Download Lecture 7

PRINCIPLES OF CROP PRODUCTION
ABT-320
(3 CREDIT HOURS)
LECTURE 7
SELECTION IN CROSS-POLLINATING CROPS
SELECTION 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 Selection
2. Recurrent Selection
MASS SELECTION
Mass 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 SELECTION
The 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 SELECTION
Recurrent 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 METHOD
This 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 METHOD
The 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 METHOD
1.
2.
3.
4.
5.
F1 Generation: The F1 plants are grown along with the parents and the
selfed progeny, if any, is removed. Sufficient F2 seeds are harvested.
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.
F5 Generation: The bulked F5 seeds are planted and desirable plants are
selected. Each selected plant is harvested separately.
F6 Generation: Single plant progeny rows are grown, uniform and
promising progenies selected and the selected progenies are bulk
harvested separately.
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