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.