For selection of a new variety from a segregating population after hybridization the methods followed in self-pollinated crop are of three types: 1. Pedigree method, 2. Bulk method, 3. Back cross method.
Method # 1. Pedigree Method:
Individual plant progeny is selected from F2 and subsequent generations, and their progenies are tested. During this process the record of parents as well as off-springs is kept, for which it is known as pedigree method.
The pedigree is defined as the description of the ancestors of an individual and it is generally helpful in finding out the amount of relatedness among two individuals, i.e., whether they are related by common parent in their descent ancestry or not.
The hybridization is done among two selected parents, after emasculation one become female parent and another male parent. After seed set and maturation, the F1 seeds are harvested separately from each plant individually. On the basis of choice of parents, the type of cross will be of two types — it will be simple cross or complex cross.
F1 generation seeds are space planted and selfing is allowed, each F1 will produce more F2 seeds. From 15-30 selected F1 plants, the F2 seeds are collected to get a reasonable size of F2 population and variation.
In F2 generation, 2000-10000 plants are space planted, 100-500 plants are selected and their seeds are harvested separately. If the parent plants are closely related varieties then the number of selected F3 plants would be smaller whereas in case of distantly related varieties the number of F3 progenies will be of relatively larger numbers.
In F3 generation also, the individual plant progenies are space planted. Each progeny should have about 30 or more plants. Individual plants with desirable characteristics are selected; disease and lodging susceptible progenies to be eliminated, and also the progenies with undesirable characters are rejected even from the selected plants. During this selection if the number of superior progenies is very small then the whole cross programme may be rejected.
The selection procedure is same as previous year, only if two or more progenies coming from the same F3 progeny are similar and comparable, then only one may be saved and others may be rejected. The emphasis is given on the selection of desirable plants from superior progenies.
Individual plant progenies of F5 generation are planted according to recommended commercial seed rate. Three or more rows for each progeny will help in comparison among progenies. Many progenies may have become reasonably homozygous genotype and may be harvested in bulk. If the progenies show variation then the individual plants are selected. The number of selected progenies should be reasonable so that preliminary yield trial with 25-100 progenies can be done.
Individual plant progenies of F6 generation are planted in multi-row plots and evaluated visually. Progenies harvested in bulk since they become homozygous. The segregating progenies may be discarded and the preliminary yield trial may be done for the progenies which are reasonably homozygous and have enough seeds.
Preliminary yield trial with three or more replications is conducted to identify few superior lines. The progenies are evaluated for plant height, lodging, disease resistance, flowering time, maturity time, etc. Quality test is done to serve as an additional basis for selection.
Ninth to Tenth or Thirteenth Year:
The superior lines are tested in replicated yield trials at several locations. The above mentioned criteria are evaluated for these lines. The line which is superior to the best commercial variety may be released as new variety.
Eleventh or Fourteenth Year:
The selected strain should get multiplied to release as a new variety. Breeder has the responsibility to supply the seeds to the state seeds corporation for production and marketing of the seeds.
1. This method is most useful as transgressive segregation for yield and other quantitative characters may be recovered in addition to improvement of specific characters.
2. This method is well suited for improvement of characters which can be easily identified and simply inherited.
3. Through the maintenance of pedigree record the breeder may be able to obtain the information about inheritance of characters.
4. Plants or progenies with weaker and visible defects are eliminated at an early stage in the breeding programme.
5. This method gives maximum importance on the breeder to use his/her skill and judgement about the selection of plants and progenies.
6. This method takes less time than bulk method to release a new variety.
1. The success of the method is mainly dependent on the skill of the breeder.
2. To keep the individual pedigree record is laborious and time consuming, it may be the limiting factor for large breeding programme.
3. Selection of large number of progenies in every generation is also laborious and time consuming.
4. In F2 and F3, the selection for yield is not effective. If sufficient number of progenies is not retained, valuable genotypes may be lost in early segregating generations.
Pedigree method is useful in selection of new superior recombinant types from a hybridization programme. This method is suitable for improving specific characteristics, such as disease resistance, plant height, maturity time, etc. as well as yield and quality characters.
Many improved varieties have been developed through pedigree method in many crops like wheat, rice, barley, pulses, oil seeds, cotton, tobacco, jowar, vegetables, etc.
