PRINCIPLES OF CROP PRODUCTIONABT-320

(3 CREDIT HOURS)

LECTURE 7SELECTION IN CROSS-POLLINATING CROPSSELECTION IN SEGREGATING POPULATION

SELECTION IN CROSS-POLLINATING CROPS

Selection is applied in cross-pollinated species for population improvement. The methods of selection used in the case of cross-pollinated crops are:

1. Mass Selection2. Recurrent Selection

MASS SELECTIONMass selection is the technique in which individual plants are selected on the basis of their performance and the bulked seeds are used to produce the next generation. Since phenotype is the sole criterion of selection in mass selection, its efficiency depends on the magnitude of heritability. Several modifications of mass selection are also being practiced:

• Selfed Plant Mass Selection• Progeny Selection

SELFED PLANT MASS SELECTIONThe available additive and dominant gene variance in a random mating population can be reshuffled through selfing. The selfed progeny rows are selected on the basis of phenotypic performance. They are pollinated by a random bulk of other S1 plants. The unit of selection and recombination are thus selfed S1 plants.

PROGENY SELECTION• This is mass selection with progeny test. The simplest form of progeny

selection is the ear-to-row method in which a number of plants are selected on the basis of the phenotype in the first season. They are allowed to open pollinate and the seeds from individual plants are harvested separately. In the second season, progeny rows are grown from each selected plant. The progeny rows are evaluated for desirable characters and superior progenies are identified. They are permitted to open pollinate and the progeny is grown in progeny rows again. The cycle is repeated and finally the seeds of superior plants are collected and bulked.

• In progeny selection, the selection is made on the basis of progeny test and not on the phenotype of individual plants. This helps in the selection of plants with better genotypes. This is a good tool to increase the yielding ability of open pollinated plants.

• The limitation in most of the progeny selection schemes is that there is no control on pollination and the plants are allowed to open pollinate. Besides, selection is based on the maternal parent only which reduces the efficiency of selection.

RECURRENT SELECTIONRecurrent selection is defined as reselection generation after generation with interbreeding of the selected plants to provide for genetic recombination.

This is a technique of selection in which one original selection cycle and one or more recurrent selection cycles are carried out for population improvement. In the original selections cycle, a number of plants with desirable characters are selected and self-pollinated. Separate progeny rows are grown from the selected plants in the next generation. The progenies are intercrossed in all possible combinations and equal amount of seeds from each is composited to produce the next generation.

SELECTION IN SEGREGATING POPULATIONS

Populations like F2, F3 etc are called segregating populations. Techniques of selection like mass selection and pure line selection cannot be applied to such populations. The methods of selection generally used in the case of segregating populations are pedigree method, bulk method and single seed descent method.

PEDIGREE METHOD• In this method of selection, individual plants are selected in F2

generation and subsequent generations based on the progeny test. During the entire operation, a pedigree record of the parents and progeny is maintained. Individual plant selection is continued till the progenies show no segregation. At this stage, selection is done among the progenies because there would be no variation within progenies.

• Pedigree Record: A detailed record of the relationship between the selected parents and their progenies can be called their pedigree record. Pedigree can be defined as a description of the ancestor of an individual and it generally goes back to its distant ancestor.

PROCEDURE OF PREDIGREE METHOD

Pedigree method of selection is an efficient method of handling segregating generations. The steps involved are explained in the coming slides.

F1 GENERATION

The F1 plants are grown to produce sufficient F2 seeds. The F1 plants are planted and assessed for their behavior with regard to easily observable characters and the selfed plants that occur accidentally are identified and removed.

F2 GENERATION

It is the most important generation that shows maximum genetic variation and provides the opportunity to select individual plants. A relatively large number of plants are raised and evaluated. Selection of plants is made based on their behavior. The selected plants are individually harvested and a detailed record is kept with respect to genetic identity of the plants and their agronomic characters.

F3 GENERATION

The progeny rows of the selected F2 plants are grown along with check variety in the F3. The plants are liberally evaluated and compared visually with check varieties. The promising individual plants are selected within the retained progenies. It must be ensured that progenies with susceptibility to diseases and pests are rejected. Plants are individually harvested and a record of genetic and agronomic characters is kept.

F4 GENERATION

The progeny of selected F3 plants is grown to produce F4 families. The progeny of one F3 plant constitutes a family. The family differences become obvious in this generation and the number of families can be reduced by visual selection. The rejection of some F4 families is needed if they show some prominent undesirable characteristics.

F5 GENERATION

In this generation, the selected plants are grown in progeny rows usually in large numbers. By this generation, variation within families considerably decreases. Progenies or even families showing appreciable plant to plant variations are rejected to set aside the chance of selecting hybrid plants. The selected families are bulk harvested.

F6-F8 GENERATIONS

Preliminary yield trials of the selected families are carried out at this stage. Screening for resistance and quality is also done.

F7-F10/F12 GENERATIONS

Multilocation trials are conducted at this stage. Screening for resistance and quality is carried out.

VARIETY RELEASE & SEED MULTIPLICATION

When the above steps are accomplished, the selected families are released as new varieties. These new varieties are multiplied and distributed to farmers.

MERITS AND LIMITATIONS OF PEDIGREE METHOD

Pedigree method provides the maximum opportunity to the breeder to exercise his skill to select plants with the most desirable gene combinations and it helps to evaluate the selections for their performance over a large number of years. At the final stage of bulking, the pedigree record helps to bulk progenies with similar yield potential. But, it is a laborious and demanding method in terms of resources and time. It involves extensive data recording. The material is to be evaluated only under optimal environmental conditions which makes off-season operations impossible.

SINGLE SEED DESCENT METHODThis is a modification of pedigree method. This envisages complete exclusion of selection among or within lines until sufficient homozygosity is attained by the F5 or F6 generation. Here, a large number of F2 plants are carried forward by maintaining their lineage through single seed from each plant till the F6 generation. The seeds of the single plants are then grown in progeny rows for their multiplication and trials.

BULK METHODThe bulk method is a short-term and labor-saving procedure for the management of segregating generations. This technique is adopted to attain sufficient level of homozygosity prior to the initiation of individual plant selection. The seeds are harvested in bulk from F2 to F4 or F6 generation and individual plant selection is practiced only afterwards.

PROCEDURE OF BULK METHOD1. F1 Generation: The F1 plants are grown along with the parents and the

selfed progeny, if any, is removed. Sufficient F2 seeds are harvested.

2. F2 to F4 Generations: sufficient number of F2 plants are grown and harvested in bulk. A composite sample of harvested seeds are collected. The process is repeated in the F3 and F4 generations. F5 seeds are harvested.

3. F5 Generation: The bulked F5 seeds are planted and desirable plants are selected. Each selected plant is harvested separately.

4. F6 Generation: Single plant progeny rows are grown, uniform and promising progenies selected and the selected progenies are bulk harvested separately.

5. F7-F9 Generations: Preliminary yield trials are conducted to reduce the number of F7 progenies. Tests are repeated at different locations in the next generation. In F9 generation, the last progeny line is selected and released as a new variety.

MERITS & LIMITATIONS OF BULK METHOD

This method is comparatively inexpensive and easy. Selection starts at a stage when the population attains homozygosity. Single plant selection is done when the plants (F5 or F6) are uniform in general. The population is expected to improve for intangible characters like adaptation to existing environment. There is a chance that a large proportion of undesirable material is carried forward.

THE END