Synthetic Variety PPT

Introduction

A synthetic variety is a population of genetically diverse plants developed by intercrossing two or more selected genotypes and then allowing the mixed population to undergo open-pollination for subsequent generations. Synthetic varieties aim to retain some amount of heterosis (hybrid vigor) while enabling farmers to multiply seed by natural open-pollination without purchasing hybrid seed each season.

Synthetic varieties are most useful in cross-pollinated crops such as maize, sorghum, pearl millet and certain forage species. They combine adaptability, stability and moderate yield advantage over pure lines, while avoiding the recurring seed cost associated with hybrids.

Definition

Definition: A synthetic variety is a population produced by intercrossing selected genotypes (usually inbred or elite lines) followed by mass multiplication through open-pollination, resulting in a genetically heterogeneous but phenotypically stable population that retains partial heterosis.

Key features:

• Genetically heterogeneous but phenotypically fairly uniform for important traits.
• Seed is produced by open-pollination and can be saved by farmers.
• Designed to combine adaptability, stability and economic yield.

Historical Background

The synthetic variety concept arose as plant breeders searched for alternatives to two extremes: (a) pure line varieties (high uniformity but limited heterosis) and (b) F₁ hybrids (high heterosis but need for repeated hybrid seed production). By mid-20th century, breeders experimented with intercrossing multiple selected lines and letting natural pollination maintain the population; this approach spread in crops adapted to cross-pollination.

Objectives of Developing Synthetic Varieties

1. To retain partial heterosis across multiple generations without the recurring cost of hybrid seed.
2. To combine complementary desirable traits — yield, disease resistance, drought tolerance and quality attributes.
3. To create a genetically diverse population that is resilient and adaptable to variable agro-climatic conditions.
4. To empower farmers to produce and save seed locally, lowering dependency on commercial seed systems.
5. To provide a population that can be improved further by mass selection or other population-based methods.

Detailed Procedure for Developing Synthetic Varieties

1. Selection of Parental Lines

Parental selection is critical. Breeders choose 4–10 (often 5–8) genetically diverse, well-adapted and complementary lines. Parents should possess target traits such as high yield, disease and pest resistance, stress tolerance and acceptable quality traits. Diversity among parents provides the genetic base that helps sustain heterosis.

2. Hybridization and Crossing Design

Parents are intercrossed using designs such as diallel, partial diallel, topcross or factorial crosses. The aim is to combine lines that show good combining ability and to identify crosses with strong heterosis or desirable trait combinations.

3. Evaluation of F₁ Hybrids

Evaluate F₁s for vigor, yield, stability, and trait expression. Select the best crosses — those showing high yield, uniformity and other desired agronomic attributes — as candidate parents for the synthetic population.

4. Intercrossing of Selected Parents

Conduct intercrossing among the chosen parents to create a composite population. This may be achieved by pairwise crossing or by bulk intercrossing in isolation. Ensure that each parent contributes approximately equally to avoid early loss of diversity.

5. Mass Multiplication (Open-pollination)

Grow the intercrossed population in an isolated field to permit unrestricted natural cross-pollination (wind, insects). Harvest seed from the population and multiply it by mass planting under isolation to produce seed lots of the synthetic variety.

6. Evaluation and Improvement

Evaluate the resulting synthetic population across locations and seasons for yield, uniformity, and stability. Over generations, breeders may apply mass selection to improve specific traits while attempting to conserve the overall genetic base.

Important: Maintaining isolation during the intercrossing and seed increase phases is crucial to avoid pollen contamination from other cultivars or wild relatives. Also, equal seed contribution from parents at the start helps preserve intended diversity.

Genetic and Agronomic Characteristics

Synthetic varieties are genetically heterogeneous populations where multiple alleles and gene combinations exist. Key agronomic and genetic characteristics include:

• Partial heterosis: Unlike hybrids that show maximum heterosis in F₁, synthetics retain moderate heterosis over generations because of maintained heterozygosity.
• Phenotypic stability: Though genetically variable, synthetics are developed to be phenotypically stable for important traits like plant height, maturity and yield.
• Adaptability: Good across a wide range of environments due to genetic diversity.
• Seed-saving: Farmers can save seed and replant for several generations without dramatic loss of performance (though some decline is possible if mismanaged).