K 65 (tall variety) ← C 591 x NP 773
K 68 (good quality grain) ← NP 773 x K 13
WL 711 (dwarf, high yield) ← (S308 x Chris) x Kalyan Sona
Malviya 12 (good grain) ← NP 876 x Cno 66
‘Jaya’ and ‘Padma’ (short duration, finer grain) ←Taichung Native 1 x T 141
‘Laxmi’ (fibre quality, early maturing, resistant to leaf blight) Gadag 1 x CC 2 (Cambodia Coimbatore 2)
Pusa early dwarf (more yield) ← Meerut x Red cloud
Method # 2. Bulk Method:
After hybridization programme when the F2 and subsequent generations are harvested in mass or as bulk to raise the next generation, it is called bulk method. At the end of bulking period, individual plants are selected and evaluated in the same manner of pedigree method. This method is applicable to any self-pollinating crop like cereals, legumes and oil seeds.
Three different advantages may be achieved from this method:
(1) Isolation of homozygous lines,
(2) Waiting for selection by environmental disaster,
(3) The long period of bulking may be helpful for natural selection to change the composition of population.
Hybridization is done between the selected parents with the desirable attributes. The cross may be simple or complex depending upon the number of selected parents.
The F1 generation seeds are space planted and seeds from these plants are harvested in bulk.
Third Year to Seventh Year:
Starting from F2 generation onwards the seeds are planted at commercial seed rate and spacing. The seeds of subsequent generations are harvested in bulk. Many environmental factors, disease outbreak, etc., may select out the particular genotype from this bulk population. The population size in each generation should be large always, 30,000 – 50,000 plants in each generation.
F7 generation seeds are space planted and superior phenotypes are selected and harvested separately. The selection is made on disease reaction, grain character, etc.
Individual plant progenies of F8 generation are grown in single or multi-row plots. Weak or inferior progenies are rejected, only 100-300 plant progenies are selected which are superior as well as showing no segregation.
Preliminary yield trial is conducted with F9 generation with standard commercial variety as check. The undesirable progenies are rejected.
Eleventh To Fifteenth Year:
Replicated yield trial at several locations is carried out using standard commercial variety. The lines are evaluated for some important characteristics in addition to yield, disease resistance, etc.; superior lines are released as new variety.
The seeds are multiplied and distributed to the farmer.
1. This method is simple, convenient and less expensive.
2. The natural disease epiphytotics, winter killing, etc., eliminate the undesirable types and increase the frequency of desirable types, which is more helpful for isolation of desirable types.
3. As natural selection acts here, so the progenies selected from long term bulk are far superior than those selected from F2 of short term bulk.
4. Less attention and labour is needed for this method.
5. Since large populations are grown, the transgressive segregants are more likely to appear and there is greater chance of isolation of transgressive segregants in this method.
6. Artificial selection may be practiced to increase the frequency of desirable types.
7. Survivability of any gene or a particular genotype can be studied in this method.
1. This method takes a long time to release a new variety, as the method of natural selection becomes operative only after F8 or F10.
2. The short term bulk method is useful for isolation of homozygous lines but the effect of natural selection has little effect on the genetic composition of the population.
3. Breeder has little opportunity to show his skill in this method, practically selection is mainly based on natural selection.
4. Inheritance of characters is not individually maintained as in pedigree method.
5. At the end of bulking period a large number of progenies are being selected.
This method has been used to a limited extent due to lack of popularity. In U.S.A the barley varieties like Arivat, Beecher, Glacier and Gem have been originated from a bulk population derived from cross Atlas x Vaughn, the bulk was maintained in this case for 7 generations.
As this method requires more time to get the effect of natural selection for developing a new variety and the lack of using any skill of breeder for selection of superior types, this method is less used by the breeder for crop improvement.
Method # 3. Back-Cross Method:
Back-cross is the event of crossing of F1 with either of the parents, but here in the back-cross method of breeding the hybrid is to be crossed with the superior parent whose genotype is to be transferred to the local variety. For example, in a particular area there is a well-adapted high yielding local variety but susceptible to a particular disease, this variety will be considered as recipient parent.
The disease resistant variety will be the donor parent. The recipient parent will be used in this method of breeding repeatedly to get all the genes to be transferred which is also known as recurrent parent and the donor parent is known as non-recurrent parent.
Genetic consequences of repeated back-crossing:
Repeated back-crossing results in decreasing the frequency of heterozygosity by 50% in each subsequent generation and rapid increase in homozygote, upto 98% by sixth back cross progeny, will be achieved. The genotype of the back-cross progeny becomes increasingly similar to that of the recurrent parent.
The plan of back-cross method differs on the gene which is to be transferred, whether it is dominant or recessive.
(a) Transfer of Dominant Gene:
Selection of Parents:
Variety A — Recurrent parent (well adapted, high yielding); Variety B — Non-recurrent parent, dominant gene controlled (stem rust resistant).
