Plant Breeding Techniques Overview

Rapid Generation Advance

• Used to advance generations of plants for selection, especially for traits controlled by multiple genes.
• Multiple planting methods can be combined.

Single Seed Descent

• Number of seeds planted per season is not fixed.
• Multiple seed descent involves planting a constant number of seeds (e.g., 3 seeds per plant) each season.

Stabilization Phase

• Achieved through continuous selfing, leading to increased homozygosity in the population.
• Determining the number of F2 plants to grow depends on the number of genes controlling the trait.
• Single locus trait:
  • F1 generation: Aa (heterozygous)
  • F2 generation: 1/4 AA : 1/2 Aa : 1/4 aa (genotypic ratio)
• Calculating desirable genotype frequency in F2:
  • (3/4 + 1/4)^2 = (3/4)^2 + 2[(3/4) * (1/4)] + (1/4)^2 = 9/16 + 6/16 + 1/16 = 16/16 = 1 (whole population)
  • Desirable (b) = 7/16
• For traits controlled by multiple genes:
  • Number of possible genotypes in F2 = 3^n (where n = number of gene pairs)
  • Number of possible phenotypes in F2 = 2^n (where n = number of gene pairs)
• Number of F2 plants needed depends on the genetic diversity of the population, typically around 2000 plants for distantly related parents.
• It can take 10-15 seasons to release a new variety, with purification requiring additional seasons.

Double Haploid Techniques

• Offer a faster approach to achieving homozygosity.
• Involves two steps:
  • Producing haploid gametes or plants from a heterozygous source.
  • Doubling the chromosome complement of haploid plants, typically using colchicine.

Methods for Obtaining Haploids

• Genetic Markers:
  • Example: Corn with purple plant color and purple aleurone.
  • Female parent: recessive purple marker.
  • Male parent: dominant purple marker.
  • Harvest purple seeds, grow, and select for green plants.
  • Green plants are selfed and haploid with purple endosperm.
• Naturally Occurring Paternal Haploid:
  • Search for green F1 plants with purple endosperm.
  • Frequency is very low.
• Indeterminate Gametophytes:
  • Stimulates male gamete development.
  • Leads to a higher frequency of green F1 plants with purple endosperm.
• Interspecific Crosses:
  • Example: Barley
  • Cross between Hordeum vulgare and Hordeum bulbosum.
  • Chromosomes of H. bulbosum are eliminated, leaving only chromosomes of H. vulgare.

Anther Culture

• Involves two processes:
  • Development of embryoids or calli from pollen cells.
  • Differentiation of embryoids into plants (regeneration).

Factors Affecting Anther Culture

• Source Plants:
  • Genotype (e.g., F1 = AaBbCcDdFf).
  • Disadvantages include limited genetic recombination and linkage breaking.
• Stages of Anther Development:
  • Tetrad stage to intermediate stage after the first pollen anthesis is most critical.
• Culture Media:
  • Proper culture medium is crucial for successful development.
  • Haploids are often short, uniform, and sterile.

Multiple Crosses for Base Population

• Large populations are needed to select desirable individuals.
• Selecting closely related plants reduces recombination.
• Aims for the formation of a broad-based population.

Improvement of Existing Pure Lines

• Backcross method: Changing one or two traits without altering others.
  • Requires two parents: adapted/established variety and a donor parent with desired traits.
• Multilines:
  • A mixture of genetically different lines with the same desirable traits.
  • Consist of isoline lines (genetically identical except for the targeted trait)
• Blends:
  • Lines with different traits.

Commercially Accepted Multilines

• Phenotypically the same lines but genetically different.
• Example: All green seed coat varieties of mungbean.
• Provide a stable alternative to crop rotation.

Multilines Yield

• Multilines often exceed the average yield of their component lines.
  • Observations:
    • 3.2% increase if components are unselected.
    • 7.45% increase if components are selected.

Advantages of Multilines

• Minimize genotype and environmental interaction effects.
• More stable.
• Resistant to diseases.
• Improved yield performance.

