How is the detailed record of relationship between the selected plants and their progenies maintained in the pedigree selection method ? Discuss the procedure for pedigree selection method, with the help of a simplified schematic diagram.

Detailed Record of Relationship in Pedigree Selection

Maintaining a detailed record, often called a pedigree record, is central to the pedigree selection method. This record serves as a comprehensive genetic catalog, tracing the lineage of selected plants back to their original F2 individual. This meticulous documentation allows breeders to understand the genetic relatedness of individuals, track the inheritance of specific traits, and make informed selections across generations. There are generally two main systems for maintaining pedigree records:

1. Pedigree based on the location of progeny row in the field:
   • In this system, each progeny is assigned a number corresponding to its location in the field plot.
   • For example, if a plant is selected from the 7th row in the F3 plot, its progeny in F4 might be designated as '7911-7-4', indicating it's the 4th progeny from the 7th row of the F3 plot (assuming '7911' is the cross number and year).
   • This method helps in physically locating the progenies and relating them to their immediate ancestors.
2. Pedigree based on serial number of selected plants:
   • Each cross is typically given a unique number (e.g., the first two digits for the year of the cross, and the remaining for the serial number of the cross).
   • Subsequently, selected individual plants within each generation are assigned serial numbers.
   • For instance, a cross made in 1979 might be '7911'. If the 7th plant is selected in F2, it becomes '7911-7'. If the 4th plant is selected from the progeny of '7911-7' in F3, it becomes '7911-7-4'.
   • This system allows for tracing each progeny back to its originating F2 individual without necessarily consulting previous year's field records, providing a clear genealogical tree.

These records are vital for:

• Tracking genetic history: Understanding the ancestry of selected lines.
• Informing selection: Making decisions based on the performance of ancestors and collateral relatives.
• Discarding undesirable lines early: Identifying and eliminating plants with visible defects or weaknesses early, saving resources.
• Genetic studies: Obtaining information about the inheritance patterns of specific qualitative and quantitative traits.

Procedure for Pedigree Selection Method

The pedigree selection method involves a systematic multi-generational process to develop new pure-line varieties. The general procedure is as follows:

Year 1: Hybridization

• Two parent plants (P1 and P2) with desirable but complementary traits are selected and crossed.
• The F1 seeds resulting from this cross are harvested.

Year 2: F1 Generation

• F1 seeds are space-planted to produce a large number of F1 plants.
• F1 plants are generally uniform and heterozygous. They are allowed to self-pollinate, and their seeds are harvested in bulk to produce the F2 generation.

Year 3: F2 Generation

• F2 seeds are space-planted, typically yielding the maximum genetic variation, as segregation occurs for various traits.
• Individual plants (usually 1-10% of the population) exhibiting desirable traits (e.g., disease resistance, plant height, early maturity, head type) that are highly heritable are selected.
• Each selected F2 plant is individually harvested, and a detailed pedigree record is initiated for each.

Year 4: F3 Generation

• Progeny rows from each selected F2 plant are space-planted.
• Comparison is made between progeny rows (family selection) and within progeny rows (individual plant selection).
• Superior plants from superior progeny rows are selected and individually harvested. Checks (standard varieties) are often included for comparison.

Years 5-6: F4 - F5 Generations

• The process of growing progeny rows and selecting superior individual plants from superior families continues.
• By F5 generation, the variation within progeny rows considerably decreases, and plants become largely homozygous (e.g., F5 plants are about 93.8% homozygous).
• Selection now primarily focuses on differences between progeny rows. Superior families may be bulk harvested.

Year 7: F6 Generation (Preliminary Yield Trials)

• Individual selected progenies from F5 (or bulked families from F5) are grown in multi-row plots for visual comparison.
• Preliminary yield trials are conducted with selected homogeneous lines, often with replications and standard check varieties, to evaluate yield and other quantitative traits.
• Lines still showing segregation are discarded or re-selected.

Year 8-10: F7 - F9 Generations (Advanced Yield Trials and Release)

• The most promising lines from F6 undergo advanced yield trials across multiple locations and years to assess their stability and performance under diverse environmental conditions.
• Quality tests, disease screening, and other relevant evaluations are performed.
• The best performing, stable, and homozygous line(s) are multiplied and officially released as a new variety.

Simplified Schematic Diagram of Pedigree Selection Method

P1 x P2 (Hybridization)
   ↓
F1 Generation (Selfing)
   ↓
F2 Generation (Space planting, individual plant selection, start pedigree record)
   ↓
F3 Generation (Progeny rows, selection within and between families)
   ↓
F4 Generation (Progeny rows, selection within and between families, increased homozygosity)
   ↓
F5 Generation (Progeny rows, selection mainly between families, near homozygosity)
   ↓
F6 Generation (Preliminary Yield Trials, evaluation of homozygous lines)
   ↓
F7-F9 Generations (Advanced Yield Trials, multi-location testing, quality checks)
   ↓
Variety Release and Seed Multiplication

Advantages of Pedigree Selection

• Detailed Genetic Information: Provides a comprehensive catalog of genetic information, aiding in the understanding of inheritance patterns.
• Effective Selection: Allows selection based on both phenotypic and genotypic characteristics, making it effective for traits with high heritability.
• Early Elimination of Undesirables: Visible defects and weaknesses can be eliminated early, saving resources and time.
• High Genetic Purity: Leads to the development of highly pure lines.
• Recovery of Transgressive Segregants: Offers opportunities to identify individuals superior to both parents for quantitative traits.

Disadvantages of Pedigree Selection

• Time-Consuming and Labor-Intensive: Extensive record-keeping and individual plant handling make it a slow and costly process.
• Requires Skill: Success heavily depends on the breeder's skill in identifying desirable plants in segregating populations.
• Not Suitable for All Species: Difficult for species where individual plants are hard to isolate and characterize.
• Limited Natural Selection: There is less opportunity for natural selection to influence the population compared to methods like bulk breeding.