Pedigree selection

Pedigree selection is a widely used method of breeding self-pollinated species.

A key difference between pedigree selection and mass selection or pure-line selection is that hybridization is used to generate variability, unlike the other methods in which production of genetic variation is not a feature. The method was first described by H.H. Lowe in 1927.

Pedigree selection is a breeding method in which the breeder keeps records of the ancestry of the cultivar. The base population, of necessity, is established by crossing selected parents, followed by handling an actively segregating population. Documentation of the pedigree enables breeders to trace parent-progeny back to an individual F2 plant from any subsequent generation. To be successful, the breeder should be able to distinguish between desirable and undesirable plants on the basis of a single plant phenotype in a segregating population. It is a method of continuous

individual selection after hybridization. Once selected, plants are reselected in each subsequent generation. This process is continued until a desirable level of homozygosity is attained. At that stage, plants appear phenotypically homogeneous.

The breeder should develop an effective, easy to maintain system of record keeping. The most basic form is based on numbering of plants as they are selected, and developing an extension to indicate subsequent selections. For example, if five crosses are made and 750 plants are selected in the F2, a family could be designated 5-175. If selection is subsequently made from this family, it can be named, for example, 5-175-10. Some breeders include letters to indicate the parental sources or the kind of crop or some other useful information. The key is to keep it simple, manageable, and informative.

Pedigree selection is applicable to breeding species that allow individual plants to be observed, described, and harvested separately. It has been used to breed species including peanuts, tobacco, tomato, and some cereals, especially where readily identifiable qualitative traits are targeted for improvement.

General guides to selection following a cross

The success of breeding methods preceded by hybridization rest primarily on the parents used to initiate the breeding program. Each generation has genetic characteristic and is handled differently in a breeding program.

F1 generations

Unless in hybrid seed programs in which the F1 is the commercial product, the purpose of the F1 is to grow sufficient F2 population for selection. To achieve this,F1 seed is usually space-planted for maximum seed production. It is critical also to be able to authenticate hybridity and identity and remove seeds from self-pollination.

Whenever possible plant breeders use genetic markers in crossing programs.

F2 generation

Selection in the plant breeding program often starts in the F2, the generation with the maximum genetic variation.The rate of segregation is higher if the parents differ by a larger number of genes. Generally, a large F2 population is planted. Of the genotypes in the F2 50% are heterozygous and, hence, selection intensity should be moderate  in order to select plants that would likely include those with the desired gene combinations. The actual number of plants selected depends on the trait and resources. Traits with high heritability are more effectively selected, requiring lower numbers than for traits with low heritability. The F2 is also usually space-planted to allow individual plants to be evaluated for selection. In pedigree selection, each F2

selected plant is documented.

F3 generation

Seed from individual plants are progeny-rowed. This allows homozygous and heterozygous genotypes to be distinguished. The homozygosity in the F3 is 50% less than in the F2. The heterozygotes will segregate in the rows. The F3 generation is the beginning of line formation. It is helpful to include check cultivars in the planting to help in selecting superior plants.

F4 generation

F3 plants are grown in plant-to-row fashion as in the F3 generation. The progenies become more homogeneous.Lines are formed in the F4. Consequently, selection in the F4 should focus more on progenies rather than individuals plants.

F5 generation

Lines selected in the F4 are grown in preliminary yield trials. F5 plants are 93.8% homozygous. These are replicated trials with at least two replications.The seeding rate is the commercial rate,receiving all the customary cultural inputs. Evaluation of quality traits and disease resistance can be included. The PYT should include check cultivars. Best performing lines are selected for advancing to the next stage in the breeding program.

F6 generation

The superior lines from F5 are further evaluated in competitive yield trials or advanced yield trials, including a check. F7 and subsequent generations Superior lines from F6 are evaluated in AYT for several years, at different locations, and in different seasons as desirable. Eventually, after F8, the most outstanding entry is released as commercial cultivar.