Breeding Methods for Cross-pollinated species

In this article we will discuss about the different methods for Breeding Cross-pollinated species

What are Cross-pollinated species?

Cross-pollinated species are plants that require pollen from a different individual of the same species to achieve fertilization and produce seeds. This process contrasts with self-pollinated species, where a single plant can fertilize itself. Cross-pollination is essential for maintaining genetic diversity and improving the adaptability of plant populations. Here’s a detailed overview:

Characteristics of Cross-Pollinated Species

1. Pollination Mechanism:
   • Dependence on External Agents: Cross-pollinated plants rely on external agents such as insects (e.g., bees, butterflies), wind, water, or animals to transfer pollen from one flower to another. This external transfer is crucial for successful fertilization.
   • Flower Structure: These plants often have specialized floral structures to attract pollinators and facilitate the movement of pollen between flowers.
2. Genetic Diversity:
   • Increased Variability: Cross-pollination promotes genetic diversity within a population, as the offspring inherit a combination of genes from different parent plants. This variability can enhance the adaptability and resilience of the species.
   • Reduced Inbreeding: By introducing genetic material from multiple individuals, cross-pollination reduces the likelihood of inbreeding and the associated risks of genetic disorders.
3. Breeding and Selection:
   • Breeding Programs: In breeding programs for cross-pollinated species, breeders often select and cross different individuals to combine desirable traits and improve overall crop performance. This process can involve complex strategies like recurrent selection or hybrid breeding.
   • Hybrid Vigor: Cross-pollinated plants can exhibit hybrid vigor (heterosis), where hybrids show superior growth, yield, or other desirable traits compared to their parent plants.
4. Examples:
   • Crops: Many major crops are cross-pollinated, including maize (corn), wheat, and some types of fruits and vegetables like apples and squash.
   • Wild Plants: Numerous wild species, including many flowering plants, rely on cross-pollination for reproduction.

Advantages and Challenges

Advantages:

• Genetic Improvement: Cross-pollination allows for the introduction of new genetic material, which can lead to the development of new varieties with enhanced traits.
• Resilience: Greater genetic diversity can improve the species’ ability to adapt to changing environmental conditions and resist diseases.

Challenges:

• Pollination Dependence: Cross-pollinated species depend on effective pollination by external agents, which can be affected by environmental factors, such as weather conditions or the availability of pollinators.
• Complexity in Breeding: Breeding cross-pollinated species can be more complex and resource-intensive compared to self-pollinated species, due to the need to manage multiple genetic combinations and maintain genetic diversity.

Breeding Methods for cross-pollinated species

1. Mass Selection: This method involves selecting a group of plants based on their phenotypic traits to improve the overall population. It is applicable to both self-pollinated and cross-pollinated species.
2. Recurrent Selection: This method aims to improve two genetically different populations for general combining ability (GCA) and specific combining ability (SCA). It involves cycles of selection and crossing to enhance desirable traits in the populations .
3. Half-Sib and Full-Sib Selection:
   • Half-Sib Selection: Involves making test crosses with half-sib families to evaluate and select superior progenies. This method allows for the assessment of the genetic contribution of each parent.
   • Full-Sib Selection: Similar to half-sib selection but evaluates full-sib families, reflecting the worth of both parental plants.
4. Synthetic Cultivar Development: This method involves creating a new cultivar by interbreeding selected plants from different populations to maintain genetic diversity and improve traits 1.
5. Backcross Breeding: This technique is used to introduce specific traits from one parent into a recurrent parent while minimizing inbreeding depression. It involves crossing a hybrid back to one of its parents.
6. Polycross Nursery: Selected clonal lines are planted in a polycross nursery to generate seed for progeny testing. This method ensures random interpollination among clones, allowing for the evaluation of general combining ability.

1. Mass Selection Method for Breeding Cross-pollinated species

Mass Selection is a breeding method used primarily for improving populations of cross-pollinated species. It focuses on selecting individual plants based on their phenotypic performance to enhance the overall genetic quality of the population.

Procedure

1. Year 1:
   • Plant the source population (e.g., local variety, synthetic variety).
   • Rogue out undesirable plants before flowering.
   • Select several hundred plants based on desirable phenotypic traits.
   • Harvest and bulk the seeds from the selected plants.
2. Year 2:
   • Grow the selected bulk in a preliminary yield trial, including a check (the unselected original population).
   • Rogue out undesirable plants again and repeat the selection process.
3. Year 3:
   • Continue the process of selection and evaluation, growing the selected bulk in trials to assess improvements.
4. Year 4:
   • Conduct advanced yield trials to evaluate the performance of the selected population compared to the original.
5. Repeat:
   • Continue the process for as long as progress is being made, adjusting selection criteria as necessary.

