PRINCIPLES OF CROP PRODUCTION ABT-320 (3 CREDIT HOURS)

1. PRINCIPLES OF CROP PRODUCTIONABT-320(3 CREDIT HOURS) LECTURE 8 BACKCROSS METHOD GENETIC BASIS OF INBREEDING PLANT HYBRIDIZATION

2. BACKCROSS METHOD • The crossing of an F1 hybrid with either of its parent is referred to as backcross. It is used as a method of breeding to improve an otherwise high yielding and popular variety for a specific character. • A superior variety which needs improvement for a special character serves as the recipient or recurrent parent. The donor or non-recurrent parent acts as a source of gene(s) to be added to the recurrent parent. Any character with good testability can be transferred in this way. Oligogenes can easily be transferred by this backcross method. However, this method can be employed even in the case of polygenic characters.

4. THE GENETIC BASIS OF INBREEDING Backcross method involves a type of inbreeding since the F1 is repeatedly backcrossed with the recipient parent. The F1 contains 50 percent genes from each of the two parents. Repeated backcrossing of the F1 with the recurrent parent would systematically increase the proportions of gene from the recurrent parent at the cost of the genes from the donor parent.

5. THE PROCEDURE OF BACKCROSS METHOD It involves: • Transfer of a single dominant gene • Transfer of a recessive gene • Transfer of a quantitative character

6. TRANSFER OF A SINGLE DOMINANT GENE • The recurrent (rr) and donor (RR) parents are crossed to produce the F1 (Rr). The recurrent parent is usually used as the female parent. If a cytoplasmic character is transferred, the recurrent parent is used as the male parent. • In the second season, the F1 plants are backcrossed to the recurrent parent to produce the first backcross generation (BC1) seeds. • In the third season, the BC1 plants which consist of Rr and rr in equal proportions are subjected to screening for the character under transfer. The desirable plants are selected and crossed with the recurrent parent to produce the BC2 seeds. • In the fourth season, the BC2 plants are grown and they will also segregate in the 1:1 (Rr:rr) ratio. The dominant plants are selected and backcrossed with the recurrent parent to produce the BC3 seeds.

7. TRANSFER OF A SINGLE DOMINANT GENE • In the fifth season, BC3 plants (Rr:rr) are grown and BC4 seeds are collected as in the case of the previous generation. In the sixth season, BC4 plants are grown and evaluated. If only four backcrosses are planned, the desirable BC4 plants are self-pollinated and BC4F2 seeds are produced. • In the seventh season, the BC4F2 segregates in the 1:2:1 (RR:Rr:rr) ratio. The recessive plants are eliminated and others are self-pollinated to produce BC4F3 seeds. • In the eighth generation, BC4F3 plants are raised in progeny rows. Progenies produced by the homozygous (RR) BC4F2 plants will be uniform for the desired characters, whereas the progenies produced from Rr plants will show segregation. All the uniform progeny rows with the desired character are bulked and released as the improved variety.

8. TRANSFER OF A RECESSIVE GENE In this case, the donor parent will be the recessive and usually it is taken as the male parent. In the first season, the dominant x recessive cross is made and F1 seeds are collected. In the second season, the F1 is grown and backcrossed with the recurrent (dominant) parent and the BC1 seeds are collected. In the third season, the BC1 is grown and selfed to produce BC1F2 seeds. In the fourth season, the BC1F2 is grown, the dominant plants are eliminated and the other plants are selected and crossed with the recurrent parent to produce the BC2. In the fifth season, the BC2 is crossed with the recurrent parent to produce the BC3 generation. In the sixth season, the BC3 is grown; selfed and BC3F2 seeds are collected. In the seventh season, the BC3F2 is grown, dominant plants are eliminated and others are selected. BC4 seeds collected from the selected plants. In the eighth generation, the BC4 plants are grown and selfed to produce the BC4F2 seeds. In the ninth season, BC4F2 is grown and recessive plants are selected, their seeds bulked and the progeny will be the desired strain of the recurrent parent with the recessive gene from the non-recurrent parent transferred to it.

9. TRANSFER OF A QUANTITATIVE CHARACTER • Since more number of loci are involved in the control of a quantitative character and the relative influence of environment in the expression of such traits is higher, a relatively large population is to be sampled in each backcross generation. The F1 is raised in the first season by crossing the recurrent parent and the donor parent. In the second season, BC1 is produced. Sufficient quantity of crossed seeds is produced so as to facilitate selection in the BC1 itself. In the third season, the BC1 plants are evaluated and those with high intensity of the character under transfer are selected since they carry the maximum number of genes for the character. BC1F2 seeds are produced by selfing. • In the fourth season, the BC1F2 is raised and is evaluated both for the character under transfer and the general features of the recurrent parent. Plants resembling the recurrent parent with the highest intensity of the character under transfer are selected and harvested individually.

10. TRANSFER OF A QUANTITATIVE CHARACTER In the fifth season, BC1F3 is raised in progeny rows from the above seeds and evaluated extensively for the character under transfer and general features of the recurrent parent. The selected F3 plants are crossed with the recurrent parent to produce BC3 seeds. In the seventh season, the BC3 plants are grown and selected for the character under transfer and the general feature of the recurrent parent. At the end, a number of line resembling the recurrent parent and showing the character under transfer are bulked to constitute the modified variety of the recurrent parent.

