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Module 10: Recurrent Selection

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1 Module 10: Recurrent Selection
PBG 650 Advanced Plant Breeding Module 10: Recurrent Selection

2 Cyclical selection of populations
Recurrent selection Cyclical selection of populations form families evaluate in trials recombine selections Pedigree selection and improvement of elite lines are also cyclical processes, but the population structure is not so clearly defined Selfing and introgression of new germplasm are common features of both selection systems Recurrent selection and development of lines can be integrated into a comprehensive system Bernardo, Chapt. 10

3 Rationale for recurrent selection
Selfing systems: Fixation of alleles is so rapid that the impact of selection is limited Probability of obtaining segregants with all of the favorable alleles controlling a quantitative trait is small Recurrent selection: systematically increases the frequency of favorable alleles maintains the genetic variation within a population to permit continual progress from selection Example: with 5 loci, all alleles have p=0.5 1/32 chance to get all of the good alleles Example: with 5 loci, all alleles have p=0.6 1/13 chance to get all of the good alleles

4 Recurrent selection in practice
Why is it not used more often? (Bernardo) Easy to apply in cross-pollinated crops; difficult in self-pollinating crops male sterility systems can be used Objectives are long-term several generations needed to complete a cycle Immediate output is an improved open-pollinated variety, not a line or hybrid Need to choose one or a few populations for selection not as much opportunity for speculation in use of germplasm Nonetheless, there are many examples of widescale use of varieties developed from recurrent selection schemes

5 Expected selection response
Source: Lecture by Jean-Luc Jannink at Iowa State, 2004

6 R=h2S Response to selection Selection differential
Realized heritability 60 70 80 90 100 110 120 130 140 150 Recombine to form C1 Select best 10% of C0 S R Falconer and Mackay, Chapt. 11

7 Predicting response to selection
R=h2S Need estimates of h2 and the selection differential In theory, h2 is only applicable for a single generation, because heritability depends on gene frequencies. In practice, predictions seem to work for 5-10 generations.

8 Selection differential
S can be predicted if we can assume: normal distribution of phenotypes truncation selection Standardized selection differential (i) S = iσP i = S/σP = z/p p = proportion selected z = height of curve at truncation point i = standard deviations from the mean

9 Values of standardized selection differential
p i 0.90 0.20 0.09 1.80 0.80 0.35 0.08 1.86 0.70 0.50 0.07 1.92 0.60 0.64 0.06 1.99 0.05 2.06 0.40 0.97 0.04 2.15 0.30 1.16 0.03 2.27 0.25 1.27 0.02 2.42 1.40 0.01 2.67 0.15 1.55 0.005 2.89 0.10 1.76 0.001 3.37 Becker, 1984 – Appendix Tables 2 and 3 (infinite population size)

10 R=h2S S = iσP R=ih2σP Response to selection
Applies to individual plants in a population Selections made before flowering + controlled matings among selected individuals Mass selection + selfing of selected plants

11 in reference population (X) Selection Unit (progeny mean)
Family selection (O) Parental plant in reference population (X) Selection Unit (progeny mean) (W) Individual in improved population (R) Recombination unit Cov(X,W) determines expected gain from selection Hallauer, Carena and Miranda (2010) Chapt. 6

12 Intrapopulation Improvement
Method Progenies tested Recombination unit Mass selection (both parents) Individual plants Mass selection (one parent) outcrossed seed Half-sib (progeny selected) Half-sib families Half-sib (parent is selfed) S1 family Modified ear-to-row Full-sib Full-sib families S2 family

13 Intrapopulation Improvement
Method Expected Gain Generations/Cycle Mass selection (both parents) 1 Mass selection (one parent) Half-sib (progeny selected) 2 Half-sib (parent is selfed) 3 S1/Testcross 4 Modified ear-to-row Full-sib S1 family* S2 family* modified ear-to-row can include within family selection σP is the square root of variance; pertains to selection units *additive variance for inbred progeny includes an additional component that is a function of the degree of dominance

14 Phenotypic variance of families
Half-sibs Full-sibs S1 families S2 families r = # replications e = # environments Error variance Variance due to genotype x environment interactions

15 Interpopulation Improvement
Method Progenies tested Recombination unit Reciprocal recurrent Half-sib families S1 families Reciprocal full-sib Full-sib families Testcross Testcrosses