Advantages

1. Cost-effectiveness: Seed can be multiplied by farmers; no annual purchase of hybrid seed.
2. Adaptability: Performs well across diverse environments; more resilient to changing conditions.
3. Genetic diversity: Lowers risk from pests, diseases and abiotic stresses.
4. Farmer empowerment: Enables local seed systems and greater farmer autonomy.
5. Sustainability: Suitable for low-input and marginal farming systems.

Limitations and Risks

• Yield ceiling: Typically lower than the best F₁ hybrids because synthetics cannot sustain maximal heterosis exhibited by hybrids.
• Need for isolation: Strict isolation is required during seed increase to prevent contamination.
• Genetic drift: Over successive farmer-saved generations, unplanned selection and drift may reduce intended diversity or lead to loss of favourable alleles.
• Management requirements: Careful monitoring and periodic rejuvenation (re-introducing parental lines) may be necessary to maintain performance.

Practical Examples and Case Studies

Synthetic varieties have been developed and released for many cross-pollinated crops. A few representative examples (used as teaching examples) are summarised below.

Crop	Synthetic Variety / Composite	Notes
Maize	HCM 1, HQPM 1 (examples)	Developed combining drought tolerance and better grain quality; farmers can save seed under isolation.
Sorghum	CSV 10, CSV 17 (examples)	Selected for adaptability and resistance to local pests.
Pearl millet	HHB 67, HHB 146 (examples)	Drought-tolerant synthetics used in semi-arid regions.
Forage crops (e.g., alfalfa)	Alfalfa synthetic populations	Maintain diversity for persistence, nutritive value and pest resistance.

Design Considerations and Breeding Strategies

When designing a synthetic variety, breeders must carefully consider the following factors:

• Number of parents: More parents increase genetic base but complicate crossing; 4–10 parents are common.
• Combining ability: Assess general and specific combining ability to choose parents that combine well for target traits.
• Equal parental contribution: Ensure roughly equal seed contribution from each parent to retain intended allele frequencies.
• Rejuvenation: Periodically reintroduce selected parents or superior lines to maintain vigor and correct drift.
• Mass selection vs. population improvement: Mass selection can improve mean performance but may reduce diversity if applied too intensely.

Seed Production and Quality Control

Proper seed production is essential to maintain the integrity of a synthetic variety. Key steps include:

1. Maintain isolation distance from other cultivars or use temporal isolation (different flowering time) to avoid pollen contamination.
2. Ensure good agronomic management to avoid unintentional selection for undesirable traits.
3. Monitor seed lots for uniformity, germination percentage and purity (both genetic and physical).
4. Record-keeping and traceability help prevent admixture and ensure seed quality across multiplication generations.

Diagram Prompt (for teaching or figure creation)

Diagram prompt:
Draw a labelled flow diagram showing development of a synthetic variety in maize:
1) Select 5 parental inbred/elite lines (label them P1, P2, P3, P4, P5).
2) Intercross parents (show arrows between parents and pairwise crossings or bulk intercross).
3) Produce F1 composite/bulk population.
4) Grow the composite in isolation and allow open-pollination (show wind/insect symbols).
5) Harvest mass-multiplied seed and evaluate across locations.
6) Optional: show periodic rejuvenation by reintroducing original parents.
Labels: Parents, Intercrossing, F1 composite, Open-pollinated seeds, Synthetic variety, Evaluation sites.

Frequently Asked Questions (Short Answers)

Q: How many parents should a synthetic variety contain?

A: Typically 4–10 parents. Fewer parents simplify management but reduce genetic base; more parents increase diversity but complicate crossing and equal contribution.

Q: Are synthetic varieties the same as composites?

A: The terms are often used interchangeably. Technically, a composite may be formed by mixing several varieties or lines directly, while a synthetic is usually formed by deliberate intercrossing of selected parents to ensure recombination among them before open-pollination.

Q: How long can farmers save seed of a synthetic variety?

A: Farmers can save seed for several generations with moderate loss in vigour; however, periodic seed renewal or rejuvenation by breeders is recommended to maintain performance and prevent accumulation of undesirable alleles.

Conclusion

Synthetic varieties provide a pragmatic middle ground between pure lines and hybrids for cross-pollinated crops. They deliver adaptability, resilience and the opportunity for farmers to produce and save seed locally. With careful parent choice, controlled intercrossing, strict isolation in seed production and managed selection, synthetic varieties remain valuable tools in crop improvement and seed systems — particularly in resource-limited contexts.