👥 Population Improvement
Population-based breeding approaches used to improve self- and cross-pollinated crops.
This lesson covers core principles and exam-focused points from this topic in plant breeding.
Population Approach to Breeding of Self -Pollinated Crops
Self-fertilization of FI hybrids leads to a very rapid increase in homozygosity. After
several generations of self-pollination, about 94 per cent of the genes would become
homozygous. Even in F2, half of the genes are in homozygous state. Thus self fertilization
quickly separates the progeny from a hybrid into a large number of purelines. As a consequence,
selection in such a segregating population only picks out the genes combinations present in the
population primarily as a result of recombination in F2.
This reduces the chance of recombination between linked, especially tightly linked genes
and of recovery of rare transgressive segregants. There is no opportunity for changing the
genotype of the plant produced by recombination in FI, F2 and to some extent, in F3.Thus the two
obvious limitations of breeding methods based on self-pollination of the hybrid (e.g., pedigree
and bulk methods) are: first, the recombination is limited to two or, at the best, three generations,
and second, there is no possibility for further changing the genotype of the segregants.
A population breeding approach has been suggested to overcome these problems. In
population breeding, outstanding F2 plants are mated among themselves in pairs or in some other
fashion. The intermating of selected F2 plants restores heterozygosity in the progeny, which
provides for a greater opportunity for recombination. This also brings together the desirable
genes from different F2 plants and would help in the accumulation of favourable genes in the
intermated population. Thus the chances of the recovery of transgressive segregants would
increase considerably. This process may be repeated one or more times.
This procedure is similar to recurrent selection in cross-pollinated crops. A variation of
this approach would be to intermate F3 or later generation progenies. This would allow a more
effective selection of desirable progenies than in the case of F2 where individual plants have to
be selected. As noted previously, selection in F2 based on individual plants is of little value,
particularly for characters like yield. Selection based on F3 or F4 progenies would be more
desirable. Intermating of selected plants may be continued for two or more generations.
This idea of population approach was first suggested by Palmer in 1953. It is not
commonly used at present, but may find a greater application in the future, as improvements due
to the pedigree method would become less and less marked. Evidently, the population approach
is akin to recurrent selection commonly used in cross-pollinated crops and often it is referred to
as such. The chief limitation of recurrent selection in self-pollinated crops is the difficulty in
making the large number of required crosses by hand (emasculation and pollination).
This difficulty may be overcome by using genetic or cytoplasmic male sterility. When
genetic male sterility is used, selection is confined to the male sterile (ms ms) plants in each
generation. Seeds from the selected male sterile plants are generally harvested in bulk. The
progeny from such plants may be expected to have both male sterile (ms ms) and male fertile
(Ms ms) plants in almost equal proportion. Further, the seeds produced on the male sterile plants
would be produced by pollination by the male fertile plants in the population. Thus the use of
male sterility effectively ensures intermating among the plants in the population and eliminates
the needs for tedious and time-consuming hand emasculation and pollination.
Results from recurrent selection are available in tobacco and soybean. In tobacco,
Matzinger and coworkers selected the plants before flowering and intermated them. A linear
response of 4.9 and 7 per cent per cycle to selection for decrease plant height and for increased
leaf number, respectively, was obtained for five cycles of selection. Further, there was no
evidence for a reduction in variability as a result of the selection. Brim and coworkers carried out
six cycles of recurrent selection for increased protein content in two segregating populations of
soybean and three cycles of selection for yield and three cycles of selection for high oil content
in another segregating population. There was an increase of 0.33 and 0.67 per cent / cycle in
protein content of the two populations, of 5.3% per cycle in yield and of 0.3% per cycle in oil
content. These findings amply demonstrate the effectiveness of recurrent,selection in improving
yield and yield traits in self-pollinated crops.
In 1970, Jensen proposed a comprehensive breeding scheme which provides for the three
basic functions of a versatile. breeding programme. Firstly, it allows the development of F2, F3
etc. (selfing series) at every stage of the breeding programme, which permits the isolation of
purelines for use as commercial varieties. Secondly, it requires intermating among the selected
plants/ lines in each stage; the progenies from these intermatings form the basis for the next stage
of the selfing series in the breeding programme.
Thus the breeding programme progresses in two different directions: (1) Vertically,
through the selfing series leading to the isolation of commercial varieties, and (2) horizontally,
through intermating among the selected plant / lines; this generates the recurrent selection series.
Thirdly, new germplasm may be introduced at any stage of the programme by intermating it with
some of the selected plants of that stage. This permits the retention and or the creation of large
amounts of variability for effective selection through several cycles, and the introduction of new
genes in the breeding material, if so desired. This breeding scheme is known as Diallel Selective
Mating Scheme (DSM) and is designed to serve both short-term and long-term breeding
objectives.
A breeder may create more than one such population for a crop, each population being
developed to fulfill a specific objective. This scheme has not been widely used primarily due to
the difficulties in making the large number of crosses required in this scheme. Jensen has
suggested the use of male sterility to overcome this difficulty in the same way as in the recurrent
selection scheme discussed earlier. Further, DSM is much more complicated than the simple
pedigree method which still is the favourite breeding method for selfpollinated crops.
