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Backcross Breeding

Backcross Breeding. History of Backcrossing. Harlan and Pope, 1922 Wanted the smooth awns from European barleys in the domestic barleys Crosses with European types were not fruitful Decided to backcross smooth awn

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Backcross Breeding

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  1. Backcross Breeding

  2. History of Backcrossing • Harlan and Pope, 1922 • Wanted the smooth awns from European barleys in the domestic barleys • Crosses with European types were not fruitful • Decided to backcross smooth awn • After 1 BC, progeny resembled Manchuria and they were able to recover high yielding smooth awn types

  3. Terminology • Recurrent parent (RP) - parent you are transferring trait to • Donor or nonrecurrent parent (DP) - source of desirable trait • Progeny test - when trait is recessive

  4. Single dominant gene for disease resistance- pre flowering • Cross recurrent parent (rr) with resistant donor parent (RR) - all F1s are Rr rr x RR Rr

  5. Single dominant gene for disease resistance- pre flowering • Cross F1 to Recurrent Parent to produce BC1 progeny which are 1 Rr: 1 rr Rr x rr R r Rr rr R allele only present in heterozygous form

  6. Single dominant gene for disease resistance- pre flowering • Evaluate BC1s before flowering and discard rr plants; cross Rr plants to Recurrent Parent Rr – keep rr - discard

  7. Single dominant gene for disease resistance- pre flowering • BC2 F1 plants evaluated, rr plants discarded, Rr plants crossed to Recurrent Parent • BC2 F1 plants evaluated, rr plants discarded, Rr plants crossed to Recurrent Parent • BC4 F1 plants evauated, rr plants discarded, Rr plants selfed to produce BC4 F2 seeds, which are 1RR: 2 Rr: 1rr

  8. Single dominant gene for disease resistance- pre flowering • BC4 F2 plants evaluated before flowering, rr discarded, R_ selfed and harvested by plant, then progeny tested. Segregating rows discarded, homozygous RR rows kept and tested.

  9. Single dominant gene - post flowering • Cross susceptible Recurrent Parent (rr) with resistant Donor Parent (RR) - all F1s are Rr rr x RR Rr X rr BC1 rr; Rr

  10. Single dominant gene - post flowering • Cross F1 to Recurrent Parent to produce BC1 progeny which are 1 Rr: 1 rr • Because we can’t evaluate the trait before flowering, a number of BC1F1 plants must be crossed to Recurrent Parent, then the trait is evaluated and susceptible plants discarded • This procedure is therefore less efficient than the pre-flowering trait because we have made crosses that we cannot use

  11. Single dominant gene - post flowering • BC2F1 plants (1 Rr:1rr) are crossed to RP, trait evaluated before harvest, susceptible plants discarded

  12. Single dominant gene - post flowering • Procedure followed through BC4 • Seeds from each BC4 F2 individual are harvested by plant and planted in rows • Segregating rows are discarded, homozygous RR rows are maintained, harvested and tested further

  13. Single recessive allele - progeny test in same season • Cross susceptible (RR) Recurrent Parent to resistant (rr) Donor Parent • F1 plants crossed to Recurrent Parent, BC 1 seeds are 1 RR:1Rr • Note now that all BC1 plants are susceptible; we are interested only in those plants which carry the resistant “r” allele • All BC1 plants crossed to Recurrent Parent and selfed to provide seeds for progeny test

  14. Single recessive allele - progeny test in same season • Screen BC1F2 plants before BC2F1 plants flower. BC1 F1 plants that are RR will have only RR progeny. BC1 F1 plants that are Rr will produce BC1F2 progeny that segregate for resistance.

  15. Single recessive allele - progeny test in same season • BC2 F1 plants from heterozygous (Rr) BC1 plants are crossed to RP; those from susceptible (RR) BC1 plants are discarded • BC2 F2 selfed seed is harvested for progeny testing • Progeny tests are conducted before BC3F1 plants flower. Only plants from (Rr) BC2 plants are crossed to Recurrent Parent

  16. Single recessive allele - progeny test in same season • Each BC4F1 plant is progeny tested. Progeny from susceptible BC3 plants are all susceptible and family is discarded • If progeny test completed before flowering, only homozygous resistant (rr) plants are selfed. Otherwise, all plants selfed and only seed from (rr) plants harvested. • Additional testing of resistant families required.

  17. Single recessive allele - progeny test in different season • Cross susceptible (RR) Recurrent Parent to resistant (rr) Donor Parent • F1 plants crossed to RP, seeds are 1 RR:1Rr • Again, we are interested in plants carrying the resistant “r” allele – we can’t distinguish them yet from RR types

  18. Single recessive allele - progeny test in different season • The difference is now that we cannot do the progeny test in the same season because the resistance is expressed late in plant’s life. • BC1 plants selfed, seed harvested by plant • BC1F2 plants grown in progeny rows, evaluated, seed from resistant (rr) rows is harvested. BC1F3 progeny crossed to Recurrent Parent to produce BC2F1 seeds.