11. MERITS & LIMITATIONS OF BACKCROSS METHOD This method is very effective to transfer a desired trait to an otherwise improved variety. Evaluation of the variety can be limited to the extent of confirming the transfer of the character under consideration. It provides the ideal solution to utilize the unique properties of an otherwise unadapted germplasm. The limitation of backcross method is that the procedure is time consuming and it does not result in the improvement of other characters.

12. PLANT HYBRIDIZATION Genotype peculiarities of two or more different varieties or species of plants could be brought together only by crossing them. The mating or crossing of two plants or lines of dissimilar genotype is called hybridization. In plant hybridization, one of the plants is taken as the female plant and the other as the male plant. Pollen grains from the male parent are made to pollinate the stigma of the flowers of the female parent. The seeds obtained from such a cross are called F1 seeds and the progeny raised from it is called F1 (First Filial) generation. The F1 is selfed to produce F2 and the subsequent generations like F3, F4 etc are raised in the same way. These generations are called segregating generations and they are handled differently based on the scope and objectives of the breeding program.

13. OBJECTIVES OF HYBRIDIZATION The major objectives of hybridization are: A. Combination Breeding Combination breeding is the transfer of one or more characters from other varieties to a particular variety. These characters may be oligogenic or polygenic. In this way, genes for disease resistance, quality traits etc can be transferred. B. Transgressive Breeding Transgressive breeding is based on transgressive variation of characters in segregating generations like F2. A cross is made between two strains of plants and the F2 is screened for transgressive variations. Transgressive segregation is the segregation of characters beyond the parental limits, in the segregating generations like F2. C. Production of Hybrids Hybrid (F1) plants show higher vigor and yield when compared to parents, in some cases. This phenomenon is called hybrid vigor. F1 seeds can be raised in bulk through hybridization and distributed directly for cultivation, especially in cross-pollinating crops.

14. TYPES OF HYBRIDIZATION Based on the genetic difference between parents, hybridization can be classified into: 1. Inter-varietal Hybridization 2. Distant Hybridization

15. INTER-VARIETAL HYBRIDIZATION The cross between the members of the same species (intra-specific) is called inter-varietal hybridization. In this type of hybridization, different cross patterns can be used. 1. Simple Cross In this case, two parents are used to produce an F1 hybrid. 2. Complex Crosses In complex crosses, more than two parents are involved. Such crosses can be called convergent crosses since they bring genes from different sources together.

16. DISTANT HYBRIDIZATION • Hybridization between the members of different species or hybridization beyond species level is called distant hybridization. Thus, it may be interspecific (intra-generic) or inter-generic. When conventional methods of hybridization fails, para-sexual methods are used in such cases. • Para-sexual hybridization is the technique of fusing somatic protoplasts when reproductive cells fail to fuse or fertilize.

17. THE PROCESS OF HYBRIDIZATION The major steps involved in the process of hybridization are: 1. Selection of Parents 2. Emasculation 3. Bagging 4. Tagging 5. Pollination 6. Harvesting F1 Seeds 7. Further handling of the plants

18. SELECTION OF PARENTS The choice of the parents depends on the objective of the cross. In combination breeding, the genetic diversity of the parents is not important. In the case of transgressive breeding, genetically diverse plants are selected as parents. If the characteristics of the parents are not completely known, they are evaluated for the agronomic features.

19. EMASCULATION In the case of crops with bisexual flowers, stamens of the flowers of the female parents are removed or the pollen grains are killed. This process is called emasculation. Mechanical, physiological or genetic methods of emasculation are used, depending upon the nature of the crop and the cross.

20. MECHANICAL METHODS OF EMASCULATION Here, the anthers are removed from the flowers of the female parents. Hand emasculation and suction method are generally used. For hand emasculation, the flower buds are opened carefully before anthesis (First opening of the flower) and the anthers are removed with the help of forceps. Care should be taken so that the gynoecium of the flowers in not damaged. In suction method, the petals are removed from the flowers before anthesis, with the help of forceps. Then, a thin rubber or glass tube attached to a suction hose is used to suck the anthers from the flowers.

21. PHYSIOLOGICAL METHODS OF EMASCULATION Here, the anthers are killed with the help of heat treatment, cold treatment or alcohol treatment.

22. GENETIC EMASCULATION Genetic or cytoplasmic male sterility factors are introduced into the female parents to make them sterile.

23. BAGGING The emasculated inflorescences of female plants are covered using butter paper bags or cloth bags. However, in the case of cross-pollinated crops, male plants may also be bagged if desired, so as to avoid pollen mixture. The bags are removed 2-3 days after pollination.

24. TAGGING Emasculated flowers are tagged properly after bagging. Circular or rectangular tags may be used. Details of the cross, date of emasculation, date of pollination and the number of flowers emasculated must be noted on the tag. Carbon pencil or permanent ink may be used for tagging.

25. POLLINATION Mature, fertile and viable pollen grains are collected from the male parent and dusted on the stigma of the female parent. Care should be taken to see that the pollen grains are dusted at the optimum stage of viability.

26. HARVESTING F1 SEEDS Crossed seeds are harvested carefully and stored to raise the F1 generation.

27. FURTHER HANDLING OF THE PLANTS Further handling of the hybrids depends on the objective of the cross. In the case of hybrid seed production, the F1 seeds are directly released to farmers. In the case of combination breeding and transgressive breeding, F2 is raised and the most appropriate solution program is used.

28. THE END