Merits of population Approach
- The population approach provides for greater opportunities for recombination. This
is made possible by restoring heterozygosity through intermating of selected
plants.
- This approach helps in the accumulation of desirable genes in the population. This is also
brought about by the intermating of selected plants from segregating generation.
Demerits of Population Approach
- The success of this approach depends upon the identification of desirable plants in F2 and
the subsequent segregating generations. This is very difficult, if not impossible, for
complex characters like yield which show low heritability. This may be avoided to some
extent by using later generation (F3 or F4) progenies; replicated yield data may also be
used.
- Another draw back of this approach is the intermating of selected plants. This may
become a major limitation in some crops because crossing in many self-pollinated species
is difficult and time consuming.
- The time taken to develop a new variety through population approach would be always
greater than that by the pedigree method.
- There is no convincing evidence for the benefits from the population approach. It has
been argued that increased recombination may be detrimental, as it would break the
desirable linkage. But such a criticism assumes that all or most of the new gene
combinations (recombinations) will be inferior to the existing ones. Such an assumption
is not entirely valid since crop improvement is based on the creation of new and desirable
gene combinations.
Breeding Methods for Cross Pollinated Crops
Populations of cross pollinated crops are highly heterozygous. When inbreeding is
practiced they show severe inbreeding depression. So to avoid inbreeding depression and its
undesirable effects, the breeding methods in the crop is designed in such a way that there will be
a minimum inbreeding. The breeding methods commonly used in cross pollinated crops may be
broadly grouped into two categories.
I. Population improvement
A. Selection
a) Mass selection
b) Modified mass selection
Detasseling
Panmixis
Stratified or grid or unit selection
Contiguous control.
B. Progeny testing and selection
a) Half sib family selection
i) Ear to row
ii) Modified ear to row.
b) Full sib family selection.
c) Inbred or selfed family selection.
i) Sl self family selection
ii) S2 self family selection.
C. Recurrent selection
a) Simple recurrent selection
b) Reciprocal recurrent selection for GCA
c) Reciprocal recurrent selection SCA
d) Reciprocal recurrent selection.
D. Hybrids
E. Synthetics and Composites.
Mass selection
This is similar to the one, which is practiced, in self-pollinated crops. A number of plants
are selected based on their phenotype and open pollinated seed from them are bulked together to
raise the next generation. The selection cycle is repeated one or more times to increase the
frequency of favourable alleles. Such a selection is known as phenotypic recurrent selection.
Merits
i) Simple and less time consuming
ii) Highly effective for character that are easily heritable.
Eg. Plant height, duration.
iii) It will have high adaptability because the base population is locally adapted one.
Demerits
-
Selection is based on phenotype only which is influenced by environment
-
The selected plants are pollinated both by superior and inferior pollens present in the
population.
- High intensity of selection may lead reduction in population there by leading to
inbreeding.
To over come these defects modified mass selection is proposed they are
a) Detasseling
This is practiced in maize. The inferior plants will be detasseled there by inferior pollen
from base population is eliminated.
b) Panmixis
From the selected plants pollen will be collected and mixed together. This will be used to
pollinate the selected plants. This ensures full control on pollen source.
c) Stratified mass selection
Unit selection
Here the field from which plants are to be selected will be divided into smaller units or
plots having 40 to 50 plants / plot. From each plot equal number of plants will be selected.
The seeds from selected plants will be harvested and bulked to raise the next generation,
by dividing the field into smaller plots, the environmental variation is minimized. This method is
followed to improve maize crop. It is also known as Grid method of mass selection
B) Progeny Testing and Selection
a) Half sib family selection
Half sibs are those, which have one parent in common. Here only superior progenies are
planted and allowed to open pollinate.
Ear to row method
It is the simplest form of progeny selection. Which is extensively used in maize. This
method was developed by Hopkins
a) A number of plants are selected on the basis of their phenotype. They are allowed to
open pollinate and seeds are harvested on single plant basis.
b) A single row of say 50 plants i.e. progeny row is raised from seeds harvested on single
plant basis. The progeny rows are evaluated for desirable characters and superior
progenies are identified.
c) Several phenotypically superior plants are selected from progeny rows. There is no
control on pollination and plants are permitted to open pollinate.
Though this scheme in simple, there is no control over pollination of selected plants. Inferior
pollen may pollinate the plants in the progeny row. To over come this defect, the following
method is suggested.
a) At the time of harvest of selected plants from base population on single plant basis, part of
the seed is reserved.
b) While raising progeny rows, after reserving part of the seeds, the rest are sown in smaller
progeny rows.
c) Study the performance of progenies in rows and identify the best ones.
d) After identifying the best progenies, the reserve seeds of the best progenies may be raised in
progeny rows.
e) The progenies will be allowed for open pollination and best ones are selected. There are
number of other modifications made in the ear to row selection.