  19. Single recessive allele - progeny test in different season • BC2F1 plants crossed to Recurrent Parent to obtain BC3F1 seeds which are 1Rr: 1 RR • BC3F1 plants are selfed, and progeny are planted in rows • BC3F2 seeds are harvested from resistant (rr) progeny rows • Resistant BC3F3 plants crossed to RP to produce BC4F1 seeds

  20. Single recessive allele - progeny test in different season • BC4 F1 plants selfed and produce 1RR:2Rr:1rr progeny • BC4F2 plants selfed and resistant ones harvested by plant • Resistant families tested further

  21. Importance of cytoplasm • For certain traits (e.g. male sterility) it is important that a certain cytoplasm be retained • In wheat, to convert a line to a male sterile version the first cross should be made as follows: Triticum timopheevi (male sterile) x male fertile wheat line. From that point on, the recurrent parent should always be used as the male.

  22. Cytoplasmic male sterility in Wheat Triticum timopheevi x Elite breeding line (Male sterile) (Male fertile) F1 (female) x RP (male) Carry out for 4 BC; use male-sterile version of elite breeding line as female parent in hybrid

  23. Probability of transferring genes • How many backcross progeny should be evaluated? • Consult table in Fehr, p. 367; for example in backcrossing a recessive gene, to have a 95% probability of recovering at least 1 Rr plant, you need to grow 5 backcross progeny.

  24. Probability of transferring genes • To increase the probability to 99% and the number of Rr plants to 3, you must grow 14 progeny • If germination is only 80%, you must grow 14/0.8 = 18 progeny

  25. Recovery of genes from RP • Ave. recovery of RP = 1-(1/2)n+1, where n is the number of backcrosses to RP • The percentage recovery of RP varies among the backcross progeny • For example, in the BC3, if the Donor Parent and Recurrent Parent differ by 10 loci, 26% of the plants will be homozygous for the 10 alleles of the Recurrent Parent; remainder will vary.

  26. Recovery of genes from Recurrent Parent • Selection for the Recurrent Parent phenotype can hasten the recovery of the Recurrent Parent • If the number of BC progeny is increased, selection for Recurrent Parent can be effective

  27. Linkage Drag • When backcrossing, we often get more than one gene from the donor parent • The additional genes may be undesirable, hence the term linkage drag • Backcrossing provides opportunity for recombination between the favorable gene and the linked unfavorable genes

  28. Linkage Drag • Recombination fraction has a profound impact: with c=0.5, probability that undesirable gene will be eliminated with 5 BC is 0.98 • with c=0.02, probability that undesirable gene will be eliminated with 5 BC is 0.11

  29. Backcrossing for Quantitative Characters • Choose Donor Parent that differs greatly from Recurrent Parent to increase the likelihood of recovery of desired trait (earliness for example) • Effect of environment on expression of trait can be a problem in BC quantitative traits

  30. Backcrossing for Quantitative Characters • Consider selfing after each BC • Expression of differences among plants will be greater • May be possible to practice selection • Single plant progeny test will not be worthwhile; must use replicated plots

  31. Other Considerations • Marker assisted backcrossing • Assume that you have a saturated genetic map • Make cross and backcross • To hasten the backcrossing process, select against the donor genotype (except for the marker(s) linked to the gene of interest) in backcross progeny

  32. Marker-Assisted Backcrossing • May improve efficiency in three ways: • 1) If phenotyping is difficult • 2) Markers can be used to select against the donor parent in the region outside the target • 3) Markers can be used to select rare progeny that result from recombinations near the target gene

  33. Model Two alleles at marker locus: M1 and M2 Two alleles at target gene: Q1 and Q2 M1 Q1 R=0.10 Q2 M2 Q2 is the target allele we want to backcross into recurrent parent, which has Q1 to begin with.

  34. Recombination • Assume recombination between marker and QTL=10% • Select one plant based on marker genotype alone, 10% chance of losing target gene • Probability of not losing gene=(1-r) • For t generations, P=1-( 1-r )t • For 5 BC generations, probability of losing the target gene is P=1-(.9)5=0.41

  35. Flanking Markers Best way to avoid losing the target gene is to have marker loci flanking it MA1 rA Q1 rB MB1 MA2 Q1 MB2

  36. Flanking Markers Probabilityof losing the target gene after selecting On flanking markers: Example: If the flanking markers have 10% recombination frequency with the target gene:, the probability of losing the gene after 1 generation is P=0.024. The probability of losing the gene after 5 generations is P=0.1182

  37. Other Considerations • Backcross breeding is viewed as a conservative approach • The goal is to improve an existing cultivar • Meanwhile, the competition moves past

  38. Backcross Populations • May be used as breeding populations instead of F2, for example • Studies have shown that the variance in a backcross population can exceed that of an F2 • Many breeders use 3-way crosses, which are similar to backcrosses

  39. Marker Assisted BC

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