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Breeding methods in cross pollinated crops

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Breeding methods in cross pollinated crops

  1. 1. Deva Ram M.Sc Ag Seed science and technology Department of Genetics and Plant Breeding SHIATS Allahabad UP. India
  2. 2. Contents •Introduction •Characters of cross pollinated crops •Methods of breeding in cross pollinated crops •Mass selection •Progeny selection (or) ear-to-row method •Modified ear-to-row method •Recurrent selection •Hybrid varieties •Synthetic varieties •Breeding methods common for both self and cross pollinated crops •Conclusion •References
  3. 3. 1. Each genotype has equal chances of mating with all other genotypes. 2. Individuals are heterozygous in nature. 3. Higher degree of inbreeding depression. 4. Wide adaptability and more flexibility to environmental changes due to heterozygosity and heterogenity. 5. Cross pollination permits new gene combinations from different sources. 6. Individuals have deleterious recessive gene which are concealed by masking effect of dominant genes.
  4. 4. 1) Population improvement 1) Without progeny testing 1) Mass selection 2) With progeny testing 1) Progeny selection 2) Recurrent selection 2) Hybrid varieties 3) Synthetic varieties Methods of breeding in cross pollinated crops
  5. 5. Mass selection is often described as the oldest method of breeding. Mass selection is an example of selection from a biologically variable population. Selection is based on plant phenotype.
  6. 6. Selection cycle may be repeated one or more times Original population FIRST YEAR (FIRST SELECTION CYCLE) SECOND YEAR (SECOND SELECTION CYCLE) i) Several plants selected on the basis of phenotype ii) Open-pollinated seed from the selected plants harvested and bulked i) Bulked seed from selected plants grown ii) Mass selection may be repeated i.e., items (i) and (ii) of the first year repeated Yield trials MASS SELECTION
  7. 7. Rapid and Simple breeding method. Selection cycle is very short. Have high heritability. Effective in improving yield of cross pollinated crops.
  8. 8. Phenotypic performance is greatly influenced by environmental factors. No control on the pollination. Inbreeding depression.
  9. 9. Progeny selection The simplest form of progeny selection is ear-to-row method Developed by Hopkins (1908) This method was extensively used in Maize
  10. 10. ORIGINAL POPULATION MAY BE REPEATED ONE OR MORE TIMES YIELD TRIALS FIRST YEAR SECOND YEAR THIRD YEAR i) Plants selected on the basis of phenotype ii) Open-pollinated seed from each plant harvested separately i) Small progeny rows grown from the selected plants ii) Superior progenies identified and selected iii) Plants allowed to open-pollinate; seed is harvested separately EAR-TO-ROW METHOD Same as in second year First selection cycle Second selection cycle Third selection cycle
  11. 11. It should be seen that ear-to-row method is relatively simple and the selection cycle is of one year only However it suffers from the defect that plants in the superior progenies are pollinated by those in both the superior and inferior progenies ; this reduces the effectiveness of selection
  12. 12. MAY BE REPEATED ONE OR MORE TIMES YIELD TRIALS FIRST YEAR SECOND YEAR THIRD YEAR FOURTH YEAR i) Plants selected on the basis of phenotype ii) Open-pollinated seed from each plant harvested separately i) Small progeny rows grown from the selected plants ii) Superior progenies identified iii) Remaining seeds from the plants (selected in first year) producing superior progenies bulked to raise the next generation Plants producing superior progenies MODIFICATION OF EAR-TO-ROW METHOD As in second year First selection cycle i) The bulk seed [yr 2, item(iii)] planted ii) Plants allowed to open-pollinate iii) Plants with superior phenotype selected and seed harvested separetely Second selection cycle
  13. 13. Merits of progeny selection 1.It is based on progeny test and not on the phenotypes of individual plants hence it is far more accurate reflection of the genotype than phenotype 2.Inbreeding may be avoided if care is taken to select a sufficiently large number of plant progenies 3.Selection scheme is relatively simple and easy
  14. 14. Demerits of progeny selection 1.There is no control on pollination and plants are allowed to open-pollinate. Thus the selection is based on the maternal parent only 2.The selection time is two years. Thus the time required for selection is as much as in the case of mass selection
  15. 15. “Reselection of generation after generation with inter breeding of selected plants to provide for genetic recombination. The procedure of Recurrent selection was described by Jenkins in 1940. The term Recurrent selection was given by Hull in 1945.
  16. 16. Simple recurrent selection. Recurrent selection for general combining ability. Recurrent selection for specific combining ability. Reciprocal recurrent selection.
  17. 17. Selection is based on Phenotypic characters of plants. Tester is not used in this scheme. It does not measure the combining ability. This method is useful only for those characters which have high heritability.