For example,
i. The selected progenies may be selfed instead of open pollination
ii. The selected plants may be crossed to a tester parent. The tester parent may be a open
pollinated variety, or inbred
iii.The progeny test may be conducted in replicated trial.
b) Full sib family selection
Full sibs are those which are produced by mating between selected plants in pairs. Here
the progenies will have a common ancestry. The crossed progenies are tested.
A x B B x A
c) Inbred or selfed family selection
Families produced by selfing.
S1 family selection
Families produced by one generation of selfing. These are used for evaluation and
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 based on progeny test and not on phenotype of individual plants.
-
In breeding can be avoided if care is taken raising a larger population for selection.
-
Selection scheme is simple.
Demerits
-
No control over pollen source. Selection is based only on maternal parent only.
-
Compared to mass selection, the cycle requires 2-3 years which is time consuming.
Recurrent selection
This is one of the breeding methods followed for the improvement of cross pollinated
crop. Here single plants are selected based on their phenotype or by progeny testing. The
selected single plants are selfed. In the next generation they are intermated (cross in all possible
combinations) to produce population for next cycle of selection.
The recurrent selection schemes are modified forms of progeny selection programmes.
The main difference between progeny selection and recurrent selection.
i) The manner in which progenies are obtained for evaluation.
ii) Instead of open pollination, making all possible inter crosses among the selected lines.
The recurrent selection schemes are of 4 different types.
Simple recurrent selection
In this method a number of desirable plants are selected and self pollinated. Separate
progeny rows are grown from the selected plants in next generation. The progenies are
intercrossed in all possible combination by hand.
Equal amount of seed from each cross is mixed to raise next generation. This completes
original selection cycle. From this, several desirable plants are selected and self pollinated.
Progeny rows are grown and inter crosses made. Equal amount of seeds are composited to raise
next generation. This forms the first recurrent selection cycle.
i) Several superior plants selected.
ii) Selected plants self pollinated
iii)Harvest on S.P. Plants
iv)Seeds evaluated superior plants
identified
i) Progeny rows raised
ii) Inter cross made in all combination
by hand
iii)Equal amount of seed bulked from
each cross
i) Composited seeds raised
ii) Repeat the operation as in first
year
Repeat as in 2 [nd] year
I Year
Original
Selection Cycle
II Year
First recurrent
Selection cycle
III Year
O O O O O O O O O O O O O O O O O O O O O
O O O O O O O O O O O O O O O O O O O O O O O O
IV Year
i) Recurrent selection is effective in increasing the frequency of desirable genes in
the population
ii) Most suited for characters having high heritability
iii) Inbreeding is kept at minimum.
Recurrent selection for general combining ability
In this case the progenies selected for progeny testing are obtained by crossing the
selected plants to a tester parent with broad genetic base.
A tester parent is a common parent mated to a number of lines. Such a set of crosses is
used to estimate the combining ability of the selected lines. A tester with broad genetic base
means an open pollinated variety, a synthetic variety or segregating generation of a multiple
cross.
Recurrent selection for GCA can be used for two basically different purposes.
- It may be used to improve the yielding ability and the agronomic characteristics of a
population. In this case the end product will be a synthetic variety.
- It may be used to concentrate genes for superior GCA. Here the end product will be
superior inbreds. Such inbreds can be developed after a few cycles of RSGCA
Recurrent Selection for Specific Combining Ability
This is similar to RSGCA except, that in the case of Tester. Here the tester will be an
INBRED instead of open pollinated variety. The other operations are similar to RSGCA. The
objective of RSSCA is to isolate from population such lines that will combine well with an
inbred. These lines are expected to give best hybrids in heterosis breeding.
Reciprocal recurrent selection
Proposed by Comstock, Robinson and Harvey. The objective is to improve two different
populations in their ability to combine well with each other. In this method we can make
selection for both GCA and SCA. Basically two populations A and B are used. Each serve as a
tester for the other.
Ist year 1. Several plants selected in population A and B.
-
Selected plants are self pollinated
-
Selected plant from A is test crossed with plants inB and Vice versa.
Harvest crossed plant on S.P. basis each.
2 [nd] year 1. Separate yield trials conducted from test cross progenies of A and B
- Superior progenies identified
3 [rd] year 1. Selfed seed from plants producing superior test cross progenies
planted.
-
All possible inter crosses made
-
Seeds harvested and composited
4 [th] year
5 [th] year
6 [th] year
Use of RRS
-
Two populations are developed by this method
-
They may be intermated to produce a superior population with broad genetic base. This
is similar to a varietal cross but in this case the populations have been subjected to
selection for combining ability (GCA and SCA)
- Inbreds may be developed from populations A and B. These inbreds may be crossed
to produce a single cross or double cross hybrids.
Summary Cheat Sheet
Quick Recall Points
- This lesson focuses on key plant breeding concepts, terminology, and exam-relevant applications.
- Review major definitions, classifications, and method-wise distinctions from the sections above.
- Revise tables and examples from this lesson for fast pre-exam recall.
Exam Traps
- Do not confuse similarly named breeding methods without checking their core selection logic.
- Pay attention to crop-specific examples because the same principle can behave differently by species.
References
1 source • [1]
References
Standard Plant Breeding Class Notes (GPBR211)
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