Hybridization (First Year):
The recurrent parent (variety A) should be used as female parent and the non-recurrent parent (variety B) as male parent.
F1 Generation (Second Year):
F1 plants are back-crossed to variety A, in this generation all the plants are heterozygous for rust resistant character.
BC1-Generation (Third Year):
50% of the BC1 plants which are resistant are selected and back-crossed to the recurrent parent, i.e., variety A.
BC2-BC5 Generation (Fourth-Seventh Year):
In each back-cross generation, segregation would occur for rust resistance. Rust resistant plants are selected and back-crossed to recurrent parent, variety A.
BC6-Generation (Eighth Year):
The plants of this generation will have 98.4% genes from variety A. Rust resistant plants are selected and selfed. Their seeds are harvested separately.
BC6-F2 Generation (Ninth Year):
Individual plant progenies of selfed plants are grown separately. The rust resistant plants are selected and the seeds are harvested separately.
BC6-F3 Generation (Tenth Year):
Individual plant progenies are grown. The plants homozygous for rust resistance and similar to the plant type variety A are harvested in bulk.
Yield Test (Eleventh Year):
The new variety is tested in replicated yield trial along with the variety A as a check, as the new variety will be same in performance with variety A only with rust resistant character.
Seed multiplication is done and distributed to the farmers.
(b) Transfer of Recessive Gene:
Procedure for transferring a recessive gene will be different from that of dominant gene, as the recessive gene will be expressed only in homozygous condition. So the selection for that recessive gene requires the F2 generation, i.e., selfing is needed after every two back-crosses, and testing for the presence of that character has to be done in that generation.
Selection of Parents:
The parents are selected in this programme as the previous one, only difference is that the rust resistance is a recessive gene controlled character:
Hybridization (First Year):
The recurrent parent is used as female parent and hybridized with the rust resistant donor plant.
F1 Generation (Second Year):
The F1 plants won’t be rust resistant, these will be back-crossed to the recurrent parent.
BC1 Generation (Third Year):
Here also no rust resistant plants will appear so no need to do any rust susceptibility test. All the plants are selfed.
BC1-F2 Generation (Fourth Year):
The plants are tested for rust resistance character. These plants are selected and back-crossed with recurrent parent.
BC2 Generation (Fifth Year):
There is no rust resistance test, the plants are selected only on resemblances with recurrent parent A, and again back-crossed with recurrent parent.
BC3 Generation (Sixth Year):
Again the plants are self-pollinated, but without any selection for resistance character.
BC3-F2 Generation (Seventh Year):
The selection for rust resistance is done in this generation. The rust resistant plants having resemblance with variety A are selected and back-crossed.
BC4 Generation (Eighth Year):
Same as BC2 generation.
BC5 Generation (Ninth Year):
Same as BC3 generation.
BC5-F2 Generation (Tenth Year):
Plants are selected against rust resistance and also the characters of variety A. The selfed seeds from the selected plants are harvested separately.
BC5-F3 Generation (Eleventh Year):
Individual plant progenies are grown and subjected to rust resistance test. Rigid selection is done for resistance to stem rust and the characteristic of variety A. Seeds from selected plants are mixed together to constitute the new variety.
Yield Test (Twelfth Year):
The variety is tested in replicated yield trial along with the variety A as check. The new variety should be same in yield performance except bearing the new character introduced.
Seed multiplication and distribution to the farmers are carried out.
Comparison between the transfer of dominant and recessive gene during back- cross method:
(c) Transfer of Quantitative Character:
Quantitative characters are governed by polygenes such as grain size, plant height, maturity time, etc. As during transfer intensity of characters will be lost, so the non-recurrent parent must have the characters more intense from than it is desired in the new variety. For transferring this type of character, each back-cross generation should be selfed and F2 generation is grown which should have large population.
Then the rigid selection is done for the character to be transferred. The system of alternate back-crossing and selfing goes on till the desired results are obtained. At the end of back-cross programme, F2 is handled like pedigree method. The progenies which are more likely to recurrent parent bearing the desirable trait are selected and mixed to make up the new variety.
(d) Transfer of Two or More Characters in a Single Parent:
For simultaneous transfer of many characters in a single back-crossing programme, the characters from different non-recurrent parents are brought together by multiple crossing and then back-crossing to recurrent parent is performed. Larger back-cross progenies would be needed than in case of transfer of a single character, and also it may take long time as in each step all the desired characters may not get expressed.