  18. 18. 1st YEAR 2nd YEAR 3rd YEAR 4 th YEAR MAY BE REPEATED AS IN 1st CYCLE Individual plant progenies are planted. All possible intercrosses are made and seeds are composited. Original Selection cycle STEPS IN BREEDING A VARIETY BY SIMPLE RECURRENT SELECTION Superior phenotypes are selected. Selected plants self-pollinated Seeds are harvested separately Individual plant progenies are planted. All possible intercrosses are made and seeds are composited. Composited intercross seeds are planted . selfing is done. 1st Recurrent selection cycle
  19. 19. In this method selection is based on heterozygous tester cross performance. Improves general combining ability of population for a character. This method is used for genetic improvement of quantitative characters. Improves those characters governed by additive gene action.
  20. 20. 6th Year MAY BE REPEATED AS IN 1st CYCLE Original Selection cycle SELF POLLINATED SEEDS SELF POLLINATED SEEDS TESTER INTER CROSS BLOCK INTER CROSS BLOCK TESTER REPLICATE D YEILD TRAIL REPLICATE D YEILD TRAIL 1st Recurrent selection cycle 1st Year 2nd Year 3rd Year 4th Year 5th Year
  21. 21. In this method selection is based on Homozygous tester cross performance. Improves specific combining ability of population for a character. This method is used for genetic improvement of polygenic characters.
  22. 22. 1st Year 2nd Year 3rd Year 4th Year 5th Year 6th Year MAY BE REPEATED AS IN 1st CYCLE Original Selection cycle 1st Recurrent selection cycle SELF POLLINATED SEEDS SELF POLLINATED SEEDS TESTER REPLICATED YEILD TRAIL TESTER REPLICATED YEILD TRAIL INTER CROSS BLOCK INTER CROSS BLOCK
  23. 23. In this method two heterozygous testers are used for crossing. This scheme was proposed by Comstock,et al. in 1949. Improves both GCA and SCA of population for a character. This method is used for genetic improvement of polygenic characters. This method is also known as recurrent reciprocal half sib selection.
  24. 24. 1st Year 2nd Year 3rd Year TEST CROSS TEST CROSS SEPARATE FIELD TRAILS SELFEDSEEDS SELFEDSEEDS Original Selection cycle COMPOSITE SEESDS OF INTERCROSS ARE RAISED AND FIRST RECURRENT CYCLE IS REPEATED
  25. 25. Recurrent selection is efficient breeding method for increasing the frequency of superior genes in a population. It helps in maintaining high genetic variability in a population due to repeated intermating of heterozygous population. The selection is made on the basis of test cross performance and only selected plants are allowed for inter mating.
  26. 26. This method involves lot of selection, crossing and selfing work. This method permits selfing which leads to loss of genetic variability. This method is not used directly for the development of new varieties.
  27. 27. HYBRID VARIETIES Hybrid varieties are the first generations (F1) from crosses between two purelines, inbreds, open pollinated varieties, clones or other populations that are genetically dissimilar Hybrid varieties were first commercially exploited in maize
  28. 28. Inbred: A nearly homozygous line obtained through continuous inbreeding of a cross-pollinated species with selection accompanying inbreeding Top cross: Cross between an inbred line and an open-pollinated variety Test cross: Cross between F1 and homozygous recessive parent Single Cross: A x B Double cross: (A x B) x (C x D) Three way cross: (A x B) x C Varietal Cross: A cross between two varieties Terminology related to production of hybrid varieties
  29. 29. 1) Development of inbreds 2) Evaluation of inbreds 3) Production of hybrid seed Operations in the production of hybrid varieties
  30. 30. Development of inbred lines Pedigree method is generally practiced in the development of inbreds First year: •A number of plants with desirable phenotypes are selected from source population and self-pollinated •The selected plants should be vigorous and free from diseases •They may be selected on the basis of their GCA estimates Second year: •About 30-40 plants are space planted from the selfed seed from each of the selected plants
  31. 31. Third to sixth years: •Process of second year is repeated •As the number of generations of self-pollination increases, individual plant progenies would become more and more homozygous Seventh year: •At this stage, individual plant progenies would be almost homozygous •Selfing may be discontinued and inbreds may be maintained by sib-pollination
  32. 32. Evaluation of inbreds If all the inbreds developed from an open- pollinated variety were mated at random, the average yield of all the single crosses would be the same as that of the open-pollinated variety Evaluation of inbreds may be divided into four steps i)Phenotypic evaluation ii)Topcross test iii)Single cross evaluation iv)Prediction of double cross performance
  33. 33. Phenotypic evaluation •It is based on the phenotypic performance of the inbreds •Highly effective for characters with high heritability •Effective in improving the yielding ability of hybrids as the yield of inbreds shows a small but positive correlation with the performance of their hybrids •Thus inbreds with poor performance can be safely rejected Topcross test •Reliable measure of the average performance of all single crosses involving the inbred •Genotypes remaining after the phenotypic evaluation are crossed to a tester with a wide genetic base •Performance of the top cross progeny is evaluated in replicated yield trials •About 50% of the inbreds are eliminated
  34. 34. Single cross evaluation: •Inbreds remaining after top cross test are crossed in a diallel manner to produce all possible single crosses •In diallel system each inbred is crossed with every other inbred •The performance of single crosses is evaluated in a replicated yield trial •Outstanding single crosses are identified and may be released as hybrid varieties Prediction of Double Cross Performance: The predicted performance of any double cross is the average performance of the four non-parental single crosses involving the four parental inbreds
