Mass and Recurrent Selections, Combining Ability - 2 | Agriculture Optional Notes for UPSC PDF Download

Breeding Methods for Cross Pollinated Crops

Cross-pollinated crop populations are typically characterized by high levels of heterozygosity. Inbreeding in these crops often leads to a significant decrease in fitness, a phenomenon known as inbreeding depression. To mitigate the undesirable effects of inbreeding depression, breeding methods for these crops are designed to minimize inbreeding.
The commonly used breeding methods for cross-pollinated crops can be broadly categorized into two groups:

Population Improvement

A. Selection

• Mass Selection
• Modified Mass Selection
  • Detasseling
  • Panmixis
  • Stratified, Grid, or Unit Selection
  • Contiguous Control

B. Progeny Testing and Selection

• Half-Sib Family Selection
  • Ear-to-Row Method
  • Modified Ear-to-Row Method
• Full-Sib Family Selection
• Inbred or Selfed Family Selection
  • S1 Self Family Selection
  • S2 Self Family Selection

C. Recurrent Selection

• Simple Recurrent Selection
• Reciprocal Recurrent Selection for General Combining Ability (GCA)
• Reciprocal Recurrent Selection for Specific Combining Ability (SCA)
• Reciprocal Recurrent Selection

D. Hybrids
E. Synthetics and Composites

Mass Selection

Mass selection, which is similar to the approach used in self-pollinated crops, involves selecting a group of plants based on their observable characteristics (phenotype). The seeds produced by these selected plants are mixed together in bulk to create the next generation. This selection cycle can be repeated one or more times to increase the frequency of desirable alleles. This method is often referred to as phenotypic recurrent selection.

Advantages

• Simple and less time-consuming.
• Highly effective for traits with high heritability, such as plant height and duration.
• Maintains adaptability because the base population is locally adapted.

Disadvantages

• Selection is solely based on observable traits (phenotype), which can be influenced by environmental factors.
• Selected plants are pollinated by a mixture of superior and inferior pollen sources within the population.
• Intensive selection can lead to a reduction in population size, potentially resulting in increased inbreeding.
• To address these limitations, a modified mass selection method has been proposed, which includes the following techniques:
  a) Detasseling: This technique is practiced in maize. Inferior plants are detasseled, removing the male flowers (tassels) from these plants, which eliminates the contribution of inferior pollen from the base population.
  b) Panmixis: Pollen is collected from the selected plants and mixed together. This mixed pollen is then used to pollinate the selected plants, providing greater control over the pollen source.
  c) Stratified Mass Selection

Unit Selection

In unit selection, the field from which plants are to be chosen is divided into smaller units or plots, each containing approximately 40 to 50 plants. An equal number of plants are then selected from each plot, and the seeds from these selected plants are harvested and combined to produce the next generation. Dividing the field into smaller plots helps minimize environmental variations. This method is commonly used to improve maize crops and is also known as the grid method of mass selection.

Progeny Testing and Selection

(a) Half Sib Family Selection

Half-sibling plants share one common parent. In this method, only superior offspring are planted and allowed to open-pollinate.
There are two variations of this method:
1. Ear to Row Method: This is a straightforward approach widely used in maize breeding. It involves several steps:

• A group of plants is selected based on their observable traits, and they are allowed to open-pollinate. Seeds are harvested from each selected plant individually.
• A single row, known as a progeny row, is grown from the seeds harvested from individual plants. These progeny rows are then evaluated for desirable traits, and superior progenies are identified.
• Several phenotypically superior plants are selected from the progeny rows. However, there is no control over the pollination process, and plants are allowed to open-pollinate.
• While this method is simple, it lacks control over the pollination of selected plants, potentially allowing inferior pollen to pollinate the progeny row. To address this issue, an alternative method is suggested:
• During the harvest of selected plants from the base population, part of the seeds is reserved.
• When raising progeny rows, after reserving part of the seeds, the rest are sown in smaller progeny rows.
• The performance of these progenies in rows is studied, and the best ones are identified.
• After identifying the best progenies, the reserved seeds of the top-performing progenies may be grown in progeny rows.
• The progenies are allowed to open-pollinate, and the best ones are selected.
• There are several other modifications that can be made to the ear-to-row selection method, including selfing the selected progenies or crossing them with a tester parent. Progeny tests may also be conducted in replicated trials.

(b) Full Sib Family Selection

• Full-sibling plants are produced by mating selected plants in pairs, resulting in progenies with a shared ancestry. These crossed progenies are tested for desirable traits.

(c) Inbred or Selfed Family Selection
In this method, families are produced by selfing or inbreeding. Two variations of inbred or selfed family selection are:

• S1 Family Selection: Families produced by one generation of selfing and used for evaluation. Superior families are intermated (Simple recurrent selection).
• S2 Family Selection: Families obtained by two generations of selfing and used for evaluation. Superior families are intermated.

Merits of Progeny Testing and Selection

• Selection is based on progeny testing, not on the phenotype of individual plants.
• Inbreeding can be avoided if a larger population is used for selection.
• The selection scheme is straightforward.

Demerits

• There is no control over the source of pollen, and selection is based solely on the maternal parent.
• The selection cycle takes 2-3 years, making it time-consuming compared to mass selection.