The multiple characters can be transferred in a stepwise method. The recurrent parent is first improved for one character then it is used as recurrent parent for introduction of another new character. Thus in stepwise manner more than one characters can be introduced into a single recurrent parent.
Generally the .back-cross method is used for transferring disease resistance character to a good and well adapted variety. Other quantitative characters can also be transferred through back-cross method in both cross- and self-pollinated crop.
1. Inter-varietal Transfer of Simply Inherited Characters:
The characters like disease resistance, seed colour, plant height, etc., which are controlled by one or two major genes are most suited for transfer through back-cross method from one variety to another of the same species. The successful transfer depends on the minimum linkage between desirable and undesirable trait.
2. Inter-varietal Transfer of Quantitative Characters:
Grain characters like seed size and shape, earliness, plant height-all can be transferred from one variety to another with the criteria of high heritability.
3. Interspecific Transfer of Simply Inherited Characters:
Mainly the character like disease resistance can be transferred from related species to cultivated species. This specially requires the relatedness between the species where the chromosomes can pair during meiosis. The transfer will be unsuccessful if the genetic environment of recurrent parent is not suitable for functioning of the gene of desirable character from donor parent.
4. Transfer of Cytoplasm:
In case of transfer of male sterile character from one parent to another requires back-cross method. The variety or species from which the cytoplasm is to be transferred is used as female parent. The recurrent parent should be the male parent. After 6-8 back-crosses, the cytoplasm will be of the donor parent with the genotype of recurrent parent.
5. Transgressive Segregation:
Modification of back-cross method will produce transgressive segregants. Few (1 to 3) back-crosses with F1 allow much heterozygosity to appear, or two or more recurrent parents may be used in back- cross programme to accumulate genes from them into the back-cross progeny. Such kind of modification would produce new variety which won’t be like the recurrent parent.
6. Production of Isogenic Lines:
Back-cross method is useful for production of isogenic lines, i.e., the lines of a crop which are identical in their genotype except for one gene. These isogenic lines are useful for studying the effect of individual genes.
1. This method does not change the genotype of the popular established variety, only it helps a single desirable character to be transferred in the existing variety.
2. As the recurrent parent is an established variety, so it is not necessary to test the yield performance which ultimately saves five years-time, as well as expenses.
3. In case of short duration plant, 2-3 generations can be raised within a year as the selection is based on inheritance of a particular character, not on performance. So this method drastically reduces the time period required to develop a new variety.
4. Back-cross method requires smaller populations than the pedigree method.
5. Defects of an established variety can be removed using this method, may be only by introducing a single character.
6. Interspecific gene transfer only can be done through this method.
7. Transgressive segregation can be obtained in case of quantitative characters by modified back-cross method.
8. This method is very much useful for cytoplasmic gene transfer to the recurrent parent, i.e., new variety will bear the cytoplasm of donor parent and genotype of recurrent parent.
1. The new variety is not superior in performance than the existing variety except the introduction of single character.
2. During transfer of such kind of single desirable gene, sometimes some undesirable genes may get transferred.
3. For introduction of more than one gene controlled character, multiple crossing programme is necessary which is often difficult, time taking and costly.
4. Once a recurrent parent taken in a back-cross method may get replaced by another superior variety of high yielding ability.
Back-cross method is very much useful for transferring of simply heritable character like disease resistance to well adapted popular local variety.
In case of wheat, ‘Kalyan Sona’ is the popular established variety to which the leaf rust resistance character has been transferred from diverse sources like Robin, KI, Bluebird, Tobari, etc. using back-cross method.
In case of Bajra, Tift 23A, a male sterile line which was susceptible to downy mildew has made resistant through back-cross method.
For interspecific transfer of genes, back-cross method is widely used for crop improvement. Cultivated sugarcane (Saccharum officincirum) is susceptible to pests and disease, crossed with S. spontanium which is resistant. This brings resistance but with undesirable characters like more fibre, low sugar, thin stem, etc. By back-crossing with noble cane these undesired characters are removed.
In cotton, the hybridization between Gossypium hirsutum and G. arboreum yielded highly sterile F hybrid, few tetraploid seeds were obtained. These plants were then back- crossed with G. hirsutum and two varieties have been selected from the back-cross progeny which are now being widely cultivated in Gujarat.
Except the characters of disease resistance other characters can also be transferred by the back-cross method. Such as in cotton, the ginning quality of fibre has been improved by back-cross method from the local variety. By using BD8 (wilt resistant, high spinning value but low ginning out turns) as recurrent parent and Goghari A26 (high ginning out turn) as non-recurrent parent, new variety Vijay was developed.