approach through their research in barley breeding. According to Harlan and his colleagues, the bulk method involves yield testing of the F2 bulk progenies from various crosses and discarding entire crosses based on their yield performance. Essentially, the main goal is to stratify crosses to select parent plants based on yield values. The current use of the bulk method has evolved to serve a different purpose.
Procedure
Steps Year 1: Identify desirable parents (cultivars, single crosses, etc.) and create a sufficient number of crosses among them (see Figure 16.5).
Year 2: After crossing suitable parents, approximately 50–100 F1 plants are planted and harvested as a bulk, with self-pollinated plants being removed.
Year 3: The seeds harvested in the second year are used to establish a bulk plot of around 2,000–3,000 F2 plants, which are then bulk harvested.
Years 4–6: A sample of the F2 seeds is planted in bulk plots, repeating the procedures from years 2 and 3 until reaching the F4 generation or achieving the desired level of homozygosity in the population. Space about 3,000–5,000 F5 plants and select approximately 10% (300–500) of the best plants for planting F6 progeny rows.
Year 7: Select and harvest about 10% (30–50) of the progeny rows that show the desired traits for preliminary yield trials in the F7 generation.
Year 8 and beyond: Conduct advanced yield trials from F8 through F10 across multiple locations and regions, including checks with adapted cultivars. Once a superior line is identified, it undergoes the standard cultivar release process.
2.6. Single-seed descent
The single-seed descent method emerged from the necessity to accelerate the breeding program by quickly inbreeding a population before initiating the selection and evaluation of individual plants, while minimizing the loss of genotypes during the segregating generations. This concept was initially introduced by C. H. Goulden in 1941, who achieved the F6 generation in just 2 years by limiting the number of generations grown from a plant to one or two, while performing multiple plantings each year, utilizing both greenhouse and off-season planting techniques. In 1962, H. W. Johnson and R. L. Bernard outlined the process of harvesting a single seed from each soybean plant. However, it was C. A. Brim in 1966 who formally described the single-seed descent procedure, referring to it as a modified pedigree method.
Procedure
Steps Year 1: Crossing is employed to establish the base population. Selected parents are crossed to produce a sufficient number of F1 plants for generating a large F2 population.
Year 2: Approximately 50–100 F1 plants are cultivated in a greenhouse, either in the ground, on benches, or in pots. They may also be grown in the field. Identical F1 crosses are harvested and bulked.
Year 3: Around 2,000–3,000 F2 plants are cultivated. Upon maturity, a single seed from each plant is harvested and bulked for planting F3. The F2 plants are then spaced adequately to ensure that each plant produces only a few seeds.
Years 4–6: Single pods from each plant are harvested for planting the F4. The F5 is planted in the field with spacing, collecting seeds only from superior plants to cultivate progeny rows in the F6 generation.
Year 7: Superior rows are harvested to conduct preliminary yield trials in the F7. Year 8 and beyond, Yield trials are performed in the F8–F10 generations. The most outstanding line is increased in the F11 and F12 as a new cultivar.
2.7. Backcross breeding
The concept of this method in plants was initially introduced by H. V. Harlan and M. N. Pope in 1922. Essentially, backcross breeding does not enhance the genotype of the resulting product, apart from the introduced gene(s).
Procedure
Dominant gene transfer
Year 1: Choose the donor (RR) and recurrent parent (rr) and perform 10–20 crosses. Collect the F1 seeds (Figure 16.6).
Year 2: Cultivate F1 plants and perform a backcross with the recurrent parent to produce the first backcross (BC1).
Years 3–7: Cultivate the relevant backcrosses (BC1–BC5) and backcross to the recurrent parent as the female. Each time, select approximately 30–50 heterozygous parents (backcrosses) that closely resemble the recurrent parent for the next backcross. Discard the recessive genotypes after each backcross. The breeder should employ suitable screening methods to identify the heterozygotes (and eliminate the homozygous recessives). For breeding for disease resistance, artificial epiphytotic conditions are established. After six backcrosses, the BC5 should closely mimic the recurrent parent and exhibit the donor trait. As generations progress, most plants will increasingly resemble the adapted cultivar.
Year 8: Cultivate BC5F1 plants for selfing. Choose several hundred (300–400) desirable plants and harvest them individually.
Year 9: Cultivate BC5F2 progeny rows. Identify and select around 100 desirable non-segregating progenies and bulk them.
Year 10: Perform yield tests of the backcross with the recurrent cultivar to assess equivalence before release.
Recessive Gene Transfer
Years 1–2: The process mirrors that of dominant gene transfer, with the donor parent possessing the recessive desirable gene.
Year 3: Cultivate BC1F1 plants, self-pollinate, harvest, and bulk the BC1F2 seed. In disease-resistance breeding, all BC1s will show susceptibility.
Year 4: Grow BC1F2 plants and evaluate them for desirable traits. Backcross 10–20 plants to the recurrent parent to produce BC2F2 seed.
Year 5: Cultivate BC2 plants. Choose 10–20 plants that closely resemble the recurrent parent and cross them with the recurrent parent.
Year 6: Grow BC3 plants, then harvest and bulk the BC3F2 seed.
Year 7: Cultivate BC3F2 plants, screen them, and select the desirable ones. Backcross 10–20 plants with the recurrent parent.
Year 8: Grow BC4 plants, then harvest and bulk the BC4F2 seed.
Year 9: Cultivate BC4F2 plants, screen them, and select the desirable ones. Backcross 10–20 plants with the recurrent parent.
Year 10: Grow BC5 plants, then harvest and bulk the BC5F2 seed.
Year 11: Cultivate BC5F2 plants, screen them, and perform backcrossing.
Year 12: Grow BC6 plants, then harvest and bulk the BC6F2 seed.
Year 13: Cultivate BC6F2 plants, screen them, and select 400–500 plants to harvest separately for progeny rows.
Year 14: Grow progenies from the selected plants, screen them, and select approximately 100–200 uniform progenies; harvest and bulk the seed.
Years 15–16: Continue following the same procedure as in breeding for a dominant gene.
The primary distinction between the transfer of dominant and recessive alleles is that phenotypic identification is not feasible after a cross in the latter case. Each cross must be followed by selfing to identify progeny with the homozygous recessive genotype, which can then be backcrossed to the recurrent parent.
3. Conclusion
Thus, various methods of breeding self-pollinated crops were studied.
4. Reference
Ramalho, M. A. P., & Araújo, L. C. A. D. (2011). Breeding self-pollinated plants. Crop Breeding and Applied Biotechnology, 11(SPE), 1-7.
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