  35. 35. Production of hybrid seed Requirements for hybrid seed production 1.Easy emasculation of the pollen parent 2.Effective pollen dispersal from the male parent. Emasculation Either hand emasculation or male sterility are used for emasculation Male sterility is economical for commercial production Pollination Artificial pollination is applied for better seed set
  36. 36. Merits •Exploit both GCA and SCA components of heterosis thus utilizing heterosis to a greater extent •Produce from hybrid varieties is more uniform as compared to that of open-pollinated, synthetic or composite varieties •Genetic composition of hybrids don’t change over time as they are maintained in the form of parental inbreds •Can be produced both in self and cross pollinated species
  37. 37. Demerits •Farmers have to use new hybrid seed every year •Hybrid production requires considerable technical skill which makes it tedious and costly •Exploitation of full potential of hybrid varieties requires an adequate and timely supply of inputs like irrigation, fertilizers, weed control, etc. many farmers are unable to ensure timely application of these inputs •In most cross-pollinated species , the requirements of isolation are rigid and difficult to fulfill except on large farms
  38. 38. SYNTHETIC VARIETIES •A variety produced by crossing in all combinations a number of lines that combine well with each other is known as a synthetic variety •Once synthesized, a synthetic is maintained by open- pollination in isolation •The lines that make up a synthetic variety may be inbred lines, clones, open-pollinated varieties, short term inbred lines or other populations tested for GCA or for combining ability with each other
  39. 39. 1. Evaluation of lines for GCA 2. Production of synthetic variety 3. Multiplication of synthetic variety Operations in producing a synthetic variety
  40. 40. Evaluation of lines for GCA •GCA of the lines to be used as parents of synthetic varieties is generally estimated by top cross or poly cross test •Poly cross refers to the progeny of a line produced by pollination with a random sample of pollen from a number of selected lines •The lines are evaluated for GCA because synthetic varieties exploit that portion of heterosis, which is produced by GCA
  41. 41. Production of a synthetic variety Method-1 •Equal amounts of seeds from the parental lines are mixed and planted in isolation •Open-pollination is allowed and is expected to produce crosses in all possible combinations •Seed is harvested in bulk; the population raised from this seed is the syn1 generation Method-2 •All possible crosses among the selected lines are made in isolation •Equal amounts of seed from all the crosses is composited to produce the syn1 generation Available experimental evidence suggests that both the above methods produce comparable results
  42. 42. •After a synthetic variety has been synthesised, it is generally multiplied in isolation for one or more generations before its distribution for cultivation •The open-pollinated progeny from the syn1 generation is termed as syn2 , that from syn2 as syn3 , etc. •The performance of syn2 is expected to be lower than that of syn1 due to the production of new genotypes and a decrease in the level of heterozygosity as a consequence of random mating •The synthetic varieties are usually maintained by open-pollination seed and may be further improved through population improvement schemes particularly through recurrent selection Multiplication of synthetic variety
  43. 43. Inbreds, synthetics, open- pollinated populations X Tester (open-pollinated variety) METHOD-I Composited seed of all lines METHOD-II Crossing block Equal seed from all crosses composited Seed multiplication Open-pollinated seed harvested Step-1 Evaluation of lines for GCA Step-2 Production of the synthetic Step-3 Seed multiplication Top cross or poly cross test for GCA; outstanding lines selected as parents Method-1. •Equal seed from all the lines mixed and planted in isolation. •Open-pollinated seed harvested as the synthetic variety (Syn1) Method-2. •The parental lines are placed in a crossing block. •All possible intercrosses are made. •Equal seed from all the crosses mixed to produce the synthetic variety (syn1) •Seed of synthetic variety may be multiplied for one or two generations before distribution. •Open-pollination in isolation (Syn1 or Syn2) STEPS INVOLVED IN PRODUCTION OF SYNTHETIC VARIETIES
  44. 44. Merits •Offers a feasible means of utilizing heterosis in crop species where pollination control is difficult •Farmers can use the grain produced from a synthetic variety as seed to raise the next crop •In variable environments synthetics are likely to do better than hybrid varieties •Cost of seed in case of synthetics is relatively lower than that of hybrid varieties •Synthetic varieties are good reservoirs of genetic variability •Offers a possibility for continuous improvement of varieties
  45. 45. Demerits •Performance of synthetics is usually lower than that of single cross and double cross hybrids •Performance of synthetics is adversely affected by lines with relatively poorer GCA. Such lines often have to be included to increase the number of parental lines making up the synthetics as lines with outstanding GCA are limited in number •Synthetics can be produced and maintained only in cross-pollinated crop species, while hybrid varieties can be produced both in self and cross-pollinated crops
  46. 46. Breeding methods common for both self and cross-pollinated Introduction Back cross method Polyploidy breeding Mutation Breeding Transgenic breeding

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