Advantages

• Simplicity: The method is straightforward and easy to implement, making it accessible for many breeders.
• Cost-Effective: It does not require complex facilities or extensive resources.
• Improvement of Population Performance: Aims to enhance the overall performance of the population by selecting superior genotypes that already exist within it.

Disadvantages

• Phenotypic Selection Limitations: Selection is based solely on observable traits, which may not accurately reflect the underlying genetic potential.
• Lack of Pollen Control: Both desirable and undesirable pollen can contribute to the next generation, potentially diluting the effects of selection.
• Inbreeding Depression Risk: High selection intensity can lead to inbreeding depression, especially if the population size is small, increasing the risk of losing individuals with desirable traits.

Applications

• Population Improvement: Mass selection is commonly used to enhance the general performance of populations in crops like maize, wheat, and other cross-pollinated species.
• Development of Varieties: It can be employed to develop new varieties that are better adapted to specific environmental conditions or have improved yield and quality traits.
• Initial Selection Phase: Often used as an initial selection phase before more advanced breeding methods, such as recurrent selection or hybridization, are applied 

2. Recurrent Selection Method for Breeding Cross-pollinated species

Recurrent Selection is a systematic breeding method used to improve populations of cross-pollinated species. It focuses on selecting individuals with desirable traits and mating them to form a new population, which is then subjected to further selection cycles. This method is particularly effective for enhancing quantitative traits and increasing the frequency of favorable alleles in a population.

Procedure

1. Cycle Initiation (C₀):
   • Start with a base population (C₀) consisting of genetically diverse individuals.
   • Cross parents in all possible combinations to create individual families.
2. Evaluation:
   • Evaluate the performance of the individual families or plants based on specific traits of interest (e.g., yield, disease resistance).
   • Select the best-performing families or individuals to serve as parents for the next cycle.
3. Intermating:
   • Intermate the selected parents to produce a new population (C₁) for the next cycle of selection.
   • This process can be repeated for several cycles (C₂, C₃, etc.), with each cycle aimed at further improving the population.
4. Repeat:
   • Continue the selection and intermating process for 3 to 5 cycles, depending on the breeding goals and the genetic variability of the population.

Advantages

• Increased Genetic Gain: Recurrent selection can significantly enhance the frequency of desirable alleles in a population, leading to improved performance over generations.
• Maintenance of Genetic Variability: The method is designed to maintain genetic diversity within the population, allowing for future improvements and adaptability.
• Flexibility: It can be adapted to various breeding objectives, including improving general combining ability (GCA) and specific combining ability (SCA).

Disadvantages

• Time-Consuming: The process requires multiple cycles of selection and intermating, which can extend the duration of the breeding program.
• Resource Intensive: It may require significant resources in terms of land, labor, and management to evaluate and select individuals over several cycles.
• Complexity: The method can be complex to implement, especially in managing the mating designs and evaluating the performance of families .

Applications

• Improvement of Quantitative Traits: Recurrent selection is particularly effective for traits controlled by multiple genes, such as yield, height, and disease resistance.
• Development of Synthetic Cultivars: It is commonly used to develop synthetic cultivars that combine the strengths of multiple parent lines while maintaining genetic diversity.
• Enhancing Hybrid Performance: The method can be used to improve the performance of hybrid varieties by selecting parents with superior GCA and SCA.

3. Half-Sib and Full-Sib Selection Method for Breeding Cross-pollinated species

Half-Sib and Full-Sib Selection are breeding methods used in the improvement of cross-pollinated species. These methods focus on evaluating and selecting individuals based on the performance of their progeny, allowing breeders to make informed decisions about which plants to use for further breeding.

Half-Sib Selection

Procedure:

1. Parent Selection: Select a group of plants to serve as parents. Each selected plant is mated with a common pollen source, resulting in half-sib families (progeny sharing one parent).
2. Family Evaluation: Grow the half-sib families in replicated trials to evaluate their performance based on traits of interest (e.g., yield, disease resistance).
3. Selection: Select the best-performing half-sib families based on their average performance. The selected families can then be used for further breeding or as parents in subsequent generations.

Advantages:

• Genetic Evaluation: Allows for the evaluation of genetic potential based on progeny performance rather than just phenotypic traits of individual plants.
• Improved Traits: Effective for improving traits with high heritability, as the selection is based on the performance of the progeny.
• Flexibility: Can be applied to a wide range of species and traits, making it a versatile breeding method.

Disadvantages:

• Common Parent Limitation: The identity of the common pollen source is not known, which can complicate the interpretation of results.
• Resource Intensive: Requires significant resources for planting, managing, and evaluating multiple families over time.