Forming Multilines

• Development of isolines (genetically similar except for the targeted trait).
• Using closely related lines (lines with a common parent).
• Using distinctly different lines (each line complements the others).

Predicting Multiline Performance

• Performance of multiline = ∑ [Performance of component i in pure stand] * [Frequency of component i in mixture]
• Example:
  • Var A: 100 units in pure stand, 25% proportion in mixture.
  • Var B: 110 units in pure stand, 75% proportion in mixture.
  • Performance of multiline = (100 * 0.25) + (110 * 0.75) = 107.5 units.

Factors Affecting Multiline Production

• Number of components and their relative frequency in the mixture.
• Performance of the component lines in pure stand.

Intergenotypic Competition

• Deviation in multiline performance can occur due to competition among genotypes.
• Types:
  • Undercompensation: Actual performance is lower than expected.
  • Complementary compensation: No competition, actual performance equals expected performance.
  • Overcompensation: Actual performance is higher than expected.

Advantages of Multilines

• Increased pest resistance.
• Greater standability and longer lifespan.

Disadvantages of Multilines

• Long time required to develop.
• Reconstruction of the mixture is needed to prevent varietal shift.
• Seed certification challenges.

Production of F1 Hybrids

• No need to produce inbred lines in advance.
• Test for combining ability (potential for different lines to produce high-yielding hybrids).
• Requires emasculation (removal of male reproductive parts) or sterile lines.

Composite Crosses

• Produce and mix different lines, allowing for natural selection.
• Genetically more diverse.
• Subject to natural selection.

Systems of Pollination Control in Plants

• Mechanisms:
  • Dioecy: Male and female flowers are on separate plants (e.g., asparagus, rambutan, papaya).
  • Self-compatibility: Gametes are functional and can self-pollinate.
  • Heteromorphic: Difference in flower morphology, with two types:
    • Thrum: Short style, high anthers.
    • Pin: Long style, short anthers. o Thrum x Pin = Compatible. o Pin x Pin = Incompatible. o Thrum x Thrum = Incompatible.
  • Homomorphic: No difference in flower morphology.
    • Gametophytic System: Incompatibility depends on the genotype of the pollen (gametophyte).
      • Known as haplo-diplo system.
      • Controlled by a single locus with multiple alleles.
      • Pollen tube growth is slow if the style has the same S allele as the pollen.
      • Alleles act individually (no interactions or dominance).
    • Sporophytic System: Incompatibility is imparted to pollen by the plant (sporophyte) upon which the pollen is borne.
      • Known as diplo-diplo system.
      • Alleles may show individual action, dominance, or competitive effects.
  • Male Sterility: Gametes are nonfunctional.
    • Genetically Controlled: Due to a single gene or chromosomal aberration (e.g., sorghum, cotton, lima bean).
      • MsMs and Msms = Fertile; msms = Sterile.
    • Cytoplasmically Controlled: Sterility is determined by the cytoplasm (e.g., corn).
    • Cyto-genetically Controlled: Controlled by both cytoplasm and genes (e.g., sorghum, onion, rice).
    • Chemically Controlled: Pollen viability is affected using chemical hybridizing agents (e.g., maleic hydrazide, ethrel, FW 450).
  • Apomixis: Seed formation without fertilization.

Hybrid Varieties

• Exploit hybrid vigor to increase yield potential, especially in modern rice varieties.
• F1 seed is obtained from a cross between two inbred parents.
• Farmers must obtain new hybrid seed for each planting from a certified source.
• Ex:
  • China: Zhen Shan 97 A/Min-Hui 63 and V20 A/Ce 64.
  • India: DRRH-1 and PA6201.
  • Philippines: PSBRc26H–Magat and PSBRc72H-Mestizo.

Explore essential plant breeding techniques such as Rapid Generation Advance and Single Seed Descent. Understand the significance of the Stabilization Phase and learn how traits influenced by multiple genes can be analyzed. This quiz will help deepen your knowledge of genetic selection in agriculture.