16 Reciprocal recurrent selection
Half-sibs evaluated (Design I matings) Full-sibs evaluated B0 HS yield trials B0 females A0 females S1 recombined B1 A0 A1 A1 x B1 (improved cross) Full-sib RRS plants must have two “ears” twice the number of plants can be evaluated continue to inbreed and evaluate specific crosses A0 S1 recombined A1 yield trials FS A1 x B1 (improved cross) B0 S1 recombined B1

17 Interpopulation Improvement
Method Expected Gain Generations per cycle Reciprocal recurrent 3 Reciprocal full-sib Testcross Depends on choice of tester, but typically Cross P1 plants to inbred line from P2 Cross P2 plants to inbred line from P1

18 Phenotypic variance of families for RRS
r = # replications e = # environments

19 Comprehensive breeding program
Development of breeding populations from diverse sources such that the performance of the population cross is maximized while maintaining high levels of genetic variance within each population Application of an effective recurrent selection procedure Development of inbreds from each population with good combining ability and recycling of superior inbreds back into the base populations Eberhart et al., 1967

20 Increasing selection response
Increase the selection differential (reduce proportion selected) Increase the coefficient of A2 Increase A2 Reduce nongenetic effects Reduce generations/cycle or increase generations/year

21 Choice of selection method
I. Breeding Objectives Open-pollinated varieties, synthetics or hybrids Status of commercial seed sector Strategy for distribution of seed Elite variety or genetic resource Target production environments Low or high inputs? Narrow or broad adaptation? Number of traits, relative importance of traits

22 Choice of selection method
II. Genetic, Environmental, External Factors Heritability of the trait(s) Extent of GXE Type of gene action Effects of inbreeding on the trait Expected gain per cycle Number of seasons per cycle Growing seasons per year and availability of off-season nurseries Seed quantities required for screening Costs and resources available

23 Maize families – seed quantity issues
Family Crosses Seed quantity Comments Half-sibs Collect pollen in bulk and cross to a female plant Take pollen from one male and cross to several females One ear ~4 ears Controlled pollinations or by detasseling Full-sib families within half-sibs Full-sibs Cross two plants One or two ears With or without reciprocals S1, S2, etc. Self pollination Seed quantities decrease with inbreeding Can increase a line by selfing or by sib-mating Testcrosses Cross one male plant to a female tester and self Cross an S1 line to a tester (population or inbred line) 1. ~4 ears 2. many ears Controlled pollinations or by detasseling (if S1 line is female) one ear  at least four single-row plots

24 Maintenance of Maize Streak Virus Resistance
Modified full-sib family selection Year 1 Main season, savanna zone Evaluate full-sib families in target environments Off-season: data entry and analysis Year 2 First season, forest zone Recombine selected full-sib families by making plant to plant crosses between families Second season (high disease pressure) Plant F1 families ear-to-row under MSV infestation Remove susceptible plants and offtypes before flowering Make reciprocal crosses between best plants in good rows to generate new full-sib families

25 Reciprocal S1 Testcross Selection (modified)
Year 1 First season Self Second season (high disease pressure) Evaluate ~500 S1 families (2 reps, 2 loc) Select for disease resistance and other highly heritable traits Testcross to the reciprocal population Year 2 Main season Evaluate ~ 200 testcrosses (3 reps, 4 loc) in the target environments Select for yield and other agronomic traits Off-season Recombine selected S1 families Selection for yield at the S1 stage applies strong selection pressure for lateness and increased plant height Can stagger populations so that one is at the S1 stage and the other is at the testcross stage each year

26 Meadowfoam - use of blue bottle flies as pollinators

27 S1 testcross selection in meadowfoam
Year 1 (spring) Self ~300 plants in the greenhouse with blue bottle flies Year 1 (fall) + Year 2 (spring) Plant rows of ~5 seeds per family in isolation with bees, 2 blocks Reject S1 families with poor agronomic characteristics (disease, insect damage, small seeds, etc) Harvest ~6000 seeds per family in bulk Year 2 (fall) + Year 3 (spring) Evaluate ~150 testcross families in yield trials, select ~30 Year 3 (fall) Recombine S1 seed of selected families in greenhouse Further selfing of selected S1 lines Evaluation of experimental varieties in yield trials

28 Balancing resources for recurrent selection
*Daylength can be controlled in the greenhouse to complete a generation in four months Makes efficient use of greenhouse space New experimental varieties can be evaluated every year


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