Applications:

• Forage Crops: Widely used in breeding perennial forage grasses and legumes, where half-sib families are evaluated for agronomic traits.
• Corn Breeding: Employed in corn breeding programs to improve agronomic traits and seed composition.

Full-Sib Selection

Procedure:

1. Parent Selection: Select two plants to serve as parents. The offspring produced from these parents are full-sibs, sharing both parents.
2. Family Evaluation: Grow the full-sib families in replicated trials to assess their performance based on desired traits.
3. Selection: Select the best-performing full-sib families for further breeding or as parents for the next generation.

Advantages:

• Controlled Genetic Background: Since full-sibs share both parents, the genetic background is more controlled, allowing for clearer evaluation of traits.
• Higher Genetic Gain: Can lead to greater genetic gain compared to half-sib selection, as both parents contribute to the traits being evaluated.

Disadvantages:

• Inbreeding Risk: The method can increase the risk of inbreeding depression if not managed properly, especially in small populations.
• Limited Genetic Diversity: Full-sib families may have less genetic diversity compared to half-sib families, which can limit adaptability.

Applications:

• Hybrid Development: Commonly used in the development of hybrid varieties, where the performance of full-sib families is critical for assessing hybrid potential.
• Specific Trait Improvement: Effective for improving specific traits in crops where both parents contribute significantly to the desired characteristics.

4. Synthetic Cultivar Development Method for Breeding Cross-pollinated species

Synthetic Cultivar Development is a breeding method used to create open-pollinated populations of cross-pollinated species. This method involves the selection of multiple parent lines that are then intermated to produce a synthetic cultivar. The goal is to combine desirable traits from various parents while maintaining genetic diversity within the population.

Procedure

1. Parent Selection:
   • Identify and select a group of genetically diverse parent lines based on their General Combining Ability (GCA) and desirable traits (e.g., yield, disease resistance).
   • The selected parents can be strains, clones, or hybrids.
2. Intermating:
   • Cross the selected parents in all possible combinations to create a seed mixture. This process involves random mating among the selected parents to ensure genetic diversity.
3. Seed Production:
   • Allow the resulting seeds to be produced through open pollination. The seeds collected from this process will form the synthetic cultivar.
4. Evaluation:
   • Evaluate the performance of the synthetic cultivar in replicated trials to assess its agronomic traits and adaptability to different environments.
5. Reconstitution:
   • Since synthetic cultivars can only be propagated for a limited number of generations, they must be periodically reconstituted from the original parent stock to maintain genetic diversity and performance.

Advantages

• Genetic Heterogeneity: Synthetic cultivars are genetically heterogeneous, which allows them to perform stably across varying environmental conditions. This heterogeneity also enables both natural and artificial selection to modify the genotypic structure over time.
• Adaptation: Over successive generations, synthetic cultivars can become better adapted to local production environments, enhancing their performance and resilience.
• Yield Stability: Yield reduction in advanced generations is generally less pronounced in synthetic cultivars compared to single or double crosses, making them more reliable for farmers.

Disadvantages

• Limited Propagation: Synthetic cultivars can only be propagated for a limited number of generations before they need to be reconstituted from the original parent stock, which can be resource-intensive.
• Complexity in Management: Managing the genetic diversity and ensuring the quality of the parent lines can be complex and requires careful planning and execution.
• Potential for Inbreeding: If not managed properly, there is a risk of inbreeding depression in synthetic cultivars, especially if the population size is small .

Applications

• Forage Species: Widely used in breeding forage crops, where synthetic cultivars can provide a stable and adaptable source of feed for livestock 13.
• Cereal Crops: Successful synthetic cultivars have been developed for crops like corn and sugar beets, where the method helps improve yield and adaptability.
• Gene Pools: Synthetic cultivars can serve as gene pools for breeding progeny, allowing breeders to select for desirable traits in future generations.

5. Backcross Breeding for Breeding Cross-pollinated species

Backcross Breeding is a method used in plant breeding to introduce specific traits from one plant (the donor parent) into the genetic background of another plant (the recurrent parent). This technique is particularly useful for improving traits such as disease resistance, pest tolerance, or other agronomic characteristics in cross-pollinated species.

