Plant Breeding 25 Jun 2026 · 23 views · By Shrikant Sir

PB Ch 28. Synthetic and Composite Varieties

Synthetic Varieties

Hybrids are incredibly powerful, but they have one massive drawback for everyday farmers: you cannot save the seeds. If a farmer plants the seeds from a hybrid crop, the next generation will suffer from severe inbreeding depression, forcing the farmer to buy expensive new seeds every single year.
But what if a breeder could create a crop that captures some of that hybrid vigor, but is stable enough that farmers can save and replant the seeds?
This is where Synthetic and Composite Varieties come in. Instead of crossing just two highly controlled lines, breeders mix multiple lines together to create a diverse, self-sustaining "team" of plants.

Synthetic Variety: A variety produced by crossing in all combinations a number of inbred lines (or other plant types) that combine well with each other, followed by open-pollination in isolation. Once synthesized, maintained by open-pollination.
It is produced by crossing a specific number of inbred lines (usually 6 to 8) that have been strictly tested to ensure they all combine well with each other. Once the initial mix is made, the crop is maintained by letting it naturally open-pollinate in the field.
Commercial utilization first suggested by Hayes and Garber in 1919

8.2 Steps in Producing a Synthetic Variety

Step 1 — Develop inbred lines or other parental types: Select from the base population; self for multiple generations to achieve near-homozygosity.
Step 2 — Evaluate GCA of lines (polycross or top cross): Only lines with HIGH GCA are selected as parents of the synthetic. GCA determines performance in random mating — synthetic varieties exploit only GCA, not SCA.
Step 3 — Production of Syn-0 generation (two methods):
- Method A: Equal amounts of seed from selected parental lines are mixed and planted in isolation; open-pollination produces crosses in all combinations; seed harvested in bulk = Syn-1 population.
- Method B: All possible crosses made in isolation; equal seed from each cross composited = Syn-1 population.

Step 4 — Multiplication: Syn-1 is multiplied in isolation for one or more generations to produce commercial seed quantities. In some crops (sugarbeet), synthetic distributed directly as Syn-1 without further multiplication.

Sewall Wright (1922) formula for predicting Syn-2 performance

Syn2 = Syn1 - (Syn1 - Syn0) / n

Where:

Syn0 = mean performance of parental inbred lines (before crossing)
Syn1 = mean performance of all possible F1 crosses between parental lines
n = number of parental lines in the synthetic

Implication:

Syn2 performance is always less than Syn1 due to random mating reducing heterozygosity. The more parental lines (higher n), the smaller the drop from Syn1 to Syn2.

How to improve Syn2 performance:

Increase number of lines;
Improve Syn1 performance
Improve performance of parental lines through population improvement.

Composite Varieties

If a synthetic variety is a highly calculated team, a Composite Variety is an all-star game.
A composite is created by mixing the seeds of several outstanding, open-pollinated lines (sometimes 20 or more) and simply letting them open-pollinate together.
The Catch: Unlike synthetics, the lines used in a composite are NOT tested for combining ability. The breeder just picks plants that look great phenotypically.
The Result: Because there is no mathematical GCA testing, you cannot predict the yield of a composite variety in advance using formulas. However, they are incredibly diverse and serve as fantastic starting material for future recurrent selection programs.

Composite Variety: A variety produced by mixing seeds of several phenotypically outstanding open-pollinated lines (or varieties) and encouraging open-pollination among the mixed lines. Unlike synthetic varieties, the lines used for composites are NOT tested for combining ability with each other.

Because no combining ability testing is done, the yields of composite varieties CANNOT be predicted in advance. However, composites are useful starting material for population improvement through recurrent selection.

Synthetic vs Composite Varieties — Comparison Table

S.No.	Synthetic Varieties	Composite Varieties
1	Fewer parental lines — generally 6-8 inbred lines or OPVs	More parental lines — even up to 20 or more
2	GCA of parental lines is tested — only high-GCA lines included	No combining ability testing — selection based on phenotypic merit only
3	Performance of Syn-2 and later generations can be PREDICTED using Sewall Wright's formula	Performance CANNOT be predicted in advance — yields of all F1 combinations not available
4	Narrower genetic base than composites — fewer, more carefully chosen parents	Broader genetic base than synthetics — more diverse parental contributions
5	Synthetic variety CAN be reconstituted at a later date from preserved parental line seeds	Composite CANNOT be reconstituted from parental components
6	Seed should be replaced after 4-5 years of open pollination (performance declines due to natural selection and genetic drift)	Seed should be replaced after 3-4 years

Germplasm Complexes

Germplasm complexes are experimental populations produced by mixing seeds from several lines or populations of diverse genetic origin.
They serve as RESERVOIRS of genetic variability and are NOT commercial varieties. They are used as base populations for recurrent selection programmes.

Merits of Synthetic Varieties

Provide a feasible means of utilising heterosis in crop species where pollination control is difficult (forage crops, clonal crops like cacao, alfalfa, clovers)
Farmer can save and re-use seed: Grain from a synthetic variety can be used as seed for the next crop — unlike F1 hybrids which lose heterozygosity upon selfing. Farmers do NOT need to purchase fresh seed each year.
In variable environments, synthetics are likely to do better than hybrid varieties — wider genetic base provides better buffering
Lower seed cost than hybrid varieties — simpler seed production
Seed production is a less skilled operation than hybrid seed production
Good reservoirs of genetic variability — composites and germplasm complexes also serve as gene reservoirs
Performance of synthetic varieties can be considerably improved through population improvement (recurrent selection) without appreciably reducing variability

Demerits of Synthetic Varieties

Performance is usually LOWER than single or double cross hybrids — synthetics exploit only GCA; hybrids exploit both GCA and SCA
Performance adversely affected by lines with relatively poor GCA — such lines may have to be included to increase parental line numbers
Can be produced and maintained only in cross-pollinated crop species — unlike hybrid varieties which can also be produced in self-pollinated crops

Indian Achievements — Synthetic and Composite Varieties

Crop	Variety Type and Name	Notes
Maize	Composites: Ambar, Jawahar, Kisan, Vikram, Sona, Vijay — released 1967	First composite varieties were released in 1967; six maize composites.' First composites in India.
Maize	Synthetics: Vijay, Sona, Kisan, Composite Ageti	Developed by mixing high-GCA inbreds.
Pearl millet	Composite VBN 2, MP 124, RHRBC 154	ICRISAT Hyderabad maize/bajra programme based on synthetics via population improvement.
Sunflower	Surya, Composite-2
Brassica	Composite 1 (Brassica campestris var. toria)	Evolved by compositing 10 elite toria strains; matures in 100 days; exhibits profuse branching; yields about 11 q/ha seed with approximately 40% oil.' First oilseed composite in India.

PYQ: IFoS 2021 (Q2c, 10M) — Explain how synthetic varieties are produced and indicate their merits.

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PB Ch 29. Clonal Selection →