Introduction
Plant breeding in cross-pollinated crops requires specialized methods that balance maintenance of variability with systematic selection of superior genotypes. Unlike self-pollinated crops, cross-pollinated species are highly heterozygous and show large genetic variation. To improve them effectively, breeders developed techniques that rely not only on the appearance of individual plants but also on the performance of their progeny.
Among the most influential of these are the Ear-to-Row Method and the Modified Ear-to-Row Method. These methods have played a historic role in the improvement of maize (Zea mays L.) and remain foundational for recurrent selection and synthetic variety development.
1. Ear-to-Row Method
Historical Background
- First proposed by Edward M. East (1908).
- Systematically applied and popularized by D.F. Jones (1918).
- Formed the basis of modern progeny testing in maize breeding.
Theoretical Basis
The method is based on the principle that phenotypic appearance alone cannot reliably predict genetic potential. By evaluating progeny rows (descendants of individual plants), breeders can more accurately estimate heritable traits and improve the population.
Stepwise Procedure
- Initial Population: Begin with an open-pollinated variety containing genetic diversity.
- Selection of Ears: Choose 100–200 desirable ears based on size, kernel quality, maturity, and disease resistance.
- Progeny Row Planting: Plant seeds from each ear in separate rows, so each row represents a family.
- Evaluation of Rows: Assess rows for yield, resistance, maturity, and uniformity; discard inferior rows.
- Within-row Selection: From superior rows, select the best plants and harvest their ears separately.
- Repeat Cycle: Continue across generations to gradually improve the population.
Advantages
- More reliable than mass selection due to progeny testing.
- Improves both quantitative and qualitative traits.
- Maintains genetic variability within the population.
Limitations
- Inter-row cross-pollination reduces accuracy of progeny evaluation.
- Time-consuming and resource-demanding.
- Environmental effects may still influence results.
2. Modified Ear-to-Row Method
Rationale for Modification
The major drawback of the original method was uncontrolled pollination. In cross-pollinated crops like maize, pollen from inferior rows could fertilize superior progenies, reducing selection efficiency. M.T. Jenkins (1929) introduced the Modified Ear-to-Row Method to overcome this limitation by adopting controlled pollination.
Stepwise Procedure
- Selection of Ears: Choose 100–200 promising ears from the base population.
- Controlled Pollination: Use selfing, full-sib mating, or half-sib mating to prevent contamination by foreign pollen.
- Row Planting: Sow controlled progeny seeds in rows, each row representing true genetic potential of one ear.
- Evaluation: Compare rows for yield, adaptability, and disease resistance; discard poor rows.
- Within-row Selection: Select best plants from superior rows for further cycles.
- Repetition: Continue over cycles to enhance population performance.
Advantages
- Eliminates pollen contamination between rows.
- Provides accurate estimates of additive genetic variance.
- Improves efficiency of selection for yield and quality traits.
- Forms the basis for recurrent selection and synthetic variety development.
Limitations
- Labor- and cost-intensive compared to the original method.
- Requires technical expertise in controlled pollination.
- Progress may be slower due to small number of controlled progenies tested.
3. Comparative Overview
| Aspect | Ear-to-Row Method | Modified Ear-to-Row Method |
|---|---|---|
| Origin | East (1908), Jones (1918) | Jenkins (1929) |
| Pollination System | Open pollination among rows | Controlled (selfing, sib-mating) |
| Accuracy of Progeny Test | Moderate | High |
| Technical Requirement | Low | High |
| Cost & Labor | Low | High |
| Selection Efficiency | Moderate | High |
| Application | Preliminary improvement of populations | Advanced improvement, recurrent selection |
4. Applications
- Maize Improvement: The main crop where these methods are applied.
- Forage Crops & Sunflower: Applied in cross-pollinated crops where variability is important.
- Synthetic Variety Development: Provides material for creating composites and synthetics.
- Recurrent Selection: Modified Ear-to-Row often serves as a base for recurrent selection cycles.