Procedure

1. Selection of Parents:
   • Identify and select a recurrent parent that has desirable traits but lacks the specific trait you want to introduce (e.g., disease resistance).
   • Select a donor parent that possesses the desired trait.
2. Initial Cross:
   • Cross the recurrent parent with the donor parent to produce the first generation (F1) hybrids. These hybrids will carry the desired trait from the donor parent.
3. Backcrossing:
   • Cross the F1 hybrids back to the recurrent parent. This step is repeated for several generations (typically 2-4 backcrosses) to ensure that the offspring retain the genetic background of the recurrent parent while incorporating the desired trait from the donor parent.
4. Selection:
   • In each generation, select individuals that exhibit the desired trait while also showing the characteristics of the recurrent parent. This selection process helps to maintain the overall quality and performance of the cultivar.
5. Evaluation:
   • Evaluate the selected individuals for performance traits and stability in various environments. Once a stable line is achieved, it can be released as a new cultivar.

Advantages

• Precision: Backcross breeding allows for the precise introduction of specific traits without significantly altering the overall genetic makeup of the recurrent parent. This precision is particularly beneficial when the recurrent parent has superior agronomic traits.
• Rapid Improvement: The method can lead to rapid improvement of specific traits, as the recurrent parent’s desirable characteristics are preserved while enhancing the population with new traits 1.
• Reduced Inbreeding Depression: By using a recurrent parent that is well-adapted to the local environment, the risk of inbreeding depression is minimized, especially when large populations are maintained 17.

Disadvantages

• Inbreeding Risk: Repeated backcrossing can lead to inbreeding depression if not managed properly, particularly if the recurrent parent is not genetically diverse 1.
• Limited Genetic Variation: The method may reduce genetic variation in the population, as it focuses on a single recurrent parent, which can limit adaptability to changing environmental conditions 17.
• Time-Consuming: The process can be time-consuming, as it requires multiple generations of crossing and selection to achieve the desired traits while maintaining the recurrent parent’s characteristics 1.

Applications

• Disease Resistance: Backcross breeding is commonly used to introduce disease resistance traits into commercially important crops, such as corn, wheat, and soybeans.
• Pest Tolerance: The method is effective for enhancing pest tolerance in crops, allowing for the development of varieties that can withstand pest pressures without the need for chemical controls 1.
• Quality Improvement: It can also be used to improve quality traits, such as nutritional content or processing characteristics, in various crops.

6. Polycross Nursery for Breeding Cross-pollinated species

Polycross Nursery is a breeding method used for developing and evaluating populations of cross-pollinated species. This method allows for the random mating of multiple parent lines to create a genetically diverse population, which can be evaluated for desirable traits.

Procedure

1. Selection of Parent Lines:
   • Identify and select a diverse group of parent lines that exhibit desirable traits. These lines should have good General Combining Ability (GCA) to ensure that their offspring will perform well.
2. Establishment of the Nursery:
   • Plant the selected parent lines in a nursery designed to facilitate random cross-pollination. The layout should allow each parent to be pollinated by pollen from all other parents in the nursery.
3. Random Pollination:
   • Ensure that the nursery is designed to promote random mating among the parent lines. This can be achieved through a square plot layout or by using a Latin square design to minimize bias in pollination.
4. Seed Collection:
   • After the pollination period, collect seeds from the nursery. Each seed will represent a unique combination of genetic material from the parent lines.
5. Progeny Testing:
   • Plant the collected seeds in replicated trials to evaluate the performance of the progeny. Assess traits such as yield, disease resistance, and other agronomic characteristics.
6. Selection:
   • Based on the performance evaluation, select the best-performing progeny for further development or for use in future breeding programs.

Advantages

• Genetic Diversity: The polycross method promotes genetic diversity within the population, which can enhance adaptability and stability across different environments.
• Evaluation of GCA: It provides an efficient estimate of General Combining Ability (GCA), allowing breeders to identify superior parent lines for future breeding efforts.
• Reduced Risk of Inbreeding: By allowing random mating among multiple parents, the risk of inbreeding depression is minimized, which is particularly important in cross-pollinated species.

Disadvantages

• Complexity in Management: Managing a polycross nursery can be complex, as it requires careful planning to ensure random pollination and to avoid bias from self-fertilization or non-random cross-pollination.
• Labor-Intensive: The establishment and maintenance of a polycross nursery can be labor-intensive, requiring significant resources for planting, monitoring, and harvesting.
• Potential for Genetic Drift: Over time, if not managed properly, there is a risk of genetic drift, which can lead to the loss of desirable traits within the population .

Applications

• Forage Crops: Polycross nurseries are commonly used in the breeding of forage crops, where genetic diversity is crucial for performance under varying environmental conditions.
• Cereal Crops: This method has been successfully applied in the breeding of cereal crops such as corn and wheat, where it helps improve yield and adaptability.
• Synthetic Cultivar Development: Polycross nurseries are instrumental in developing synthetic cultivars, which are open-pollinated populations that can provide stable and high-yielding varieties for farmers.